Methods and compositions for assaying homocysteine
This invention relates generally to the field of homocysteine detection. In particular, the invention provides a method for determining homocysteine presence or concentration in samples, which method comprises: contacting a sample containing or suspected of containing Hcy with a Hcy co-substrate and a Hcy converting enzyme in a Hcy conversion reaction to form a Hcy conversion product and a Hcy co-substrate conversion product; and assessing the Hcy co-substrate conversion product to determine the presence, absence and/or amount of the Hcy in the sample. The Hcy co-substrate conversion product may be assessed directly, or it may be assessed by further conversion of the Hcy co-substrate conversion product into another material by the action of one or more additional enzymes. A kit for assaying homocysteine based on the same principle is also provided.
1. A method for assaying homocysteine (Hcy) in a sample without chromatographic separation, which method comprises:
a) contacting a sample containing or suspected of containing Hcy with a Hcy co-substrate and a Hcy converting enzyme in a Hcy conversion reaction to form a Hcy conversion product and a Hcy co-substrate conversion product, wherein the Hcy co-substrate is S-adenosylmethionine (SAM), the Hcy converting enzyme is a S-adenosylmethionine (SAM)-dependent homocysteine S-methyltransferase, the Hcy conversion product is methionine (Met) and the Hcy co-substrate conversion product is S-adenosyl-L-homocysteine (SAH);
b) contacting the mixture formed in step a) with an adenosylhomocysteine nucleosidase to convert SAH into adenine and S-ribosyl homocysteine;
c) contacting the mixture formed in step b) with an S-ribosyl homocysteinase to cycle the S-ribosyl homocysteine into homocysteine; and
d) assessing the amount of adenine in the mixture formed in step b) and/or c),
whereby the amount of Hcy in the sample is determined from the amount of adenine.
2. The method of claim 1 , wherein the step of assessing the amount of adenine comprises the step of converting the adenine by adenine deaminase into ammonia and hypoxanthine, wherein the amount of at least one of ammonia and hypoxanthine is assessed in order to assess the amount of adenine.
3. The method of claim 2 , wherein the assessment of the amount of adenine comprises assessing the amount of ammonia formed.
4. The method of claim 3 , wherein the amount of ammonia formed is measured by assessing appearance of NAD+ or disappearance of NADH when ammonia is converted into glutamate by the action of glutamate dehydrogenase.
5. The method of claim 2 , wherein the amount of adenine is assessed by assessing the amount of hypoxanthine formed.
6. The method of claim 5 , wherein the amount of hypoxanthine is assessed by monitoring its conversion into xanthine, which conversion is catalyzed by xanthine oxidase.
7. The method of claim 1 , wherein the amount of adenine is assessed by monitoring the formation of either or both of AMP and diphosphate catalyzed by the action of adenine phosphoribosyltransferase acting on the adenine and 5-phospho-α-D-ribose 1-diphosphate (PRPP).
8. The method of claim 1 , wherein the sample is a body fluid or a biological tissue.
9. The method of claim 8 , wherein the body fluid is selected from the group consisting of urine, blood, plasma, serum, saliva, semen, stool, sputum, cerebral spinal fluid, tears, mucus and amniotic fluid.
10. The method of claim 8 , wherein the body fluid is blood.
11. The method of claim 10 , wherein the blood sample is further separated into a plasma or serum fraction.
12. The method of claim 1 , wherein the SAM is produced from ATP and methionine by a SAM synthase.
13. The method of claim 1 , wherein prior to or concurrently with the contact between the sample and the Hcy co-substrate and the Hcy converting enzyme, oxidized or conjugated Hcy in the sample is converted into reduced Hcy.
14. The method of claim 1 , wherein the amount of adenine is assessed without chromatographic separation.
15. A method for assaying homocysteine (Hcy) in a sample, which method comprises:
a) contacting a sample containing or suspected of containing Hcy with a Hcy co-substrate and a Hcy converting enzyme in a Hcy conversion reaction to form a Hcy conversion product and a Hcy co-substrate conversion product, wherein the Hcy co-substrate is S-adenosylmethionine (SAM), the Hcy converting enzyme is a S-adenosylmethionine (SAM)-dependent homocysteine S-methyltransferase, the Hcy conversion product is methionine (Met) and the Hcy co-substrate conversion product is S-adenosyl-L-homocysteine (SAH),
b) contacting the SAH generated in step (a) with SAH nucleosidase and S-ribosyl homocysteinase to generate Hcy, which is cycled into the Hcy conversion reaction by the SAM-dependent homocysteine S-methyltransferase to form a Hcy co-substrate based enzyme cycling reaction system, and adenine; and
c) assessing the adenine formed to assess the presence or amount of Hcy in the sample.
16. The method of claim 15 , wherein the assessment of adenine does not involve an enzymatic reaction generating H 2 O 2 and detection of H 2 O 2 .
17. A kit for assaying Hcy in a sample, which kit comprises:
a) a S-adenosylmethionine (SAM)-dependent homocysteine S-methyltransferase;
b) S-adenosylmethionine (SAM);
c) a SAH nucleosidase;
d) a reagent for assessing adenine; and
e) S-ribosyl homocysteinase.
18. The kit of claim 17 , wherein the reagent for assessing adenine comprises an adenine converting enzyme other than SAH hydrolase.
19. The kit of claim 18 , wherein the adenine converting enzyme is an adenine deaminase.
20. The kit of claim 18 , wherein the adenine converting enzyme is an adenine phosphoribosyltransferase.