U.S. patent number 3,896,008 [Application Number 05/209,741] was granted by the patent office on 1975-07-22 for electrochemical potentiometric method for selectively determining alkaline phosphatase content in aqueous fluids.
This patent grant is currently assigned to Owens-Illinois, Inc.. Invention is credited to Melvin H. Keyes.
United States Patent |
3,896,008 |
Keyes |
July 22, 1975 |
Electrochemical potentiometric method for selectively determining
alkaline phosphatase content in aqueous fluids
Abstract
Electrochemical apparatus and potentiometric method for rapidly,
accurately and selectively determining alkaline phosphatase content
in aqueous fluids. Electrochemical apparatus of this invention can
comprise a D-serine deaminase electrode (hereinafter also referred
to as a D-serine specific or enzyme electrode) referenced to either
a constant or variable potential reference electrode. According to
the method of this invention, a known quantity of D-phosphoserine
is added to an aqueous fluid sample containing an unknown
concentration of alkaline phosphatase. The D-phosphoserine is
hydrolyzed by the alkaline phosphatase to inorganic phosphate and
D-serine at a rate proportional to the concentration of alkaline
phosphatase in the sample, while the sample is continuously
monitored with the apparatus of this invention at regular intervals
for changes in D-serine content. The incremental increases in
D-serine content in the fluid sample are reflected in increases in
electrical potential as a result of the deamination of D-serine at
the enzyme electrode. The particular combination of enzyme
electrode, together with the ionic composition of the aqueous fluid
sample to be subjected to such assay, will directly affect the
selection and sequence of steps in conducting such analysis. For
example, where the aqueous fluid sample contains sodium, potassium
and/or ammonium ions in concentrations sufficient to generate an
electrical potential at the ammonium ion membrane of the enzyme
electrode, the sample need be contacted with cation-exchange resin
either prior to or concurrent with such analysis. The apparatus and
method of this invention are readily adaptable for clinical assay
of clinical samples of biologic fluids for alkaline phosphatase
content in the diagnosis of various forms of obstructive jaundice
and other hepatic diseases.
Inventors: |
Keyes; Melvin H. (Sylvania,
OH) |
Assignee: |
Owens-Illinois, Inc. (Toledo,
OH)
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Family
ID: |
26889019 |
Appl.
No.: |
05/209,741 |
Filed: |
December 20, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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193468 |
Oct 28, 1971 |
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Current U.S.
Class: |
205/778; 435/176;
435/180; 204/420; 435/21; 435/178; 435/817; 204/403.1 |
Current CPC
Class: |
C12Q
1/005 (20130101); Y10S 435/817 (20130101) |
Current International
Class: |
C12Q
1/00 (20060101); G01n 027/46 () |
Field of
Search: |
;204/1T,195G,195B
;195/13.5R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Montalvo et al., "Anal. Chem.," Vol. 41, No. 13, Nov. 1969, pp.
1897-1899. .
Mosbach, "Scientific America," Vol. 224, No. 3, March 1971, pp.
26-33. .
Guilbault et al., "Anal. Chim. Acta," 52, (1970), pp. 287-294.
.
Guilbault et al., "JACS," April 22, 1970, pp. 2533-2538. .
Guilbault et al., "Anal. Chem.," Vol. 41, No. 4, April 1969, pp.
600-605. .
White et al., "Principles of Biochemistry," 1959, 3rd ed., p.
501..
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Primary Examiner: Tung; T.
Attorney, Agent or Firm: Heberling; Richard D. Holler; E.
J.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of my copending
application of the same Title, Ser. No. 193,468, filed Oct. 28,
1971, now abandoned.
Claims
What is claimed is:
1. A potentiometric method for selectively determining the
concentration of alkaline phosphatase of an aqueous fluid sample,
comprising;
a. diluting the aqueous fluid sample with a buffered aqueous
solution having a pH in the range of about 8.5 to about 9;
b. providing a potentiometer electrically connected in series to a
D-serine specific electrode and a reference electrode, wherein the
D-serine specific electrode is provided with an ammonium
ion-sensitive membrane in contact with and substantially
encapsulated by a coating of insolubilized D-serine deaminase, said
coating containing a catalytically effective amount of D-serine
deaminase and being permeable to both the aqueous fluid sample and
products of the reaction of the aqueous fluid sample and the
D-serine deaminase;
c. contacting the D-serine specific electrode and the reference
electrode with the diluted sample;
d. recording the ionic background activity of the sample;
e. adding to the diluted sample about 5.times.10.sup..sup.-4 to
about 5.times.10.sup..sup.-1 moles D-phosphoserine per liter of
diluted sample;
f. and recording the incremental change in potential at regular
intervals for a period ranging from about 5 to about 15
minutes.
2. The potentiometric method as defined in claim 1, wherein the
D-serine specific electrode is referenced to a constant potential
reference electrode.
3. The potentiometric method as defined in claim 2, wherein the
diluted aqueous fluid sample is pre-treated with a strongly acidic
cation-exchange resin.
4. The potentiometric method as defined in claim 1, wherein the
diluted sample is periodically agitated after addition of
D-phosphoserine during recordation of changes in enzyme electrode
potential.
5. A potentiometric method for selectively determining the
concentration of alkaline phosphatase in an aqueous fluid sample
having a pH in the range of about 8.5 to 9, the method comprising
the steps of:
a. providing a potentiometer electrically connected to an enzyme
electrode and a variable potential reference electrode, in which
the enzyme electrode is provided with an ammonium ion-sensitive
membrane in contact with and substantially encapsulated by a
coating of immobilized enzyme, said coating containing a
catalytically effective amount of D-serine deaminase, said coating
being permeable to both the aqueous fluid sample and products of
the reaction of the aqueous fluid sample and the D-serine
deaminase; and in which the variable potential reference electrode
is provided with an uncoated ammonium ion-sensitive membrane of
substantially the same ionic sensitivity and selectivity as the
enzyme electrode; and
b. contacting the ammonium ion-sensitive membranes of both the
enzyme electrode and the variable potential electrode with the
sample of aqueous fluid and about 5.times.10.sup..sup.-4 to
5.times.10.sup..sup.-1 moles D-phosphospherine per liter of
sample.
