Reaction mechanism for the inactivation of the quinoprotein methylamine dehydrogenase by phenylhydrazine.

Phenylhydrazine has previously been shown to be an irreversible inactivator of the tryptophan tryptophylquinone (TTQ) enzyme methylamine dehydrogenase [Davidson, V.L. and Jones, L.H. (1992) Biochim. Biophys. Acta 1121, 104-110]. Structure-reactivity correlations have been performed to elucidate the mechanism by which this inactivation occurs. The reactions of a series of p-substituted phenylhydrazines with methylamine dehydrogenase were examined. Correlation with electronic substituent effects was observed. A Hammett plot of the second order inactivation rate constants versus sigma p exhibited a positive slope. Plots of these rate constants against substituent constants which reflected either resonance or field/inductive parameters for each p-substituent indicated that the rate was primarily influenced by resonance electronic effects. A Brønsted plot of the inactivation rate constant against pKa of each substituted phenylhydrazine yielded a beta-value (slope) of 0.7. Based upon these results, a reaction mechanism is proposed for the inactivation of methylamine dehydrogenase by phenylhydrazines, and a structure is proposed for the putative transition state for the rate-limiting step in the overall processes of binding and adduct formation by phenylhydrazine. The relevance of these results to the process of imine formation between substrate amines and TTQ during the normal catalytic process is also discussed.