Histidinemia Urocanic acid

Catabolism of histidine in most organisms proceeds via an initial elimination of NH3 to form urocanic acid (Eq. 14-44). The absence of the enzyme L-histidine ammonia-lyase (histidase) causes the genetic disease histidinemia 284/285 A similar reaction is catalyzed by the important plant enzyme L-phenylalanine ammonia-lyase. It eliminates -NH3+ along with the pro-S hydrogen in the (3 position of phenylalanine to form frans-cinnamate (Eq. 14-45). Tyrosine is converted to p-coumarate by the same enzyme. Cinnamate and coumarate are formed in higher plants and are converted into a vast array of derivatives (Box 21-E,... 

In histidinemia, the missing enzyme is histidase. This was established in two ways administration of urocanic acids, which is followed by histidinemia and his-tinuria and direct assay for the enzyme. The metabolic block leads to histidine accumulation in the blood and excessive excretion in the urine. 

Histidinemia is an autosomal recessive disorder that is benign in most affected individuals. The incidence from newborn screening is 1 10000, making histidinemia one of the most frequent of the inborn errors of metabolism. The enzyme defect is histidase, an enzyme normally expressed only in skin and liver. The block in conversion of histidine to urocanic acid results in an increased concentration of histidine in blood and urine and the abnormal presence of histidine metabolites in urine. 

The responses of histidine and its metabolites in blood and urine to histidine and urocanic acid loads in patients with histidinemia, urocanase deficiency and formiminotransferase deficiency. 

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