NONENZYMATIC REDUCTION OF TETRACHLORO-1,4-BENZOQUINONE BY REDUCED NICOTINAMIDE ADENINE-DINUCLEOTIDE PHOSPHATE IN AN AQUEOUS SYSTEM

NADPH was shown to reduce tetrachloro-1,4-benzoquinone (TCBQ) to tetra chloro-1,4-benzene diol which was identified on the basis of cochromat ography with synthetic standard in a reverse phase HPLC system, electr onic absorption spectroscopy, and mass spectrometry. Conversely, NADPH was shown to undergo TCBQ-dependent oxidation as evidenced by uv spec troscopic analysis. ESR spectroscopy demonstrated that NADPH-dependent reduction of TCBQ proceeds through the intermediacy of a semiquinone radical intermediate. In addition, molecular mechanics and molecular o rbital calculations were carried out on TCBQ to obtain partial atomic charges and the results suggest that the first electron transfer from the reduced nicotinamide moiety to TCBQ occurs at a carbonyl carbon. K inetic analysis indicated that the rate of NADPH oxidation is first or der with respect to [TCBQ], second order with respect to [NADPH], and an Arrhenius plot yielded an activation energy of 26 kJ . mol(-1) for the rate limiting step. Mechanisms are proposed which are consistent w ith these results. (C) 1996 Academic Press, Inc.