Identification of the enol tautomer of imidazolone propionate as the urocanase reaction product.

A fluorescent product was transiently formed during catalysis by urocanase from Pseudomonas putida. The fluorophore showed an emission maximum at 430 nm when excited at 330 nm, essentially identical to that exhibited by the enol tautomer of imidazolone propionate. The keto isomer was not fluorescent under these conditions. In aqueous acid solutions where imidazolone propionate is relatively stable, an equilibrium mixture of tautomeric forms contained approximately 1% of the enol isomer. In ethanolic solutions, the equilibrium concentration of enol tautomer increased to approximately 25%. The differing content of imidazolone propionate tautomers as a function of solvent conditions permitted a comparison of the keto and enol forms as substrates for the reverse reaction. This revealed an almost complete preference for the enol tautomer. These results are taken as direct proof that enol imidazolone propionate is the true urocanase reaction product.