6. A method as defined in claim 5 in which the further step is
provided of measuring the generated electrical potential on the
potentiometer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved electrochemical apparatus and
a potentiometric method for selectively determining alkaline
phosphatase content of aqueous fluids. More specifically, the
apparatus and method of this invention enable selective
determination of alkaline phosphatase content of industrial and
biologic fluids also containing ionic and proteinaceous materials
ordinarily considered incompatible with this type analysis.
Analysis of alkaline phosphatase content is achieved by determining
its rate of hydrolysis of D-phosphoserine. As D-phosphoserine is
hydrolyzed by the alkaline phosphatase, D-serine is produced at a
rate and in concentrations proportional to alkaline phosphatase in
the sample. The rate of change in D-serine concentration in the
sample is subsequently monitored at regular intervals by selective
deamination of this compound at a D-serine specific enzyme
electrode and the recording of the potential generated as a result
of change in ammonium ion concentration in the sample with an
electrode sensitive to such changes.
2. Description of the Prior Art
Traditionally, analytical electrochemical apparatus have had a
sensing electrode and a constant potential-type reference
electrode. The sensing electrode of this type of apparatus is
ordinarily designed to respond to ionic activity of a compound in
solution whose concentration is sought to be determined or from
whose activity the concentration of another compound can be
determined; whereas, the constant potential-type reference
electrode (e.g. a saturated Calomel electrode) only puts out a
constant electrical signal irrespective of the ionic activity of
the background materials in solution.
With the development of various types of cation-sensitive glass
electrode membranes by Eisenman (U.S. Pats. Nos. 2,829,090 and
3,041,252) and other researchers has come a resurgence in interest
in the field of analytical potentiometry. Electrodes are now
currently available that directly measure ionic activity, that are
nondestructive, easy to use and relatively inexpensive. These
ion-selective electrodes generally fall into three major
categories: (a) glass electrodes; (b) solid-state or precipitate
electrodes; and (c) liquid-liquid membrane electrodes.
Paralleling the development of the ion-sensitive glass membrane has
been the increasing interest in the use of enzymes in analytical
chemistry as a result of the ability of researchers to effectively
insolubilize these biocatalysts in a form which permits subsequent
recovery and reuse without substantial loss of the enzyme's
activity. Early attempts to combine these two technologies achieved
only marginal success because of the inability of the ion-sensitive
membranes of the so-called "enzyme electrodes" to effectively
discriminate between products of the enzyme-substrate reaction and
those materials indigenous to the aqueous sample which were also
capable of generating an electrical potential at the enzyme
electrode. Initial attempts to reduce the effect these indigenous
materials have on enzyme electrode response through the
pre-treatment of the aqueous fluid sample with ion-exchange resins
has resulted in substantial improvement in accuracy in enzyme
electrode response, although such pre-treatment dramatically
increases the time required to complete such analysis.
Even after separate pre-treatment of the sample solution with
cation-exchange resin, those apparatus wherein the enzyme electrode
is referenced to a traditional type of constant potential reference
electrode are still unable to accurately determine low
concentrations (.about.10.sup..sup.-4 to 10.sup..sup.-6 M) of
substrate in test samples without substantial error. The deficiency
in this type of system is believed attributable, in part, to the
fact that all traditional types of constant potential reference
electrodes have a narrow aperture in their immersion tip which
provides for the controlled flow of an electrolyte, ordinarily a
potassium salt, from the internal chamber of the immersion tip into
the sample being analyzed. This constant low-volume flow of
potassium ions into the sample solution can result in the
independent generation of an electrical potential at the enzyme
electrode reflecting this potassium ion leakage.
Attempts to compensate for this leakage by the addition of
cation-exchange resin directly to the sample during analysis have
not proven very practical or successful. Cation-exchange resin,
when added directly to the solution being assayed, reportedly
results in the generation of an electrical potential independent of
the concentration of other cations also present in the sample,
Guilbault et al., Anal. Chim. Acta, 52, 287 (1970). This change in
potential has been attributed by Eisenman to a change in liquid
junction potential in solutions of colloids or in suspensions, G.
Eisenman, Glass Electrodes for Hydrogen and Other Cations, Marcel
Dekker, New York, (1967). The independent generation of potential
by the ion-exchange resin can, however, be controlled by limiting
the quantities of these resins that are present in the sample
during analysis and by rigorous buffering of the test sample. This
limited addition of cation-exchange resin, however, appears to be
inadequate in controlling both the interfering cations indigenous
to the sample and the electrolyte seepage from the traditional type
of reference electrode.
The development of an analytical system apparently free from the
problems associated with reference electrode contamination of the
sample has recently been reported in the literature, Guilbault et
al., Anal. Chim. Acta, 52, 287 (1970). Guilbault observed that by
the use of reference electrode having a continuous cation-sensitive
glass membrane at its immersion tip in place of a standard
saturated calomel electrode, urea content of test samples could be
potentiometrically determined relatively free from the problems
discussed previously.
The Guilbault system is, however, subject to error where successive
samples, having variable concentrations of ionic contaminants
(Na.sup.+ , K.sup.+ , NH.sub.4.sup.+ etc.), are analyzed. The
variable potential reference electrode of his system will vary in
its response with those variations in ionic composition and,
therefore, result in an artificial shift in the magnitude of the
potential unless a series of calibration curves are also made
reflecting the shifting concentrations of these so-called ionic
contaminants.
In order to avoid the problems encountered by electrolyte
contamination from the reference electrode and yet provide a system
capable of measuring successive samples without recalibration of
the apparatus for each and every ionic contaminant, a system is
needed wherein the reference electrode has a continuous nonporous
membrane and is sensitive to an ion in solution whose concentration
is constant from sample to sample or whose concentration in such
samples can be readily maintained at a constant level by buffering.
The referencing of an enzyme electrode to a pH electrode, for
example, provides one possible solution to this problem.
SUMMARY OF THE INVENTION
This invention is an electrochemical apparatus comprising an enzyme
electrode and a reference electrode electrically connected in
series to a potentiometer, wherein the enzyme electrode is provided
with an ammonium ion-sensitive membrane in contact with and
substantially encapsulated by a coating of immobilized D-serine
deaminase, said coating containing a catalytically effective amount
of D-serine deaminase and being permeable to both the aqueous fluid
sample and reaction products of the aqueous fluid sample and the
D-serine deaminase. The D-serine specific electrode of this
apparatus can be referenced to either a constant potential or
variable potential electrode.
In the preferred apparatus of this invention, the D-serine specific
electrode is referenced to either a pH or a cation-sensitive
variable potential electrode.
