Fully Reversible Redox Cycling of 2,6-Dimethoxy-1,4-benzoquinone Induced by Ascorbate

V. A. Roginsky^1*, G. Bruchelt², and H. B. Stegmann³

¹Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119977 Russia; E-mail: rogin@cc.nifhi.ac.ru

²Children Hospital, Tuebingen University, Ruemelinstrasse 23, 72070 Tuebingen, Germany; fax: +49 (07071) 298-4448

³Institute of Organic Chemistry, Tuebingen University, Auf der Morgenstelle 18, 72076 Tuebingen, Germany; fax: +49 (07071) 29-5014

^* To whom correspondence should be addressed.

Received August 11, 1997; Revision received October 29, 1997

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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°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^·- + H⁺ (1), followed by semiquinone (Q^·-) oxidation: Q^·- + O₂ --> Q + O₂^·- (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^·-). 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^·-] was as much as 380 ± 40 and 280 ± 30 M^-1·sec^-1, respectively. When DMOBQ was mixed with the corresponding hydroquinone, QH₂, in oxygen-free buffer, the ESR signal of Q^·- which formed due to the equilibrium Q + QH₂ <--> 2Q^·- + 2H⁺ (3) was observed. The equilibrium constant K₃ of (2.6 ± 0.4)·10^-5 and the change in the reduction potential, deltaE₃ = E(Q/Q^·-) - E(Q^·-/QH₂), of -280 mV were calculated from the steady-state concentration of Q^·- at pH 7.4 and 37°C. From combination of deltaE₃ determined in this study with E₇(Q/Q^·-) reported in the literature, a value of +190 mV was calculated for the standard second one-electron reduction potential E(Q^·-/QH₂). 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^·-/QH₂) was suggested to be the basic reason for the perfect work of DMOBQ as a redox cycling agent and its pronounced anticancer activity.

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KEY WORDS: xenobiotics, quinones, ascorbate, anticancer agents, redox cycling, one-electron reduction, catalytic oxidation

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