Vitamin adenosyltransferase

Figure 10.9. Metabolism of methionine. Methionine adenosyltransferase, EC 2.5.1.6 methionine synthetase, EC 2.1.1.13 (vitamin B12-dependent) and EC 2.1.1.5 (betaine as a methyl donor) cystathionine, 6-synthetase, EC 4.2.1.22 and y-cystathionase, EC 4.4.I.I.

Deficiencies of methionine adenosyltransferase, cystathionine 8-synthase, and cystathionine )/-lyase have been described. The first leads to hypermethioninemia but no other clinical abnormality. The second leads to hypermethioninemia, hyperhomocysteinemia, and homo-cystinuria. The disorder is transmitted as an autosomal recessive trait. Its clinical manifestations may include skeletal abnormalities, mental retardation, ectopia lentis (lens dislocation), malar flush, and susceptibility to arterial and venous thromboembolism. Some patients show reduction in plasma methionine and homocysteine concentrations and in urinary homocysteine excretion after large doses of pyridoxine. Homocystinuria can also result from a deficiency of cobalamin (vitamin B12) or folate metabolism. The third, an autosomal recessive trait, leads to cystathioninuria and no other characteristic clinical abnormality. 

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