Accordingly, the potentiometric method of this invention for
selectively determining alkaline phosphatase content of an aqueous
fluid comprises initially diluting the sample in an appropriate
buffer having a pH in the range of from about 8.5 to about 9. The
degree of dilution of the sample with buffer should be sufficient
to reduce the native concentration of alkaline phosphatase in a
typical sample of, for example, blood serum, from about 60 to 100 m
Units (International Units) per milliliter at 25.degree.C. to about
30 to about 50 m Units. Subsequent to dilution, the sample is
contacted with an enzyme electrode and reference electrode of an
apparatus of the type described above and the background activity
of the sample recorded. Once an activity base-line for the sample
has been ascertained, D-phosphoserine is added to the diluted
sample in concentrations ranging anywhere from about
5.times.10.sup..sup.-4 to about 5.times.10.sup..sup.-1 moles per
liter for each 50 m Units of alkaline phosphatase activity. After
the D-phosphoserine is added, incremental changes in electrical
potential are recorded at regular intervals for a period ranging
anywhere from about 5 to about 15 minutes or until sufficient data
is available for determination of the rate of hydrolysis of
D-phosphoserine by the alkaline phosphatase present into the
sample.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a side-elevational view of the electrochemical apparatus
of the invention;
FIG. 2 is an elevational view, partly in section, of the constant
potential reference electrode of FIG. 1;
FIG. 3 is an elevational view, partly in section, of the enzyme
electrode of FIG. 1;
FIG. 4 is a side-elevational view of the preferred embodiment of
the electrochemical apparatus of the invention;
FIG. 5 is an elevational view, partly in section, of the variable
potential reference electrode of FIG. 4;
FIG. 6 is an elevational view, partly in section, of the enzyme
electrode of FIG. 4 (substantially identical to FIG. 3).
DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS
Electrodes
An electrode which can be used in the preparation of the D-serine
specific electrode and as a variable potential reference electrode
of the apparatus of this invention can be any ion-sensitive device
capable of detecting changes in ammonium ion activity in aqueous
solution and converting it to an electrical signal; e.g.
cation-sensitive glass electrode, Model No. 39137, and ammonium
ion-sensitive solid-state electrode, Model No. 39626, both
manufactured by Beckman Instruments, Inc., Fullerton, Calif.
Structurally, the D-serine specific enzyme electrodes 1, 1', shown
in FIGS. 3 and 6, each comprise a hollow glass stem 2, 2' having an
opening at each end thereof. One end of each glass stem is tightly
capped with a lid 3, 3' which acts both as a closure and as a
support for an electrically conductive lead 4, 4' which forms part
of the internal reference electrode 5, 5'. The opposite end of each
glass stem, the immersion tip 9, 9' of the enzyme electrodes is
sealed with a continuous ammonium ion-sensitive membrane 6, 6',
which is itself encapsulated by a coating of insolubilized enzyme
7, 7'. A mesh material 8, 8' anchored to the immersion tip of each
electrode by means of an elastic O-ring 10, 10' provides additional
mechanical support for the insolubilized enzyme coating. The
interior chamber 11, 11' of each immersion tip is partially filled
with an electrolytic medium 12, 12' capable of transmitting the
electrical potential, generated by the presence of ammonium ions at
the outer surface of the glass membrane, from the interior wall of
the glass membrane to the internal reference electrode.
The variable potential reference electrode 25 illustrated in
partial section in FIG. 5 is of substantially identical physical
structure to those D-serine specific enzyme electrodes 1, 1'
illustrated in partial section in FIGS. 3 and 6 respectively; its
physical structure, however, differing, of course, in that the
ion-sensitive membrane 6" of its immersion tip 9" remains fully
exposed to the aqueous test sample 20.
The ion-sensitive membrane of these electrodes can be bulbous in
shape, that is substantially spherical and of a diameter equal to
or larger than the terminal end of the glass stem of the electrode,
or blunt ended. The ionic sensitivity and selectivity of this
membrane will vary directly with its permselectivity, which in turn
is determined by the membrane's chemical composition. The
ion-sensitive membranes of these variable potential electrodes can
be continuous non-porous diaphragms selectively permeable to one or
more cations and/or anions. For example, the glass composition of
the ion-sensitive membrane of Beckman's cation-sensitive glass
electrode (Model No. 39137) is believed to be based on an
alkali-alumina-silica composition in the nature of that disclosed
in U.S. Pat. No. 3,278,399. The cation-sensitive electrode referred
to above has been reported by its manufacturer to be equipped with
an ion-sensitive membrane having the following ionic selectively
H.sup.+ > Ag.sup.+ >K.sup.+, NH.sub.4.sup.+ > Na.sup.+
> Li.sup.+ >> Mg.sup.2.sup.+, Ca.sup.2.sup.+. The variable
potential electrode of the apparatus of this invention can be
alternatively equipped with a highly selective membrane sensitive
only to hydrogen ions (and in some instances, sodium ions, but only
at higher pH levels). These electrodes, commonly referred to as pH
electrodes, are readily available commercially in a variety of
shapes and sensitivities, e.g. Model No. 39000, (general purpose
electrode) and 39004 (Type E-2 electrode having low-sodium ion
error), Beckman Instruments, Inc., Fullerton, Calif.
A second type of electrode which can be used as a variable
potential electrode of the apparatus and in the method of this
invention is the highly selective solid-state ammonium
ion-sensitive electrode briefly referred to previously. This
particular type of solid-state electrode is not, however, as
durable as the glass electrode, the organic sensor material of its
immersion tip being gradually chemically and physically eroded over
relatively brief periods of use. Other solid-state variable
potential electrodes which, for example, can also be referenced to
the enzyme electrode of this invention are selective for the
detection of activity of calcium ions; chloride ions; or fluoride
ions.
The constant potential electrode that can be referenced to the
D-serine specific electrode of the apparatus of this invention can
be any device which has a chemically-inert junction, a low
electrical resistance, a low-volume flow rate of electrolytic
solution and, of course, enables measurements of the potential
developed at the enzyme electrode by completion of the electrical
circuit across the test sample, e.g. Quartz Junction reference
electrode (Calomel Internal) Model No. 39400 or Fiber Junction
reference electrode (Calomel Internal) Model No. 39170, Beckman
Instruments, Inc., Fullerton, Calif.
