Proposal for the mechanism of action of urocanase. Inference from the inhibition by 2-methylurocanate.

Incubation of urocanase with 2-methylurocanate leads, after an initial normal reaction, to a time dependent inactivation of the enzyme. It is suggested that a tautomeric form of the product, 2-methyl-imidazolone propionate, is the actual inhibitor. On the basis of these and of published experimental data a novel mechanism is proposed for the urocanase reaction. The crucial and initial step is the electrophilic addition of enzyme-bound NAD to the 2-position of the imidazole nucleus of urocanate.