The action of bacterial cytidine deaminase on 5,6-dihydrocytidine.

Cytidine deaminase from Escherichia coli was found to catalyze the hydrolytic deamination of 5,6-dihydrocytidine, at a rate slightly lower than its rate of action on the normal substrate. The results suggest that nucleophilic addition by the enzyme at the 5,6 position of the substrate is not an essential part of catalysis, unless the active site is so flexible that deamination can occur with addition in one case (cytidine) and without addition in another case (5,6-dihydrocytidine). 3,4,5,6-Tetrahydrouridine bears a close structural resemblance to a hypothetical "tetrahedral" intermediate formed by direct water addition to 5,6-dihydrocytidine. The hydrolytic activity of the enzyme toward 5,6-dihydrocytidine and its potent inhibition by 3,4,5,6-tetrahydrouridine are presumably related by the ability of the active site to stabilize structures of this kind by tight binding. Cytidine deaminase shows no detectable activity as a catalyst for the dehydration of 6-hydroxy-5,6-dihydrouridine.