GB1601094A

GB1601094A - Simultaneous radioassay of folate and vitamin b12

Info

Publication number: GB1601094A
Application number: GB3284/78A
Authority: GB
Original assignee: Becton Dickinson and Co
Current assignee: Becton Dickinson and Co
Priority date: 1977-01-27
Filing date: 1978-01-26
Publication date: 1981-10-21
Also published as: CA1098015A; IT1153998B; DE2803154C2; DE2803154A1; FR2379068A1; SE7800974L; IT7819687A0; BR7800477A; CH636206A5; FR2379068B1

Abstract

A sample which contains free folic acid and/or tetrahydrofolic acid and also free vitamin B12 is brought into contact with (1) a binding agent for folic acid and/or tetrahydrofolic acid, (2) a binding agent for vitamin B12, (3) a substance used for labelling, which is selected from folic acid, 5-methyltetrahydrofolic acid and analogues of folic acid and is labelled with a first radioactive isotope, and (4) a vitamin B12 used for labelling, which is labelled with another radioactive isotope. The bound fractions of folic acid and/or tetrahydrofolic acid and also vitamin B12 are separated from the unbound fractions. The radioactivity of at least one of the bound fractions and unbound fractions is measured.

Description

(54) SIMULTANEOUS RADIOASSAY OF FOLATE AND VITAMIN B,2 (71) We, BECTON, DICKINSON AND COMPANY, a corporation organized and existing under the laws of the State of New Jersey, United States of America, of Rutherford, New Jersey 07070, United States of America, do hereby declare the invention, for which we pray that patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to radioassay, and more particularly, to a radioassay for folate and vitamin B,2.

Currently, endogenous folate is measured by a competitive protein-binder technique. In brief, competitive protein-binding (CPB) for the assay of folate involves the ability of unlabeled folate in serum or other media to compete with labeled folic acid for a specific folate binder, present in usable concentrations in such sources as cow's milk, hog kidney, etc., and thereby inhibit the binding of labeled folic acid. As a result of the competitive inhibition, the ratio of bound labeled folic acid to free labeled folic acid diminishes as the concentration of unlabeled folate is increased. Accordingly, the concentration of folate in an unknown sample; e.g., a patient's serum, is obtained by comparing the ihhibition with that produced by known amounts of folate, as presented in a standard curve.

Endogenous vitamin B12 is also determined by a similar competitive proteinbinding technique, with the vitamin B12 binder generally being hog intrinsic factor.

The present invention is directed to an improved radioassay wherein the folate and vitamin B,2 present in a sample may be determined simultaneously.

In accordance with the present invention, a sample containing folate and vitamin B12 is contacted with a binder for the folate, a binder for the vitamin B,2, a folate tracer labeled with a first radioactive isotope and a vitamin B12 tracer labeled with a second radioactive isotope different from the first radioactive isotope, resulting in competitive binding between the labeled and unlabeled folate and the labeled and unlabeled vitamin B,2 for their respective binder sites.The bound portions of labeled and unlabeled folate and bound portions of labeled and unlabeled vitamin B,2 are simultaneously separated from the unbound portions of labeled and unlabeled folate and unbound portions of labeled and unlabeled vitamin B12, following by counting the radioactivity of at least one of the bound or unbound portions, with the amounts of folate and vitamin B12 then being determined from standard curves.

In accordance with the present invention, folate and vitamin B,2 are released from their endogenous binders by heating a sample, containing folate and vitamin B,2 e.g., a serum or plasma sample, with the endogenous binders being destroyed by such heating. The heating can be effected at a wide variety of pH values, with the pH generally being 5 or greater. The pH is most generally at least 7.0 (neutral or alkaline pH), with the pH preferably being at least 9.0 and preferably no greater than 9.6. The most preferred pH is from 9.2 to 9.4 (generally 9.3) in that this permits the release to be effected at the same pH as the subsequent assay in which folic acid is employed as a standard, instead of the reduced methyl derivative of folic acid.

The heating to release vitamin B,2 and folate from their binders is generally effected at a temperature of from 95"C to 1 100C, and preferably of from 98"C to 105"C.

The release of folic acid and vitamin B,2 from their endogenous binders is also effected in the presence of a suitable reducing agent in order to preserve the endogenous folate present in the serum in reduced form. The reducing agent which is included during the heating step is a reducing agent which maintains the reduced folate without adversely affecting the vitamin BlZ and which is stable under the assay conditions.As representative examples of suitable reducing agents there may be mentioned: ascorbate, Cleland's reagent (dithiothreitol); dithioerythritol, monothioglycol; thioglycol; thioglycollic acid; cysteine: homocysteine: glythathione, mercaptoethanol; sulfhydryl reducing agents; inorganic reducing agents, such as sodium sulfite; sodium dithionite; sodium sulfide, sodium metabisulfite, with the organic reducing agents being preferred.

