Fully reversible redox cycling of 2,6-dimethoxy-1,4-benzoquinone induced by ascorbate

The kinetics of cyclic redox transformation of 2,6-dimethoxy-1,4-benzoquinone (DMOBQ)-the well-known effective anticancer agent-induced by ascorbate (AscH(-)) were studied in phosphate buffer, pH 7.40, at 37 degrees C using the Clark electrode and ESR techniques. The process is due to the electron transfer from AscH(-) to quinone (Q): Q + AscH(-) --> Q: + Asc(radical anion) + H+ (1), followed by semiquinone (Q(.)) oxidation: Q + O-2 --> Q + O-2(radical anion) (2). DMOBQ, taken even at submicromolar concentrations, effectively catalyzed AscH(-) oxidation that manifested itself by intensive oxygen consumption and an increase in the steady-state concentration of the ascorbyl radical (Asc(radical anion)). The rate of oxygen consumption, R-OX, was kept almost constant for a long time. R-OX was found to be proportional to the [Q][AscH(-)] product and not dependent on the concentrations of the individual reagents. The rate constant for reaction (1) determined from R-OX and [Asc(radical anion)] was as much as 380 +/- 40 and 280 +/- 30 M-1 . sec(-1), respectively. When DMOBQ was mixed with the corresponding hydroquinone, QH(2), in oxygen-free buffer, the ESR signal of Q(.) which formed due to the equilibrium Q + QH(2) <-> 2Q(.) + 2H(+) (3) was observed. The equilibrium constant K-3 of (2.6 +/- 0.4) 10(-5) and the change in the reduction potential, Delta E-3 E(Q/Q:) -E(Q:/QH(2)), of -280 mV were calculated from the steady-state concentration of Q: at pH 7.4 and 37 degrees C. From combination of Delta E-3 determined in this study with E-7(Q/Q:) reported in the literature, a value of +190 mV was calculated for the standard second one-electron reduction potential E(Q:/QH(2)). The latter is lower by 270-230 mV than that for all the studied 1,4-hydroquinones, The very beneficial combination of E(Q/Q:) and E(Q:/QH(2)) was suggested to be the basic reason for the perfect work of DMOBQ as a redox cycling agent and its pronounced anticancer activity.