homocysteïne

Homocysteine is a sulfur-containing amino acid that is not incorporated into proteins. It is an intermediate in the metabolism of methionine, an essential amino acid, and is formed during demethylation of methionine. In the body, homocysteine can be remethylated to methionine or diverted to cysteine through transsulfuration, forming a key part of the one-carbon metabolism network.

Remethylation to methionine occurs mainly via methionine synthase, which uses 5-methyltetrahydrofolate as a methyl donor and requires vitamin B12. An alternative remethylation pathway uses betaine as a methyl donor in the liver. Transsulfuration converts homocysteine to cystathionine through cystathionine beta-synthase, a reaction that requires vitamin B6, and is subsequently converted to cysteine.

Plasma or serum homocysteine levels are used in clinical assessment. Normal reference ranges are roughly 5 to 15 micromoles per liter, with higher values termed hyperhomocysteinemia. Levels can be influenced by age, kidney function, nutritional status, and genetic factors. Common contributors include deficiencies of folate, vitamin B12, or vitamin B6; renal impairment; smoking; and certain medications or genetic variants such as MTHFR polymorphisms. Rare inherited disorders of homocysteine metabolism, such as cystathionine beta-synthase deficiency, cause marked elevations and a risk of thrombotic events and other complications.

Elevated homocysteine has been associated with increased risk of cardiovascular disease, stroke, and cognitive decline in observational studies, but causal interpretation remains debated. Management focuses on correcting deficiencies with B vitamins and folate, addressing renal disease, and reducing modifiable risk factors. Evidence that lowering homocysteine alone reduces cardiovascular events is mixed, and recommendations emphasize a comprehensive risk-reduction approach.