After release of folic acid and vitamin B12 from their endogenous binders and the destructign of such endogenous binders, the sample is contacted with a dual tracer; i.e., a tracer for folate and a tracer for vitamin B,2, and a dual binder: i.e., a binder for folate and a binder for vitamin B12. The receptors, both naturally occurring and antibodies, for folate are well known in the art, and any one of such receptors may be employed in the assay of the present invention. As representative examples of such receptors, there may be mentioned: receptors or binders extracted from various animal organs, particularly kidneys and pancreas: i3- lactoglobulin preparations; cow's milk, dolphin serum and the like, with milk binder being preferred.Similarly, binders or receptors for vitamin B12 are known in the art, e.g., saliva, chicken serum, intrinsic factor, with the preferred binder being intrinsic factor.

The folate tracer is either folic acid (pteroylmonoglutamic acid) (PG [or the reduced 5-methyl derivative of folic acid, 5-methyltetrahydrofolic acid (MTFA)l or appropriate analogs thereof, labeled with a radioactive isotope, which is preferably a radioactive isotope of iodine. The term folate tracer generically refers to radiolabeled folic acid, the radiolabeled reduced methyl derivative and the radiolabeled analogs thereof. The 5-methyl derivative is generally not employed as a tracer as a result of the instability thereof. The radiolabeled folate employed as a tracer is preferably radiolabeled folic acid, and the term radiolabeled folic acid includes the radiolabeled analogs thereof i.e., folic acid substituted with a radiolabeled radical.The preferred radioactive isotope is a radioactive isotope of iodine and most preferably I'25 with the tracer being a folic acid including a -substituent which includes a radioiodinated phenol or imidazole group such as histidine, histamine, tyrosine or tyramine, which is substituted with a radioactive isotope of iodine. A particularly preferred tracer is one in which the cg-carboxyl group of the glutamyl moiety is substituted with radioiodinated tyrosyl or histidyl; however, it is to be understood that the tracer can also be one in which the y carboxyl group is so substituted, as disclosed in U.S. Patent No. 3,989,812.

The vitamin B12 tracer is preferably a radiolabeled vitamin B12, with the vifamin B12 preferably being labeled with 51Co.

In accordance with the preferred procedure, the assay is effected at an alkaline pH; i.e., the assay and release of folate and vitamin B12 from binders are both effected at alkaline pH, with the release being effected at temperatures known in the art; i.e., generally in the order of from 98"C to 105 C. It is to be understood that the release and subsequent assay could be effected at different pH values; however, identical pH values are preferably employed in that this eliminates-the necessity for a pH adjustment.

In accordance with the most preferred procedure for effecting the assay, folate and vitamin B12 are released from their endogenous binders at an alkaline pH in the order of from 9.2 to 9.4 followed by effecting the assay at a pH of from 9.2 to 9.4 with the folate tracer being in the form of a radioiodinated folic acid, with the radioiodinated folic acid preferably being a radioiodinated analog of folic acid: e.g., radioiodo-substituted histidyl, histamyl, tyrosyl or tyramyl, preferably tyrnsyl.

The tracer for vitamin B12 is 57Co. Labeled vitamin Br2. The use of radiolabeled folic acid and assay of endogenous folates at such a pH permits the use of folic acid as a standard, instead of MTFA, as described in U.S. Patent No. 3,988,431.

It is to be understood that in accordance with the preferred procedure wherein folic acid is employed as a standard, even though the assay should be effected at pH 9.2-9.4, the release from binder can be effected at another pH value. Similarly, if the standard for the assay is MTFA, the assay and/or release can be effected at pH .values other than 9.2-9.4.

The bound and unbound portions are separated by procedures known in the art, with such portions generally being separated by the use of a particulate adsorbent. The preferred adsorbent is dextran-coated charcoal; however, it is to be understood that any one of a wide variety of other adsorbents, such as ion exchange resins, inorganic adsorbents, etc., may be employed for separating the bound and free portions. In addition, the assay may be effected by a so-called solid phase assay technique, wherein the receptors for folate and vitamin B12 are previously coated on or bound to a solid support, such as a test tube, or insoluble polymer, whereby the bound and free portions may be readily separated from each other.The techniques for effecting separation of bound and free portions, whether by the addition of a particulate adsorbent, or by the use of a receptor bound to a solid phase, forms no part of the present invention.

After separation of the bound and free portions, the radioactivity of either the bound or free portion or both portions is determined, and the determined radioactivity is compared with a standard curve, by procedures known in the art.

