Introduction:

Homocysteine is a thiol-containing amino acid produced by the intracellular demethylation of methionine. HCY is exported into plasma where it circulates mostly in its oxidized forms bound to plasma proteins. Smaller amounts of reduced homocysteine and disulfide homocystin (HCY-SS-HCY) are present. Total homocysteine (tHCY) represents the sum of all HCY species found in plasma and serum. HCY is either metabolised to cysteine or to methionine. In the vitamin B6 dependent transsulphuration pathway HCY is irreversibly catabolized to cysteine. A major part of HCY is remethylated to methionine, mainly by the folate and cobalamin-dependent enzyme methionine synthase. HCY accumulates and is excreted into the blood when these reactions are impaired^(4-5). The elevated concentration of homocysteine in the blood is considered an independent risk factor of CVD.

Epidemiological studies have investigated the relationship between HCY levels in blood and CVD. A meta analysis of 27 epidemiological studies, including more than 4000 patients, estimated that a 5 gM increase in HCY was associated with an odds ratio for coronary artery disease (CAD) of 1.6 for men and 1.8 for women. Peripheral arterial disease also showed a strong association⁽⁶⁾.

Certain patient groups with anemia and/or asthenia also demonstrate increased levels of plasma or serum HCY. Patients with chronic renal disease experience an excess morbidity and mortality due to arteriosclerotic CVD. Elevated concentration of HCY is a frequently observed finding in the blood of these patients. Although such patients may lack some of the vitamins involved in the metabolism of HCY, the increased levels of HCY are mainly due to impaired removal of HCY from the blood by the kidney⁽⁷⁾.

Severely elevated concentrations of HCY are found in subjects with "homocystinuria", a rare genetic disorder of the enzymes involved in the metabolism of HCY. Patients with homocystinuria exhibit mental retardation, early arteriosclerosis and arterial and venous thromboembolism(8). Drugs such as methotrexate, carbamazepine, phenytoin, nitrous oxide and penicillamine interfere with the HCY metabolism and may give elevated levels of HCY⁽⁹⁾.

Principle of the method:

The Formosa Biomedical Technology Total Homocysteine Biochemical Assay is an enzymatic method rather than the immunoassay employing antibodies which measures total homocysteine in plasma.

Bound HCY is reduced to free HCY which then catalyzed by recombinant methionine α,γ-lyase (HCY enzyme) to produce H₂S. The H₂S subsequently reacts with a chromophore, and the product is measured optically at 660nm. The optical density is proportional to the tHCY level.

Reference:

1. McCully KS. Homocysteine and vascular disease. Nat Med 1996; 2: 386-389.

2. Selhub J, Jacques PF, Bostom AG, et al. Association between plasma homocysteine concentrations and extracranial carotid-artery stenosis. N Engl J Med 1995; 332: 286-291.

3. Nygard O, Nordrehaug JE, Refsum H, et al. Plasma homocysteine levels and mortality in patients with coronary artery disease. N Engl J Med 1997; 337: 230-236.

4. Ueland PM. Homocysteine species as components of plasma redox thiol status. Clin Chem 1995; 41:173-176.

5. Finkelstein JD, Methionine metabolism in mammals. J Nutr Biochem 1990; 1: 228-237.

6. Boushey CJ, Beresford SAA, Omenn GS, et al. Quantitative assessment of plasma homocysteine as a risk factor for vascular disease. JAMA 1995; 274: 1049-1057.

7. Bostom AG, Lathrop L. Hyperhomocysteinemia in end-stage renal disease: prevalence, etiology, and potential relationship to arteriosclerotic outcomes. Kidney Int 1997; 52: 10-20.

8. Malinow MR. Plasma homocysteine and arterial occlusive disease: a mini-review. Clin Chem 1995; 41: 173-176.

9. Ueland PM, Refsum H, Stabler SP, et al.. Total homocysteine in plasma or serum: methods and clinical applications. Clin Chem 1993; 39: 1764- 1779.