A homologue of cysteine, produced by the demethylation of methionine, and an intermediate in the biosynthesis of L-cysteine from L-methionine via L-cystathionine. Elevated levels of homocysteine have been associated with certain forms of heart disease. See Also: folic acid.
Elevation of the level of homocysteine in the plasma is an independent risk factor for cardiovascular disease (including myocardial infarction, congestive heart failure, stroke, thromboembolic disease, and intermittent claudication) and (in pregnant women) for fetal neural tube defects such as spina bifida and meroanencephaly. Approximately 25% of people with atherosclerosis are found to have elevation of plasma homocysteine above 15 mmol/L. Because homocysteine rises after myocardial infarction and remains elevated for months, some have questioned the causal role assigned to it in vascular disease. Several prospective studies have failed to establish a connection between homocysteine levels and coronary disease risk. Homocysteine appears to exert a direct toxic effect on the intima of arteries, besides inducing oxidation of low-density lipoproteins and predisposing to thrombus formation by activating platelets and coagulation factors. In animal reproduction studies it promotes neural tube defects, cardiac anomalies, and failure of ventral closure. Elevation of plasma homocysteine occurs in various conditions, including genetic disorders, nutritional deficiencies, and chronic diseases. The level is higher in men and tends to rise with advancing age. Premature cardiovascular disease was first linked to elevation of homocysteine in people with homocystinuria, a rare genetic disorder in which deficiency of the enzyme cystathionine *-synthase leads to elevation of homocysteine in plasma and of its oxidation product, homocystine, in urine. A more common genetic disorder associated with abnormally high levels of homocysteine results from mutation of the gene that encodes the enzyme methylene tetrahydrofolate reductase. The marked increase in homocysteine levels after menopause may play a role in the increased incidence of vascular disease, cancer, and osteoporosis in postmenopausal women. Dietary deficiency of folic acid, vitamin B6 (pyridoxine), and vitamin B12 is also associated with elevation of homocysteine, as are chronic renal failure, hypothyroidism, and some malignancies. Lowering the serum concentration of homocysteine by administration of folic acid has been shown to reduce the risk of adverse cardiovascular events in patients with homocystinuria. In animal studies, administration of folic acid prevents the teratogenic effect of homocysteine. Screening for elevated homocysteine levels is advised for patients with coronary artery disease that is out of proportion to known risk factors, or with a family history of premature atherosclerotic disease. Administration of folic acid in a dose of 1 mg/day or more reduces homocysteine levels nearly to normal and protects against both vascular disease and birth defects.
Reference: Stedman's Medical Dictionary