As should be apparent, since different radioactive isotopes are employed for labelling the folate tracer and vitamin B,2, the respective tracers may be counted in different channels of a counter or if the counter has one channel, it can be calibrated so that different counter settings will count one isotope at a time.

The invention will be further described with respect to the following example; however, the scope of the invention is not to be limited thereby: EXAMPLE The following reagents are used in the dual assay: 1. Dual Tracer 1.5 ,u Ci -(pteroylglutamyl)l25I-L-tyrosine (prepared as described in Application No. 40609/77, Serial No. 1,581,075, and 0.75 p Ci vitamin B,2 [57Co.], human serum albumin, sodium borate, dextran and preservatives.

2. Dual Binder Folate binder from bovine milk and hog intrinsic factor, both formulated for a trace binding (Bo) of 55+15%, human serum albumin, dextran and preservatives.

3. Dual Standards containing human serum albumin, sodium borate, sodium chloride and preservatives.

3A Dual Standard A Zero level 3B Dual Standard B 1.0 ng/ml Folic acid; 100 pg/ml B12 3C Dual Standard C 2.0 ng/ml Folic acid; 200 pg/ml B12 3D Dual Standard D 4.0 ng/ml Folic acid; 400 pg/ml B,2 3E Dual Standard E 10 ng/ml Folic acid; 1000 pg/ml B12 3F Dual Standard F 20 ng/ml Folic acid; 2000 pg/ml B12 4. Buffer pH 9.3, 0.05M sodium borate with 6.25 stung potassium cyanide/ml.

5. Cleland's Reagent (dithiothreitol) solution, 5%.

5A Assay Buffer A mixture of I ml of Reagent 5 to 50 ml of Reagent 4.

6. Dextran-coated charcoal suspension, 4.4+0.1 g dextran-charcoal dry mix (1:10) suspending agent and sodium chloride in 100 ml sterile distilled water.

Protocol Preparation of a Standard Curve Clinical Determinations 1. Number 16 polypropylene tubes 1. Starting with 17, consecu sequentially from 1--16. tively number two poly propylene tubes for each clinical sample.

2. Add assay Buffer (Reagent 5A) 2. Add 1000 F" 1 Assay Buffer as follows: (Reagent 5A) to each tube.

Tubes Buffer 1,2 1600,us 3-16 1000fl11 3. Add Dual Standards (Reagents 3. Add 100 yI patient sample to 3A-3F) as follows: each of two tubes. Mix gently.

Folic Acid Vitamin B12 Tube No. Standard as ng/ml as pg/ml 3,6 100,ulA 0 0 7,8 l00NlB B 1.0 100 9,10 100,ulC 2.0 200 11,12 100,ulD 4.0 400 13,14 1001E 10 1000 15,16 100,ulF 20 2000 Mix gently.

4. Cover all tubes loosely with plastic caps (except I and 2).

5. Heat all tubes (except 1 and 2) in a glycerin or water bath at 100"C for 45 minutes.

6. Removal all tubes from 100"C bath. Cool to 20--250C in a running water bath. Do not continue assay until the tubes are within this range. Uncap all tubes.

7. Add 100 yI Dual Tracer 7. Add 100,ul Dual Tracer (Reagent 1) to all tubes. Mix (Reagent 1) to each tube.

gently by hand. Set tubes 1 Mix gently by hand.

and 2 aside at room temperature until Step 15.

8. Add 100ul Dual Binder 8. Add 100L1 Dual Binder (Reagent 2) to tubes 5-16. (Reagent 2) to each tube. Mix Mix gently by hand. gently by hand.

From this point, all tubes are treated as follows: 9. Incubate at room temperature for 45 minutes from the time of the last addition of the binder. Cover the rack of tubes with aluminum foil to exclude light or keep in the dark.

10. Add 0.4 ml dextran-coated charcoal to tubes 3-16 and to all patient sample tubes (17,18, etc.). Do not add to tubes 1 and 2. This reagent is "squirted" into each tube to obtain a uniform suspension in the reaction mixture.

I 1. Keep at room temperature for 10 minutes from the time of last addition in Step 10.

12. Centrifuge at a minimum of 1240xg for 15 minutes, preferably in the cold.

Shorter times may be sufficient in equipment of higher centrifugal force.

13. Consecutively number a set of clean tubes, beginning with 3.

14. Gently decant each clear supernatant into the similarly numbered tube prepared in Step 13. Maximal transer is obtained by hitting the rims together.

Avoid decanting over any charcoal to the counting tube. Discard the charcoal residues.

15. Count the radioactivity in the supernatants and tubes 1 and 2 in sequence for one or more minutes with a scintillation (gamma) counter.