The constant potential reference electrode 13 illustrated in
partial section in FIG. 2 comprises a hollow glass stem 14 tapered
to a narrow aperture 15 at the immersion tip end 16 and open at the
opposite end thereto. The open end of the glass stem is capped by a
lid 17, serving both as a closure and as a supportive means for an
electrically conductive lead 18 which forms part of the internal
reference electrode 19. The aperture at the immersion tip of the
constant potential reference electrode permits communication
between the solution 20 being assayed and the electrolyte 21,
normally a saturated potassium chloride solution, occupying the
internal chamber 22 of the immersion tip of the electrode. An
asbestos fiber wad 23 in the lower portion of the internal
reference electrode allows a controlled flow of Calomel 24 through
the aperture 25 at the base of the internal reference electrode
into the internal chamber of the immersion tip, thereby
establishing electrical contact between the test solution and the
internal reference electrode.
The D-serine specific electrode used in the apparatus of this
invention is prepared by encapsulation of the ammonium
ion-sensitive portion of the immersion tip of an electrode with a
substrate permeable layer of insolubilized D-serine deaminase. The
manner of insolubilization of the D-serine deaminase does not
appear to be of critical importance to the apparatus and method of
this invention, to the extent that the D-serine deaminase retains
substantial catalytic activity and can be intimately affixed to the
sensing portion of the immersion tip, either by physical or
chemical means.
Several of the standard methods currently available for the
insolubilization of enzymes can be used in the preparation of the
encapsulating layer of the enzyme electrode of the apparatus of
this invention. Among those methods which have been found suitable
in this regard are entrapment of the enzyme in a polymer hydrogel,
covalent bonding of the enzyme to the functional groups of both
natural and synthetic polymeric substances or absorption of a
solution of the enzyme on a netting and then covering the netting
with a dialysis membrane permeable to substrate trasport but
impermeable to the passage of the enzyme. The insolubilization of
enzymes by entrapment in a highly cross-linked polymer hydrogel,
especially polyacrylamide, can often adversely affect the
conformation of the enzyme macromolecule resulting in denaturation
and loss of biocatalytic activity, Degani and Miron, Biochim.
Biophys. Acta 212,362-364 (1970). Therefore, adjustment of the
concentration of cross-linking agents should be carefully
controlled, as suggested in the above article, in order to permit
retention of maximum biocatalytic activity of the enzyme. Such
adjustment in the degree of cross-linking of the polymer hydrogel
need also take into full account the permeability requirements of
the system. An excellent review of the techniques and mechanisms
involved in enzyme insolubilization has recently been published by
Klaus Mosbach; his article appearing in the Mar. 1971 issue of
Scientific American, Vol. 224:3, 26 (1971); see also generally
Katchalski's article, appearing in Structure Function Relationship
of Proteolytic Enzymes (Desnuelle, Neurath and Ottesen, Eds.)
Munksgaard, Copenhagen, Denmark, pp. 198-220 (1970); and Inman and
Dintzis, Biochemistry 8, 4074 (1969).
During insolubilization of the D-serine deaminase, especially
during entrapment in a polymer hydrogel, the co-enzyme, pyridoxal
5-phosphate, may become dissociated from the enzyme protein and
dialyzed away, rendering the insolubilized enzyme partially
inactive. In order to provide for this contingency, it is advisable
to add this coenzyme to the polymerization medium during
insolubilization in the hydrogel. Ordinarily,
5.times.10.sup..sup.-6 mole per liter of coenzyme added directly to
the polymerization medium will minimize such activity loss. As a
further precaution, a like amount of the coenzyme can also be added
directly to the sample during assay.
The physical form of the insolubilized enzyme may, however,
determine the form of the encapsulating layer, and the means
required to achieve encapsulation of the sensing tip of the
electrode. The physical shape of the sensing tip of the electrode
can also influence the form of the encapsulating layer, and the
technique required to achieve such encapsulation. Encapsulation of
the ammonium ion-sensitive membrane of the immersion tip of the
electrode need not be complete, nor need the enzyme itself be in
intimate contact with the ammonium ion-sensitive portion of the
electrode. AS long as the insolubilized D-serine deaminase
substantially (.about.90%) and intimately covers the ammonium
ion-sensitive tip of the electrode and the insolubilized enzyme is
itself permeable to both the substrate and ionic transport of
ammonium ions, hydrolysis of D-serine by D-serine deaminase either
occurring within the insolubilized enzyme matrix or on the surface
of the encapsulating layer will generate an increase in electrical
potential at the sensing membrane surface. Encapsulation of the
ammonium ion-sensitive membrane of the immersion tip of the enzyme
electrode should, however, be as complete as possible; especially
in those instances where the test sample has not been pre-treated
with ion-exchange resin for the complete removal of ionic
interferants which are also capable of generating a potential at
the enzyme electrode. If this ammonium ion-sensitive membrane is
not totally encampsulated, only that portion of the membrane so
enveloped should be exposed to the aqueous fluid sample.
The physical and chemical nature of the insolubilizing medium
selected will, to a certain extent, be determined by the operating
conditions to which the enzyme electrode is exposed. For example,
if the insolubilizing medium is sensitive to degradation upon
exposure to alkali, it would probably not prove suitable where
potentiometric measurement is to be made with the enzyme electrode
in a highly caustic solution. Similarly, an insolubilizing medium
that did not have sufficient physical durability would probably not
be suitable in the preparation of an enzyme electrode for a
continuous or a semicontinuous monitoring system because of
consequent hydrodynamic erosion of the encapsulating layer from the
continuous flow-through of the solution being analyzed. The
problems encountered by hydrodynamic erosion of the encapsulating
layer can be ameliorated to a limited degree by covering the
encapsulating layer with a dialysis membrane anchored to the stem
of the electrode by an elastic O-ring. Of course, the dialysis
membrane used to shield the enzyme layer should itself be permeable
to substrate molecules.
Because physical and chemical durability of the enzyme layer is
essential in a semicontinuous or continuous monitoring system, the
method of choice for enzyme insolubilization in the preparation of
the enzyme electrode favors covalent bonding of the enzyme to the
insolubilizing medium.
In some instances, it may be advisable and necessary to provide a
mesh material to lend additional physical support to the enzyme
layer or as a means of attaching the insolubilized enzyme to the
ion-sensitive membrane of the enzyme electrode.
