Hydroxykynureninuria, also known as xanthurenic aciduria is autosomal recessive disorder of tryptophan metabolism. It is characterized by excessive output of xanthutrenic acid, 3-hydroxykynurenine, and kynurenine in urine. This disease is caused by homozygous mutation in the KYNU gene, which encodes kynureninase. Kynureninase is an enzyme in the catabolic pathway of tryptophan metabolism. Some of these deficiencies lead to pellagra or mild pellagra-like symptoms, while it was reported in some patients no symptom of niacin deficiency was observed. The different clinical outcomes could be explained by differences in the intake of niacin. The sufficient intake of niacin could prevent depletion in some patients despite their impaired niacin synthesis.
