cystathioninuria

▪ genetic defect

      hereditary metabolic disorder involving the amino acid methionine. The normal metabolic pathway of methionine is its conversion in successive steps to homocysteine, cystathionine, and cysteine, each step being effected by a specific enzyme. In cystathioninuria, the enzyme cystathionine gamma-lyase, which normally catalyzes the hydrolysis of cystathionine to cysteine, is defective. As a result, abnormally high concentrations of cystathionine appear in the urine. Although the enzyme appears to be present in the tissues of affected persons, it is inactive until an excess of vitamin B₆ is available. In some cases, the administration of vitamin B₆ brings about a dramatic response, which results in a marked fall in cystathionine excretion.

      Most affected individuals experience cystathioninuria as a mild disorder, evincing only the abnormal urinary excretion of cystathionine; sometimes, however, the disorder is associated with intellectual disability. Compare homocystinuria.

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