Homocysteine is a sulfhydryl-containing amino acid, synthesized from methionine. It is a non-essential, non-proteinogenic amino acid. It is an important determinant of the methylation cycle and is present in the plasma in four forms. An abnormally high level of homocysteine leads to hyperhomocysteinemia and also promotes atherosclerosis.
Increased plasma homocysteine is a risk factor for coronary heart disease and carcinogenesis; may be a marker for tumor cell growth.