Kinetics of redox interaction between substituted quinones and ascorbate under aerobic conditions

One-electron reduction of quinones (Q) by ascorbate (AscH(-)); (1) AscH(-) + Q --> Q(.-) + Asc(.-) + H+, followed by the oxidation of semiquinone (Q(.-)) by molecular oxygen, (2) Q(.-) + O-2 --> Q + O-2(.-), results in the catalytic oxidation of ascorbate (with Q as a catalyst) and formation of active forms of oxygen. Along with enzymatic redox cycling of Q, this process may be related to Q cytotoxicity and underlie an antitumor activity of some Qs. In this work, the kinetics of oxygen consumption accompanied the interaction of ascorbate with 55 Qs including substituted 1,4- and 1,2-benzoquinones, naphthoquinones and other quinoid compounds were studied in 50 mM sodium phosphate buffer, pH 7.40, at 37 degrees C by using the Clark electrode technique. The capability of Q to catalyze ascorbate oxidation was characterized by the effective value of k(EFF) calculated from the initial rate of oxygen consumption (R-OX) by the equation R-OX = k(EFF)[Q][AscH(-)] as well as by a temporary change in R-OX. The correlation of k(EFF) with one-electron reduction potential. E(Q/Q(.-)), showed a sigma-like plot, the same for different kinds of Qs. Only the Qs which reduction potential E(Q/Q(.-)) ranged from nearly -250 to +50 mV displayed a pronounced catalytic activity, k(EFF) increased with shifting E(Q/Q(.-)) to positive values. The following linear correlation between k(EFF) (in M-1 s(-1)) and E(Q/Q(.-)) (in mV) might be suggested for these Qs: lg(k(EFF)) = 3.91 + 0.0143E(Q/Q(.-)). In contrast, Qs with E(Q/Q(.-)) < -250 mV and E(Q/Q(.-)) > +50 mV showed no measurable catalytic activity. The Qs studied displayed a wide variety in the kinetic regularities of oxygen consumption. When E(Q/Q(.-)) was more negative than -100 mV. Q displayed a simple ('standard') kinetic behavior-R-OX was proportional to [AscH(-)][Q] independently of concentration of individual reagents, [AscH(-)] and [Q]; R-OX did not decrease with time if[AscH-] was held constant; Q recycling was almost reversible. Meanwhile, Qs with E(Q/Q(.-)) > -100 mV demonstrated a dramatic deviation from the 'standard' behavior that was manifested by the fast decrease in R-OX with time and non-linear dependence of even starting values of R-OX on [Q] and [AscH(-)]. These deviations were caused basically by the participation of Q(.-) in side reactions different from (2). The above findings were confirmed by kinetic computer simulations. Some biological implications of Q-AscH(-) interaction were discussed. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.