REACTION-MECHANISM FOR THE INACTIVATION OF THE QUINOPROTEIN METHYLAMINE DEHYDROGENASE BY PHENYLHYDRAZINE

Phenylhydrazine has previously been shown to be an irreversible inacti vator of the tryptophan tryptophylquinone (TTQ) enzyme methylamine deh ydrogenase [Davidson, V.L. and Jones, L.H. (1992) Biochim. Biophys. Ac ta 1121, 104-110]. Structure-reactivity correlations have been perform ed to elucidate the mechanism by which this inactivation occurs. The r eactions of a series of p-substituted phenylhydrazines with methylamin e 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 t hese rate constants against substituent constants which reflected eith er resonance or field/inductive parameters for each p-substituent indi cated that the rate was primarily influenced by resonance electronic e ffects. A Bronsted plot of the inactivation rate constant against pK(a ) of each substituted phenylhydrazine yielded a beta-value (slope) of 0.7. Based upon these results, a reaction mechanism is proposed for th e 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 cat alytic process is also discussed.