Although the nature and physical parameters with respect to the
mesh size of such supporting materials do not appear to be of
critical importance, the thickness of such supporting materials
can, to a limited degree, affect the response time of the enzyme
electrode by indirectly affecting the thickness of the
encapsulating enzyme layer. Mesh materials are also available that
can be used as an absorption medium or carrier for the soluble
enzyme; however, unless the mesh material is itself subsequently
covered, after attachment to the electrode tip, with a layer of
dialysis type material impermeable to diffusion of these larger
proteinaceous molecules, the soluble enzyme will be readily
stripped from the mesh upon immersion of the enzyme electrode into
an aqueous medium. Mesh materials which have very fine sizes
(.about.60 micron pores or less) can be used to attach the
insolubilized enzyme (ordinarily a powder or granular material) to
the electrode, provided the pore size of the mesh is sufficiently
retentive of the insolubilized enzyme. The thickness of the
encapsulating enzyme layer should not exceed one-tenth of a
millimeter and preferably should be about half that thickness in
order for the system's response times to be within the range of
from about 60 to about 120 seconds.
The quantity of enzyme that need be affixed to the ammonium
ion-sensitive portion of the enzyme electrode is determined
ultimately by the enzyme's activity. If, for example, the activity
of a deaminase enzyme, such as urease, is 375 Sumner Units per
gram, the concentration of urease needed to effectively and yet
economically catalyze the deamination of urea in sufficient
quantities to generate a reproducible Nernstian response which
accurately reflects (to the extent practically possible) the
concentration of urea in the sample, will generally not exceed
about 20 milligrams enzyme per cubic centimeter of polymeric
insolubilizing medium.
Although higher concentrations of urease on the enzyme electrode
have reportedly been used to generate higher potentials and
therefore a more accurate measurement of the sample's urea content,
this slight increase in accuracy, except for special limited
situations, does not justify the added increase in cost expended on
the additional enzyme in extensive clinical use, Guilbault and
Montalvo, J. Am. Chem. Soc. 90, 2533 (1970). This level of optimum
enzyme concentration and point of diminishing returns can be
determined for the D-serine specific electrodes of this invention
in the same manner described in the Guilbault article.
D-serine deaminase isolated from any source could conceivably be
used (provided it is specific for D-amino acids) in the preparation
of the D-serine specific electrode of the apparatus of this
invention. Two of the better-known sources of this enzyme and
techniques for their isolation can be found in the technical
literature in separate articles by Dupourque et al. and Labow et
al. appearing in J. Biolog. Chem., Vol. 241:5, 1233 (1966) and J.
Biolog. Chem., Vol. 241:5, 1239 (1966) respectively.
For most practical clinical applications of the apparatus of this
invention, anywhere from about 5 to about 50 milligrams of highly
purified D-serine deaminase (prepared according to the Dupourque
article -- 300 Units of activity per milligram of enzyme) per cubic
centimeter of polymeric insolubilizing medium should prove adequate
to effectively catalyze the selective deamination of the D-serine
produced as a result of the hydrolysis of D-phosphoserine by the
alkaline phosphatase present in the sample. Because of the relative
thinness of the encapsulating enzyme layer on the enzyme electrode,
usually less than 0.1 millimeter, the total amount of active enzyme
present in such layer will generally not exceed about 1 to about 2
milligrams.
METHOD
The electrochemical apparatus of this invention enables the
accurate and selective determination of alkaline phosphatase
content in aqueous fluids. Those aqueous fluids which can be
subjected to potentiometric assay according to the method of this
invention are industrial fluids and biologic fluids whose liquid
content comprises in excess of 95% water by volume. Biologic fluids
which can be assayed according to the method of this invention are
those aqueous fluids extracted from any animal (vertebrate or
invertebrate), vegetable or bacterial source and excretions from
such sources.
According to the method of this invention, an aqueous fluid sample
having an unknown concentration of alkaline phosphatase is diluted
with an appropriate buffer; contacted with a pair of electrodes of
an apparatus of this invention; a base-line potential for the
sample recorded; about 5.times.10.sup..sup.-1 to about
5.times.10.sup..sup.-4 mole per liter of D-phosphoserine for each
50 m Units of activity of alkaline phosphatase added to the sample;
and incremental changes in electrical potential recorded at regular
intervals at the enzyme electrode.
The aqueous fluid sample is ordinarily prepared for assay by the
method of this invention by dilution with an aqueous buffer such
that the approximate concentration of alkaline phosphatase in the
resultant solution will not generally exceed approximately 30 to 50
m Units of activity per milliliter at 25.degree.C. as monitored by
p-nitrophenyl phosphate, Rich and Hausamen, Zeitschr. Anal. Chem.
212, 267 (1965). Ordinarily, dilution of a biologic fluid, such as
blood serum with an equivolume amount of buffer provides this
sufficient degree dilution of the sample.
Often during extraction and dilution of the aqueous fluid sample
prior to actual assay, some of the metal activator groups of
alkaline phosphatase can be washed away or bound to ion-exchange
resin, thereby rendering the enzyme partially inactive. This can be
readily remedied by the addition of, for example, selected divalent
metal salts of Mg.sup.2.sup.+, Ca.sup.2.sup.+ and Sr.sup.2.sup.+ to
the sample during assay in concentrations of up to 10.sup..sup.-2
moles per liter of sample. In the event that selected
Zn.sup.2.sup.+ salts are used to restore the alkaline phosphatase
to full potency, the concentration fo the Zn.sup.2.sup.+ should not
exceed .about.10.sup..sup.-6 to .about.10.sup..sup.-5 moles per
liter of sample or inhibition of the alkaline phosphatase may
occur.
The above sequence of steps, of course, assumes that the sample
does not contain any ionic interferants, e.g. monovalent of
divalent cations, having sufficient ionic activity to independently
generate an electrical potential at the D-serine specific electrode
of sufficient magnitude to significantly alter or distort the
potential generated by the increase in ammonium ions resulting from
the deamination of D-serine.
