THE REACTION OF COENZYME PQQ WITH HYDRAZINES

The reaction of coenzyme PQQ with hydrazines, which are known to be inhibitors of quinoprotein amine oxidases, has been investigated in vitro. Only the redox reaction is observed in the reaction with phenylhydrazine, methylhydrazine, N,N-dimethylhydrazine, and N,N′-dimethylhydrazine. However, the PQQ adduct formation occurs concomitantly in the reaction with 4-nitrophenylhydrazine, and only hydrazone formation is observed in the reaction with 2,4-dinitrophenylhydrazine [(3) and (4), respectively]. Kinetic studies reveal that the order of the redox reactivity is phenylhydrazine ≃ methylhydrazine > N,N′-dimethylhydrazine ≫ N,N-dimethylhydrazine, which does not correlate with the two-electron redox potentials of these hydrazines. The reactivity of the PQQ model compounds [PQQ, PQQTME, and (5)-(10)] in the oxidation of methylhydrazine is also examined, the relative reactivity seems to be related to the reactivity of the quinone functional group toward nucleophilic addition (hydration), but does not reflect the two-electron redox potentials. These results suggest that reduction of PQQ with hydrazines proceeds via covalent addition of hydrazines to the quinone to form a carbinolamine type intermediate followed by electron flow from the nitrogen of hydrazines into the quinone moiety.