Histidase and histidinemia. Clinical and molecular considerations.

Histidase (histidine ammonia-lyase, EC 4.3.1.3) catalyzes the deamination of L-histidine to trans-urocanic acid in the liver and skin of mammals. Histidase deficiency results in increased histidine and histamine in blood, and decreased urocanic acid in blood and skin. In this review we discuss current research on: (1) the mechanism of formation of an unusual residue, dehydroalanine, at the active site of histidase; and (2) the role of urocanic acid as an ultraviolet light-induced immunoregulator in the skin, and the implications of urocanic acid deficiency for human histidinemia. Genetic mechanisms that may account for the 1% of histidinemic patients with neurological impairments are considered briefly.