Kinetics of redox interaction between substituted 1,4-benzoquinones and ascorbate under aerobic conditions: critical phenomena.

Redox cycling is believed to be the most general molecular mechanism of quinone (Q) cytotoxicity. Along with redox cycling induced by a reductase, a similar process is known to occur via electron transfer from ascorbate (AscH-) to Q with formation of a semiquinone radical (Q.-): (1) Q + AscH- (k1)--> Q.- + Asc.- + H+ (2) Q.- + O2 --> Q + O2.-. The net effect of reactions (1) and (2) provides for the catalytic oxidation of AscH-, with Q serving as a catalyst. In this work, the kinetics of oxygen consumption accompanying this process were studied with several substituted 1,4-benzoquinones (BQ) at 37 degrees C in phosphate buffer, pH 7.40, using the Clark electrode technique. The value of k1 determined from the initial rate of oxygen consumption was typically found to increase when the one-electron reduction potential E(Q/Q.-) shifted to more positive values. With Q, for which E(Q/Q.-) is less than -100 mV, the rate of oxygen uptake (R(OX)) was found to be directly correlated with the [Q][AscH-] value independent of the concentration of individual reagents, remaining constant for a long period. With mono- and dialkyl-substituted 1,4-BQs, for which E(Q/Q.-) is higher than -100 mV, significant deviations from the above simple kinetic regularities were observed. In particular, R(OX) decreased dramatically with time and critical phenomena (the existence of certain concentrations of Q and/or AscH- above or below which the catalytic oxidation of AscH- ceased completely after a non-stationary period of short duration) were observed. These abnormalities can be explained on the basis of the kinetic scheme which contains, in addition to reactions (1) and (2), several side reactions including that between Q.- and AscH-. Implications of critical phenomena discovered in this study for the problems of Q toxicity and vitamin C avitaminosis are discussed.