In the event that ionic interferants are present in the sample, the
sample need be pre-treated with cation-exchange resin prior to
potentiometric assay; or, alternatively, the sample assayed with an
apparatus that can be selectively discriminate between these
interferants and the ammonium ions generated by deamination of the
D-serine. In most instances, the selectivity of such apparatus
cannot help but be enhanced by some pre-treatment of the sample,
either prior to or during analysis with similar cation-exchange
resins. Whether or not pre-treatment of the sample, known to
contain ionic interferants, with cation-exchange resin is required
is largely dependent upon the type of apparatus being used to
conduct the assay of the sample. For example, if the apparatus is
provided with a D-serine specific solid-state enzyme electrode,
referenced to a constant potential electrode, the presence of
sodium, potassium and/or ammonium ions in the sample should have
little, if any, effect on the highly selective ammonium
ion-sensitive membrane of the D-serine specific electrode. Where,
however, the more durable ammonium ion-sensitive glass electrode is
used in place of a solid-state ammonium ion-sensitive device in
preparation of the D-serine specific electrode, the aqueous fluid
sample need be contacted with ion-exchange resin prior to or during
potentiometric assay, depending upon whether or not the D-serine
glass electrode is referenced to a constant or variable potential
reference electrode. When the apparatus is provided with a constant
potential or pH reference electrode, the sample should be contacted
with cation-exchange resin prior to potentiometric assay. If, on
the other hand, the D-serine specific glass electrode is referenced
to a variable potential electrode having substantially the same
ionic sensitivity and selectivity as the enzyme electrode, the
sample can be contacted with cation-exchange resin during
analysis.
Ordinarily, one to two grams of strongly acidic cation-ion exchange
resin present in about 50 milliliters of buffered sample solution
during analysis is sufficient to remove approximately 90% of the
ionic interference caused by the presence of sodium, potassium
and/or ammonium ions in a typical sample of blood serum. The
addition of ion-exchange resin in excess of the above concentration
to the sample during analysis has been found to reduce the
sensitivity of the enzyme electrode and often results in
fluctuation of the sensing electrode's signal.
The cation-exchange resins which can be added to the test solution
to further reduce the effect of sodium, potassium and/or ammonium
ion interferants on enzyme electrode response generally have a high
degree of sulfonic acid functionality. Among those strongly acidic
resins which can be effectively used to improve enzyme electrode
sensitivity are Amberlite CG50 (Rohm and Haas, Philadelphia, Pa.),
Dowex 50W-X.sub.2 (Dow Chemical Co., Midland, Mich.), and Dowex
50W-X.sub.2 (Dow Chemical Co., Midland, Mich.).
The strongly acidic cation-exchange resin marketed by Dow Chemical
Company under the brand name "Dowex 50W-X.sub.2 " is preferred over
other similar materials presently commercially available because of
the selectivity, exchange capacity and compatiblity of these resins
with the equipment and materials being analyzed.
Because the resins used in the reduction in concentration of ionic
interferants are highly acidic, they tend to substantially lower
the pH of the system. Since the activity of both alkaline
phosphatase and D-serine deaminase, as is true for all enzymes, is
pH dependent and the ammonium ion-sensitive membranes of D-serine
specific electrodes and pH reference electrode are sensitive to
hydrogen ion activity, the pH of the sample need be adjusted and
maintained within a pH range which both favors maximum enzyme
activity and yet minimizes hydrogen ion activity. The maintenance
of pH stability is achieved through the use of any of a number
well-known buffering systems, see generally Good, N.E., et al.,
Biochem. 5, 467-477 (1966). For the purpose of this invention, the
pH range of the buffered test sample during analysis should be
maintained within about 8.5 to about 9. Representative of those
buffering systems which can be used in the dilution and maintenance
of pH stability of the aqueous fluids being assayed according to
the method of this invention are Bicene (N, N-bis [2-hydroxyethyl]
glycine); tricene (N-tris-[hydroxymethyl]]methyl glycine; and TAPS
(tris [hydroxymethyl] methylaminopropane sulfonic acid). The use of
a TRIS buffering system should generally be avoided because of the
tendency of this buffer to cause inhibition of the D-serine
deaminase on the enzyme electrode.
Once the buffered solution containing the D-phosphoserine has been
added to the sample, and contacted with the unknown quantity of
alkaline phosphatase present therein, the D-phosphoserine will
undergo enzymatic hydrolysis to inorganic phosphate and D-serine.
Either concurrent with or immediately after this contacting of the
D-phosphoserine with the sample, this sample should be subjected to
semicontinuous potentiometric assay for detection of changes in
D-serine concentration; the potential at the D-serine specific
electrode being noted at regular intervals for a period ranging
from about 1 to 10 minutes. The rate of change in D-serine
concentration in the sample, as determined by such analysis, is
proportional to the log of the concentration of alkaline
phosphatase in the sample. Of course, the temperature of the test
solution during such analysis must be maintained at a constant
value.
The temperature of the test solution during assay should be kept
below about 37.degree.C. in order to prevent denaturation of either
the alkaline phosphatase in solution or D-serine deaminase on the
enzyme electrode. As a practical matter, the temperature of the
test solution should be maintained in excess of 15.degree.C. in
order to facilitate the enzymatic hydrolysis of both
D-phosphoserine in the test solution and the D-serine at the enzyme
electrode. Contacting the electrodes and the sample at room
temperature generally provides the preferred mode of conducting
such analysis both from the standpoint of speed of electrode
response times and simplicity.
In order to further improve the response times of both the enzyme
and reference electrodes, it is often advisable to hydrate their
ion-sensitive membranes overnight in either distilled water, 0.1 M
Bicene buffer, pH 8.5 or other suitable buffer. After analysis of
each successive sample, the electrodes, especially the enzyme
electrode, need be thoroughly washed in buffer or distilled water
to remove traces of substrate and ionic materials that may be
adjacent to the ammonium ion-sensitive membrane of the electrode or
dispersed within the insolubilization medium encapsulating the
electrode. The enzyme electrode should be stored in buffer when not
in use. Permitting the encapsulating enzyme coating of the enzyme
electrode to dry out often results in loss of biocatalytic activity
as a result of denaturation of the enzyme. The enzyme electrode
employed in such method can be reused to preform numerous analyses,
often having a useful life of about 21 days.
Where the enzyme electrode is intended to be used over such
extended periods of time to conduct numerous analyses of several
samples or in a continuous analysis routine, it is advisable to add
.about.5.times.10.sup..sup.-6 moles per liter sample of the
D-serine deaminase cofactor, pyridoxal 5-phosphate directly to the
sample during analysis in order to restore that original complement
of this compound associated with the enzyme protein which may have
been washed out during insolubilization and/or potentiometric
analysis.
The examples which follow further illustrate the preparation and
operation of the apparatus and the method of this invention. The
parts and percentages appearing in these examples are by weight
unless otherwise stipulated.
