The amino acid histidine, when catalyzed by the enzyme histidase, forms<unk>  acid. Disruptions in this pathway, caused by a deficiency of histidase, is the underlying cause of histidinemia. This results in reduced levels of skin and serum<unk>  acid, the primary indicator of insufficient histidase activity. In<unk> , increased<unk>  acid in the urine indicates a deficiency of the enzyme urocanase. With normal to only slightly elevated levels of histidine present in the liver during<unk> , the only true metabolic indicator of the disorder can be found in the urine.
Urocanic aciduria
Hawkinsinuria
Orotic aciduria
Alpha-aminoadipic and alpha-ketoadipic aciduria
Cystathioninuria
Alkaptonuria
Saccharopinuria
Homocystinuria