Imidazolepropionic acid as a urinary metabolite of L-histidine

Imidazolepropionic acid has often been proposed as a metabolite of L-histidine, but convincing data on its formation by mammalian tissue has been lacking. Hanke and Koessler (1922) originally described imidazolepropionic, -acetic, -lactic, and -acrylic acids as metabolites of L-histidine in bacteria, but their methods were inadequate to distinguish these products. One route of L-histidine metabolism was subsequently established as proceeding through urocanic acid to formiminoglutamic acid and other aliphatic derivatives (Fruton and Simmonds, 1958). Recently, Baldridge and Tourtellotte (1958) showed that imidazolepropionic acid was found in rat urine following the administration of a large load of L-histidine by stomach tube. Auerbach et,al. (1962) identified imidazolepropionic acid in the urine following administration of a load of urocanic acid to an infant with a presumed defect in histidine metabolism. This paper is to present evidence showing that imidazolepropionic acid is a normal constituent of human urine, and that it is derived from urocanic acid.