EXAMPLE I
An enzyme immobilizing medium is prepared by dissolving 4.0 grams
of acrylamide monomer and 0.2 grams of N,
N'-methylene-bisacrylamide (Eastman Kodak Company, Rochester, N.Y.)
in 25 milliliters 0.1 M Bicene buffer, pH 7.0. Once the monomer and
cross-linking agent have been substantially dissolved in the
buffer, 0.0027 grams riboflavin and 0.0027 grams potassium
persulfate are added to the buffered solution. This buffered
monomer stock solution can be used immediately or stored in the
dark at room temperature for up to two days.
One milliliter of monomer stock solution is pipetted into a 5
milliliter centrifuge tube containing 0.100 grams D-serine
deaminase (Mutant C.sub.6, E. Coli W. -- activity 300 International
Units) and 5.times.10.sup..sup.-5 M pyridoxal phosphate. The
resulting enzyme-monomer suspension is stirred for about two
minutes and then set aside for about 20 minutes at room temperature
to permit more complete dissolution of the enzyme. After the enzyme
has dissolved in the monomer solution to the extent permitted under
the above conditions, the tube and its contents are refrigerated at
2.degree.C. for 10 minutes, centrifuged for 10 minutes at 3000 rpm
(1470 g's), and the supernatant drawn off in a pipette.
Immediately preceding the preparation of the enzyme-monomer
solution, the immersion tip of an ammonium ion-sensitive glass
electrode, (Model 39137, Beckman Instruments, Inc., Fullerton,
Calif.) is washed in distilled water, wiped dry with lintless
tissue paper and mounted on a supporting bracket in an inverted
position. A two-inch square of nylon netting, 350.mu. pore size
(from a sheer nylon stocking, J.C. Penney, Inc.) is draped over the
cation-sensitive membrane of the immersion tip of the electrode,
where it is anchored in place by means of an elastic O-ring.
Two drops (.about.0.1 milliliters) of monomer-D-serine deaminase is
applied to the cation-sensitive membrane and nylon netting covering
the immersion tip of the electrode. The electrode, still in the
inverted position, is placed in a water-jacketed tube provided with
a nitrogen gas inlet. The air in the space between the interior
wall of the water-jacketed tube and the inverted electrode is
displaced by purging with nitrogen for about 15 minutes. Once the
oxidizing environment has been replaced with nitrogen, the
immersion tip of the electrode, still under a blanket of nitrogen,
is irradiated with a General Electric BBA photoflood lamp equipped
with a reflector. The temperature of the photochemical
polymerization is carefully monitored with a mercury bulb
thermometer that is strapped to the bracket supporting the
electrode. During this exposure to the photoflood lamp, nitrogen is
continually passed over the immersion tip of the electrode, for the
purpose of both maintaining an oxygen-free polymerization
environment and controlling the temperature of the polymerization
which should be maintained between about 22.degree. and
28.degree.C. Ordinarily, the D-serine deaminase is entrapped and
monomer fully polymerized within the first 10 to 15 minutes of
exposure to the photoflood lamp; however, irradiation of the
monomer-enzyme coating for at least 60 minutes is recommended.
After the D-serine specific electrode has been prepared in the
manner described above, it, together with a Quartz Junction
constant potential reference electrode (Model 39400, Beckman
Instruments, Inc., Fullerton, Calif.), are electrically connected
to a Beckman Zeromatic Model II pH meter which in turn is
electrically connected to a Brown potentiometric recorder. The
cation-sensitive glass membrane of the D-serine specific electrode
is then hydrated prior to use by immersion in distilled water for
about 24 hours. After hydration of the ion-sensitive membrane of
the electrode, the cell is calibrated by measuring the D-serine
concentratrion of a number of solutions having known and varying
concentrations of D-serine in 0.5 M Bicene buffer containing 2.0
grams Dowex 50W-X.sub.2 cation-exchange resin.
Ten milliliters of a sample of human blood serum is contacted with
1 gram Dowex 50W-X.sub.2 cation-exchange resin for the removal of
ionic interferants. Subsequent to this exposure to ion-exchange
resin, the sample is placed in a thermostated chamber (22.degree.C.
.+-. 0.1C..degree.), and 8.5, with an equivolume amount of Bicene
buffer, pH 8.5, containing 5.times.10.sup..sup.-4 M MgCl.sub.2 per
liter of sample. The solution is mildly agitated with a magnetic
stiring bar while a base-line potential is recorded for the sample.
After the ionic background activity of the sample is recorded,
D-phosphoserine is added to the diluted solution in an amount such
that its concentration in the sample is about
5.times.10.sup..sup.-3 moles per liter of solution. With the
addition of D-phosphoserine to the solution, the D-serine content
of solution immediately begins to rise. As the D-serine content of
the solutions increase, so too does the potential at the enzyme
electrode. This change in potential of the D-serine specific
electrode is continually monitored for a period of 10 minutes as
the solution is agitated periodically; the results being recorded
on the Brown potentiometric recorder.
EXAMPLE II
The analytical rountine described in Example I is repeated except
for the referencing of the D-serine specific electrode to a pH
electrode (Model No. 39000, Beckman Instruments, Inc., Fullerton,
Calif.
EXAMPLE III
The analytical routine described in Example I is repeated except
for: (a) the addition of one gram of Dowex 50W-X.sub.2
cation-exchange resin directly to the diluted sample immediately
preceding analysis instead of the separate pretreatment step used
previously and, (b) the referencing of the D-serine specific
electrode to a variable potential electrode of substantially the
ionic sensitivity and selectivity as the enzyme electrode (Model
No. 39137, Beckman Instruments, Inc., Fullerton, Calif.).
EXAMPLE IV
0.1 grams agarose, reagent grade, is added to 50 milliliters
distilled water, and allowed to swell for 2 hours with constant
agitation. The suspension is permitted to settle for two minutes,
the supernatant decanted off and the swollen agarose resuspended in
sufficient quantities of distilled water such that total volume of
the suspension is 5 milliliters. The pH of the agarose suspension
is then adjusted to a pH 11.0 with 0.1 M NaOH. One-tenth of a gram
of finely divided cyanogen bromide, reagent grade, is then added to
the suspension while the pH is maintained at 11.0 by constant
titration with 2.0 N NaOH, and the temperature held at about
20.degree.C. (room temperature) by the addition of ice to the
reaction mass as necessary. The reaction is deemed complete when
there is no additional base consumed. The suspension is then
quickly transferred to a Buchner funnel and washed with equal
portions of cold (.about.4.degree.C.) distilled water and cold
(.about.4.degree.C.) 0.1 M Bicene buffer, pH 8.0. Washing of the
filtrate is performed by aspiration and ordinarily is complete
within two minutes.
