homocysteine /-sis″tēn/ , an amino acid containing sulfur and a homolog of cysteine, produced in the demethylation of methionine. It is also an intermediate product in the biosynthesis of cysteine from L-methionine via L-cystathionine in the breakdown of proteins. High levels of homocysteine are associated with an increased risk of collagen cardiovascular disorders, particularly thromboembolic stroke. It is believed the amino acid may have a toxic effect on cells lining the blood vessels. Studies also indicate that low levels of homocysteine are found in people with high intake of B vitamins. See also homocystine.
