Rat liver imidase.

Imidase, an enzyme variously identified as dihydropyrimidinase (EC 3.5.2.2), hydantoinase, dihydropyrimidine hydrase, and dihydropyrimidine amidohydrolase, has been purified to electrophoretic homogeneity from rat liver. Although a component in the chain of pyrimidine catabolism, imidase is capable of serving in a broader role that includes detoxication of xenobiotics. The enzyme catalyzes the hydrolytic cleavage of imides that range from the linear to the heterocyclic and that include hydantoins, dihydropyrimidines, and phthalimide. For some substrates, the reaction is experimentally reversible. The pH activity curves are a function of the pKa of the individual substrate's imino group, with cleavage favored at a pH near the respective pKa value. There is evidence for stereoselectivity and for stereospecificity. A mechanism is proposed for the enzyme-catalyzed reaction.