Kinetic and mechanistic studies on the reaction of melilotate hydroxylase with deuterated melilotate

Melilotate has been synthesized from melilotate by iodiantion followed by reductive deiodination in the presence of deuterated hydrazine. The deuterated melilotate has been employed in investigations of the reaction mechanism of melilotate hydroxylase. Stopped-flow spectrophotometry has revealed no isotope effect in the formation or decay of the oxygenated intermediate which is observed when reduced melilotate hydroxylase reacts with molecular oxygen. Steady-state analysis has corroborated this result, and in addition shows that there is no isotope effect in the reductive cycle of the enzyme mechanism. This analysis does reveal a reproducible 8% decrease in Vmax for the enzyme when using deuterated melilotate. These observations are compatible with the thesis that the above intermediate is an oxygenated form of the reduced flavine prosthetic group and that the last step of the proposed mechanism is rapid and involves a primary isotope effect. The existence of the NIH shift mechanism has been studied using combined gas chromatography-mass spectrometry. No evidence could be obtained for intramolecular migration of deuterium during the hydroxylation reaction. However, the small amount of migration expected when phenols are hydroxylated precludes elimination of the NIH shift as a possibility.