Category: drug_metabolism
Methionine is the methyl-group donor source: methionine → SAM (S-adenosylmethionine; MAT) → SAH (S-adenosylhomocysteine; after methyl transfer) → homocysteine (SAH hydrolase) → methionine (methionine synthase, B12 + 5-methyl-THF dependent) or → cystathionine → cysteine (B6-dependent transsulfuration). B12 OR folate deficiency raises homocysteine; chronically elevated homocysteine is a CV risk marker. MTHFR C677T polymorphism reduces 5,10-methylene-THF → 5-methyl-THF flux, raising homocysteine in homozygotes.