Category: amino-acid
Aliases: Hcy, L-Homocysteine
Non-proteinogenic thiol AA generated from methionine via S-adenosyl-methionine (SAM) → S-adenosyl-homocysteine (SAH) → Hcy following every methyl-transfer reaction. Fate: (1) remethylation to methionine via methionine synthase (requires B12 + 5-methyl-THF — folate cycle) or via BHMT (requires betaine/TMG) in liver/kidney; (2) trans-sulfuration via cystathionine-β-synthase (requires B6) → cystathionine → cysteine → glutathione / taurine. Elevated plasma Hcy (hyperhomocysteinemia) associated with endothelial dysfunction, thrombosis, dementia, and CAD — though RCTs of folate/B-vitamin lowering have not cleanly shown MACE reduction (NORVIT, HOPE-2, VISP). Severe cases (>100 μmol/L) → CBS deficiency (classical homocystinuria) with ectopia lentis, skeletal abnormalities, early thrombosis.