The activated agarose is transferred from the funnel to a beaker
containing five milliliters 0.1 M Bicene buffer, pH 8.0, 0.100
grams D-serine deaminase (same activity as in Example I) and
5.times.10.sup..sup.-5 M pyridoxal phosphate. The resulting
monomer-enzyme suspension is stirred for 16-20 hours at 0.degree.
to 3.degree.C. Once formation of the polymer-enzyme conjugate is
complete, the suspension is thoroughly washed with alternating
solutions of Bicene buffer and distilled water.
Immediately preceding the preparation of the polymer-enzyme
conjugate, the immersion tip of a cation-sensitive electrode, Model
39137 (Beckman Instruments, Inc., Fullerton, Calif.) is washed in
distilled water and wiped dry with a lintless tissue paper.
Approximately one milliliter of cyanogen bromide modified
agarose-D-serine deaminase conjugate is placed in the center of a
two-inch square of nylon netting, 60.mu. pore size (Pharmacia Fine
Chemicals Inc., Piscataway, N.J.). The immersion tip of the
electrode is then centered in the polymer-enzyme conjugate mass,
the nylon netting folded up along the barrel of the electrode and
anchored in place by means of an elastic O-ring. The netting is
then manually manipulated so that an even layer of conjugate is
distributed over the cation-sensitive bulb of the electrode.
The D-serine specific electrode prepared in the manner described
above can be referenced to either a constant or variable potential
electrode and potentiometric assay of an aqueous fluid sample
conducted according to the procedures described in the foregoing
Examples.
EXAMPLE V
Ten grams of polyacrylamide beads (Bio-Gel P-300, Bio Rad
Laboratories, Richmond, Calif.) together with 15 milliliters
distilled water are placed in a siliconized flask, the flask
stopped and beads allowed to swell overnight (.about.12 hours). The
swollen polyacrylamide hydrogel is then crushed in a Waring blender
and the hydrogel separated into major fractions of like particle
size by standard gravity sedimentation techniques. Two milliliters
polyacrylamide hydrogel is added to a flask containing a
stoichiometric amount of anhydrous hydrazine, based on
polyacrylamide hydrogel, reagent grade, (Matheson, Coleman and
Bell, Norwood, Ohio, assayed 99-100%, 20.4 M); the flask containing
the anhydrous hydrazine being pre-heated in a constant temperature
oil bath at 49.degree.C. for 45 minutes prior to the addition of
the polyacrylamide hydrogel. The hydrogel-hydrazine mixture is
reacted at 49.degree.C. for about 20 hours, during which time the
mixture is continually agitated by an immersible magnetic stirring
bar. At the end of the reaction interval, the hydrazine modified
hydrogel is washed with 0.1 N NaC1 aqueous solution until the
supernatant tests negative for the presence of free hydrazine, as
indicated by a failure of the development of a pale violet color in
the reaction of the supernatant with trinitrobenzenesulfonate. The
hydrazine modified hydrogel, obtained as described above, is
reacted with stoichiometric quantities of cold (.about.4.degree.C.)
1 N HNO.sub.2 for about 5 minutes and then the acyl azide modified
hydrogel washed with 0.1 M Bicene buffer until the hydrogel pH is
raised to between 7 and 8.
The acyl azide modified hydrogel is then transferred to a beaker
containing 5 milliliters 0.2 M Bicene buffer, pH 8.0 and 0.20 grams
D-serine deaminase (same activity as in Example I), the
buffer-enzyme solution being pre-cooled to about 0.degree.to
4.degree.C. The resulting mixture is maintained at about this same
temperature while continually stirred for 5 hours. The resulting
hydrogel-enzyme conjugate is separated from the unreacted enzyme by
centrifugation, and then washed with 200 milliliters 0.01 N glycine
buffer.
A D-serine specific electrode is prepared utilizing the
insolubilized D-serine deaminase obtained above according to the
technique described in Example IV. The D-serine specific electrode
thus prepared is referenced to a constant potential electrode and
potentiometric assay of an aqueous fluid sample conducted as
described in Example I.
EXAMPLE VI
Two grams of a sample of powdered porous 96% silica glass (950A
.+-. 50A pore size, 16m..sup.2 /gm. surface area) is washed in 0.2
N HNO.sub.3 at 80.degree.C. with continuous sonication for at least
three hours. The glass sample is washed further by decantation with
distilled water and dried by heating to 625.degree.C. overnight
(.about.12 hours) in an oxidizing environment.
The following day the glass sample is cooled, placed in a round
bottom flask with 100 milliliters of a 10% solution of .gamma.
-aminopropyltriethyloxysilane in toluene. The mixture is refluxed
overnight (.about.12 hours), cooled, and the particulates separated
from the reaction mass by filtration. The filter cake is then
washed with acetone to remove traces of solvent and unreacted
silane, air dried and then stored, if desired, or used immediately
thereafter. The product of this reaction (hereinafter referred to
as aminoalkylsilane modified silica), is calculated as having 0.171
meq. of silane residues/gram of glass sample as determined by total
nitrogen.
One gram of aminoalkylsilane modified silica is added to a flask
containing 3.5 milliliters distilled water and 0.10 grams D-serine
deaminase (same activity as in Example I). This mixture is then
combined with a second solution comprising 0.5 milliliter N,
N'-dicylohexylcarbodiimide (DCCI) in 0.5 milliliters
tetrahydrofuran (THF). The resultant solution is then stirred
overnight (.about.12 hours) at room temperature
(.about.22.degree.C.), the reaction product separated from solution
by filtration, and washed exhaustively with alternating solutions
of NaHCO.sub.3, 0.001 M HCl and distilled water. The insolubilized
D-serine deaminase is stored in 0.1 M Bicene buffer, pH 7.0, at
0.degree.to 3.degree.C.
A D-serine specific electrode is prepared utilizing the
insolubilized D-serine deaminase obtained above according to the
technique described in Example IV. The D-serine specific electrode
thus prepared is referenced to a constant potential electrode and
potentiometric assay of an aqueous fluid sample conducted as
described in Example I.
* * * * *