A number of inborn errors of metabolism of the tryptophan oxidative pathway (see Figure 8.4) have been reported, all of which result in the development of pellagra that responds to high doses of niacin. These conditions include vitamin B6-responsive xanthurenic aciduria, caused by a defect of kynureni-nase (Section 9.4.3); hydroxykynureninuria, apparently caused by a defect of kynureninase; tryptophanuria, apparently caused by tryptophan dioxygenase deficiency; a hereditary pellagra-like condition, apparently caused by an increase in activity of picolinate carboxylase; and Hartnup disease.
Hartnup disease is a rare genetic condition in which there is a defect of the membrane transport mechanism for tryptophan and other large neutral amino acids. The result is that the intestinal absorption of free tryptophan is impaired, although dipeptide absorption is normal. There is a considerable urinary loss of tryptophan (and other amino acids) as a result of the failure of the normal reabsorption mechanism in the renal tubules - renal amino-aciduria. In addition to neurological signs that can be attributed to a deficit of tryptophan for the synthesis of serotonin in the central nervous system, the patients show clinical signs of pellagra, which respond to the administration of niacin.
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