Homocysteine
An amino acid produced by the body, usually as a byproduct of consuming meat. Homocysteine is made from another amino acid, methionine, and then in turn is converted into other amino acids. Elevated levels of homocysteine in the blood appear to make for an elevated risk of cardiovascular (heart and vessel) disease. Levels of homocysteine as low as 12 micromoles per liter of blood plasma have been found associated with an increased risk of heart attack, stroke, peripheral vascular disease and venous thromboembolism (blood clots in the veins). A homocysteine level of 15 micromoles or above has a heart attack rate three times as high as normal. Even a level of 12 micromoles can double the coronary risk. Homocysteine can damage blood vessels in several ways. It injures the cells that line arteries and stimulates the growth of smooth muscle cells. Homocysteine can also disrupt normal blood clotting mechanisms, increasing the risk of clots that can bring on a heart attack or stroke. Elevated levels of homocysteine also appear to increase the chance of Alzheimer's disease. In a remarkable study published in The New England Journal of Medicine in 2002, a large contingent of elderly people free of dementia were studied prospectively (in a forward-looking way). The results were quite startling. An elevated blood plasma level of the amino acid homocysteine was found to be an independent predictor of the development of clinical dementia, usually due to Alzheimer's disease. The higher the homocysteine level, the more likely a person was to later develop Alzheimer's. When plasma homocysteine is at a high level (over 14 micromoles per liter of blood), the risk of Alzheimer's disease doubled. The strength of this association is very powerful. The ways to bring down homocysteine are by eating less meat and by taking supplements of the B vitamins folic acid (folate), B6, and B12 that are needed by the enzymes that process homocysteine. (Homocysteine builds up when the amino acid methionine cannot be converted to cysteine because an enzyme is lacking or is present in inadequate amounts. The B vitamins folic acid (folate), B6, and B12 are crucial to these conversion enzymes.) Since folic acid supplementation can reduce plasma homocysteine levels, a possible strategy to help prevent both heart disease and Alzheimer's may be to provide more folic acid. Concern for homocysteine as a cardiovascular risk factor came first. It dates to 1969 when Dr. Kilmer S. McCully reported that children born with a genetic error of metabolism called homocystinuria that causes the homocysteine levels to be very high sometimes died at a very young age with advanced disease in their arteries. However, until the 1990's the importance of homocysteine in cardiovascular disease was overshadowed by cholesterol story.
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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.

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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

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n.
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.

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ho·mo·cys·te·ine (ho″mo-sisґte-ēn) a sulfur-containing amino acid homologous with cysteine and produced by demethylation of methionine. It can serve as an intermediate in the biosynthesis of cysteine from methionine via cystathionine or can be remethylated to methionine.

Medical dictionary. 2011.

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