Homocysteine

Homocysteine is a sulfur-containing amino acid formed during methionine metabolism. Cells recycle it to methionine or convert it to cysteine.

Methionine cycle: A metabolic cycle that transfers methyl groups and regenerates methionine from homocysteine. Homocysteine is produced within this cycle and can return to methionine through remethylation.
Hyperhomocysteinemia: An abnormally elevated concentration of homocysteine in blood. This is the clinical finding that prompts evaluation of homocysteine metabolism.
Cardiovascular disease: A group of disorders affecting the heart and blood vessels. High homocysteine is associated with cardiovascular risk, though lowering it does not reliably prevent events.
Amino acid: An organic molecule with amino and carboxyl groups; amino acids are protein building blocks and metabolic intermediates. Homocysteine belongs to this broad chemical family but is not incorporated into proteins.
Mendelian randomization: A method that uses genetic variants as instruments to test causal relationships between exposures and outcomes. Genetic studies have tested whether lifelong differences in homocysteine cause vascular disease.
Transsulfuration pathway: A metabolic pathway that converts homocysteine into cysteine through several enzyme-catalyzed reactions. This route disposes of homocysteine by directing its sulfur toward cysteine synthesis.
Homocystinuria: A group of inherited metabolic disorders that cause homocysteine and related compounds to accumulate. Severe elevations can signal an inherited defect in homocysteine metabolism.
Stroke: Brain injury caused by interrupted blood flow or bleeding in the brain. Elevated homocysteine is associated with stroke risk, but causality and treatment benefit remain uncertain.
Methionine: An essential sulfur-containing amino acid and the metabolic precursor of homocysteine. Homocysteine forms when methionine loses a methyl group during metabolism.
Cochrane: An international organization that produces systematic reviews of health-care evidence. Its reviews assess whether B-vitamin homocysteine lowering prevents cardiovascular events.

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