Effect of pyridoxine on the metabolism of tryptophan and branched-chain amino acids in two mentally retarded sibs.

Abstract

Two mentally retarded sibs, a girl aged 5 years and a boy aged 3 years, are described. Both children had slightly elevated levels of serum leucine, and the boy also had an elevated level of valine. In both children, tryptophan loading studies revealed a disorder of metabolism which involved the pyridoxal-dependent enzyme, kynureninase. The administration of oral pyridoxine supplements corrected the abnormality of tryptophan metabolism and reduced to normal the serum levels of leucine found in both sibs, and of valine in the boy. The relation between the defects of tryptophan and branched-chain amino acid metabolism was demonstrated when quinolinic acid was measured in the urine both before and after the administration of pyridoxine. The possible relation between the biochemical and clinical findings is discussed.