If the counter has two or more channels it should be calibrated to count ['25I] in one channel and [57Co] in another channel. If the counter has one channel it should be calibrated so that different counter settings will count one isotope at a time. In the case of the latter, it will be necessary to count the tubes twice, once setting the counter for ['25I] and obtaining the folate curve counts and sample counts, then setting the counter for [57Col to obtain the Vitamin B12 curve counts and sample counts. If in counting one isotope there is spillover from the other isotope, counter adjustments must be made. Decreasing channel widths can minimize or eliminate such spillover.

The folate curve and values are calculated from the counts obtained by counting [1251]; the vitamin B12 curve and values are obtained by counting [57Co.].

As the radioactive tracer decays with age, increased counting times may be required. (This will be unnecessary with efficient equipment). The volume of tracer used must be that specified in this protocol; tracer volume should not be increased to compensate for decay. Binding of each vitamin to its respective binder remains essentially unchanged with time using the volume of tracer specified. Reproducible results will be obtained by following the protocol which has been described.

The Standard Curve covers the range of 1.0 to 20 ng/ml folate and 100 to 2000 pg/ml of Vitamin Bt2. A "Blank" (tubes 3 and 4) is used to correct for background counts and radioactive tracer which is not adsorbed onto the charcoal.

The present invention is particularly advantageous in that it is possible to effect a simultaneous assay for folate and vitamin Bl2. Moreover, in accordance with the particularly preferred embodiments, Applicant unexpectedly found that vitamin B12 and folate can both be effectively released from their endogenous binders and effectively assayed at an alkaline pH.

WHAT WE CLAIM IS: 1. An improved simultaneous assay for folate and vitamin B12 comprising: contacting a sample containing folate and vitamin B12 released from endogenous binders therefor with a binder for the folate and a binder for the vitamin Bt2, a folate tracer labeled with a first radioactive isotope and vitamin B12 tracer labeled with a second radioactive isotope different from the first radioactive isotope; separating bound portions of folate and vitamin B,2 from unbound portions of folate and vitamin B12; and counting the radioactivity of at least one of the bound and unbound portions to determine both folate tracer and vitamin B12 tracer.

2. A process according to Claim 1 wherein, prior to the contacting, folate and vitamin B12 are heat released from endogenous binders therefor.

3. A process according to Claim 2 wherein said heat release is effected at a pH of at least 7.0.

4. A process according to Claim 3 wherein the heat release is effected at a pH of at least 9.0 and no greater than 9.6.

5. A process according to Claim 4 wherein the heat release is effected at a pH of from 9.2 to 9.4.

6. A process according to any one of the preceding claims wherein the contacting is effected at a pH of at least 7.0.

7. A process according to any one of the preceding claims wherein the vitamin B12 is labeled with 57Co.

8. A process according to any one of the preceding claims wherein the folate tracer is a radioiodinated folic acid.

9. A process according to any one of the preceding claims wherein the folate tracer is folic acid in which a carboxyl group of the glutamyl moiety is substituted with a substituent which includes a radioiodinated phenol or imidazole.

10. A process according to any one of the preceding claims wherein the folate tracer is folic acid in which a carboxyl group of the glutamyl moiety is substituted with a member selected from radioiodinated histidyl, radioiodinated tyramyl, radioiodinated histamyl and radioiodinated tyrosyl.

11. A process according to Claim 10 wherein said member is '25I-L-tyrosyl.

12. A process according to any one of claims 2-5 and claims 6-11 as appendent thereto wherein the heating and subsequent contacting are effected at the same pH.

13. A process according to any one of claims 2--5 or claim 6--11 as appendent thereto or claim 12 wherein said heating and said contacting are effected at a pH of from 9.2 to 9.4.

!4. A process according to any one of the preceding claims wherein the binder for vitamin B12 is a naturally occurring receptor and the binder for folate is a naturally occurring receptor.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The present invention is particularly advantageous in that it is possible to effect a simultaneous assay for folate and vitamin Bl2. Moreover, in accordance with the particularly preferred embodiments, Applicant unexpectedly found that vitamin B12 and folate can both be effectively released from their endogenous binders and effectively assayed at an alkaline pH. WHAT WE CLAIM IS:

1. An improved simultaneous assay for folate and vitamin B12 comprising: contacting a sample containing folate and vitamin B12 released from endogenous binders therefor with a binder for the folate and a binder for the vitamin Bt2, a folate tracer labeled with a first radioactive isotope and vitamin B12 tracer labeled with a second radioactive isotope different from the first radioactive isotope; separating bound portions of folate and vitamin B,2 from unbound portions of folate and vitamin B12; and counting the radioactivity of at least one of the bound and unbound portions to determine both folate tracer and vitamin B12 tracer.

11. A process according to Claim 10 wherein said member is '25I-L-tyrosyl.