Effect of the microsomal system on interconversions between hydroquinone, benzoquinone, oxygen activation, and lipid peroxidation

Our previous results indicated that cytochrome P450 destruction by benzene metabolites was caused mainly by benzoquinone (Soucek et al., Biochem. Phar macol. 47 (1994) 2233-2242). The aim of this study was to investigate the i nterconversions between hydroquinone, semiquinone, and benzoquinone with re gard to both spontaneous and enzymatic processes in order to test the above hypothesis. We have also studied the participation of hydroquinone and ben zoquinone in OH radicals formation and lipid peroxidation as well as the ro le of ascorbate and transition metals. In buffered aqueous solution, hydroq uinone was slowly oxidized to benzoquinone via a semiquinone radical. This conversion was slowed down by the addition of NADPH and completely stopped by microsomes in the presence of NADPH. Benzoquinone was reduced to semiqui none radical at a significantly higher rate and this conversion was stimula ted by NADPH and more effectively by microsomes plus NADPH while semiquinon e radical was quenched there. In microsomes with NADPH, both hydroquinone a nd benzoquinone stimulated the formation of OH radicals but inhibited perox idation of lipids. Ascorbate at 0.5-5 mM concentration also produced signif icant generation of OH radicals in microsomes. Neither hydroquinone nor ben zoquinone did change this ascorbate effect. In the contrary, 0.1 -1.0 mM as corbate stimulated peroxidation of lipids in microsomes whereas presence of hydroquinone or benzoquinone completely inhibited this deleterious effect of ascorbate. Iron-Fe-2' apparently played an important role in lipid perox idation as shown by EDTA inhibition, but it did not influence OH radical pr oduction. In contrast, Fe3+ did not influence lipid peroxidation. but stimu lated OH radical production. Thus, our results indicate that iron influence d the above processes depending on its oxidation state, but it did not infl uence hydroquinone/benzoquinone redox processes including the formation of semiquinone. It can be concluded that interconversions between hydroquinone and benzoquinone are influenced by NADPH and more effectively by the compl ete microsomal system. Ascorbate, well-known antioxidant produces OH radica ls and peroxidation of lipids. On the other hand, both hydroquinone and ben zoquinone appear to be very efficient inhibitors of lipid peroxidation. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.