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HSCH₂CH₂CH(NH₃)+COO-; a homolog 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 h. have been associated with certain forms of heart disease. SEE ALSO: folic acid.Elevation of the level of h. in the plasma is an independent risk factor for cardiovascular disease (including myocardial infarction, stroke, thromboembolic disease, and intermittent claudication) and (in pregnant women) for fetal neural tube defects such as spina bifida and anencephaly. An increased plasma total h. level has been reported to confer an independent risk of vascular disease similar to that of smoking or hyperlipidemia, and to compound the risk associated with smoking and hypertension. Approximately 25% of people with atherosclerosis are found to have elevation of plasma h. above 15 mmol/L. Because h. 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 h. levels and coronary disease risk. H. 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 h. 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 h. in people with homocystinuria, a rare genetic disorder in which deficiency of the enzyme cystathionine β-synthase leads to elevation of h. in plasma and of its oxidation product, homocystine, in urine. A more common genetic disorder associated with abnormally high levels of h. results from mutation of the gene that encodes the enzyme methylene tetrahydrofolate reductase. The marked increase in h. 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 h., as are chronic renal failure, hypothyroidism, and some malignancies. Lowering the serum concentration of h. by administration of folic acid has been shown to reduce the risk of adverse cardiovascular events in people with homocystinuria. In animal studies, administration of folic acid prevents the teratogenic effect of h.. Screening for elevated h. levels is advised for people with coronary artery disease out of proportion to known risk factors, or for those with a family history of premature atherosclerotic disease. Administration of folic acid in a dose of 1 mg/day or more reduces h. levels nearly to normal and protects against both vascular disease and birth defects.

ho·mo·cys·te·ine .hō-mō-'sis-tə-.ēn, .häm-ō- n an amino acid C₄H₉NO₂S that is produced in animal metabolism by the demethylation of methionine and forms a complex with serine that breaks up to produce cysteine and homoserine and that appears to be associated with an increased risk of cardiovascular disease when occurring at high levels in the blood

a sulphur-containing amino acid that is an intermediate in the synthesis of cysteine. A deficiency in the enzyme cystathionine synthetase results in elevated levels in the blood of homocysteine and homocystine (an oxidized form of homocysteine), resulting in elevated urinary levels (see homocystinuria). It is becoming increasingly recognized that elevated levels of homocysteine in the blood are a risk factor for vascular disease independent of diabetes, hypertension, elevated levels of cholesterol in the blood, and smoking.