Homokysteiini

Homokysteiini is the Finnish term for homocysteine, 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 participates in methylation reactions and the synthesis of other sulfur-containing compounds.

Biochemistry and metabolism: Homocysteine is produced from S-adenosylhomocysteine during methylation reactions. It can be remethylated to methionine via methionine synthase, a process that requires vitamin B12 and folate. Alternatively, it can enter the transsulfuration pathway and be converted to cystathionine by cystathionine beta-synthase, a reaction that requires vitamin B6. Remethylation also occurs in the liver and kidney via betaine-homocysteine methyltransferase using betaine as a methyl donor.

Clinical significance: Plasma levels of homocysteine (total homocysteine) can be elevated, a condition known as hyperhomocysteinemia. Higher levels are associated with an increased risk of cardiovascular disease, thrombosis, and certain neurocognitive issues. Inherited disorders such as homocystinuria, often caused by cystathionine beta-synthase deficiency, lead to markedly elevated homocysteine and present with ocular lens dislocation, skeletal abnormalities, osteoporosis, and thromboembolic events. Secondary causes include renal or liver disease, nutritional deficiencies of vitamins B6, B12, or folate, and certain medications.

Diagnosis and management: Diagnosis is based on measurement of plasma total homocysteine. Management focuses on lowering levels through vitamin supplementation (cobalamin, folate, and pyridoxine), sometimes betaine to promote remethylation of homocysteine to methionine, and dietary adjustments. In hereditary cases, specific therapies and ongoing risk-reduction strategies are tailored to the underlying defect.