NADPH-CYTOCHROME REDUCTASE CATALYZED REDOX CYCLING OF 1,4-BENZOQUINONE - HAMPERED AT PHYSIOLOGICAL CONDITIONS, INITIATED AT INCREASED PH VALUES

In the present study the inability of 1,4-benzoquinone to support NADP H-cytochrome reductase catalysed redox cycling was investigated. The r esults obtained demonstrate that NADPH-cytochrome reductase is able to initiate a rapid two-electron reduction of 1,4-benzoquinone resulting in formation of the hydroquinone. The intermediate one-electron reduc ed semiquinone form does not pass its electron on to molecular oxygen, i.e. giving rise to redox cycling, but is reduced by a second electro n, either by NADPH-cytochrome reductase upon protonation of the semiqu inone or through disproportionation, both giving rise to the two-elect ron reduced hydroquinone. At pH values below the pK(a) of the hydroqui none, the electrons of the hydroquinone are also not passed on to mole cular oxygen due to efficient protonation. However, at pH values aroun d or above the pK(a) (9.85) of the two-electron reduced hydroquinone f orm, significant redox cycling activity is observed in a 1,4-benzoquin one containing incubation. Further experiments demonstrate a similarit y in both the concentration and pH dependence of 1,4-benzoquinone or 1 ,4-hydroquinone supported NADPH-cytochrome reductase catalysed redox c ycling. From these observations it is concluded that 1,4-benzoquinone is able to redox cycle from its deprotonated two-electron reduced hydr oquinone form, but only at relatively high pH values. Together the dat a provide an insight into why the NADPH-cytochrome reductase catalysed redox cycling of 1,4-benzoquinone is inhibited at physiological condi tions, but initiated at increased pH values.