NADPH-cytochrome reductase catalysed redox cycling of 1,4-benzoquinone; hampered at physiological conditions, initiated at increased pH values

Abstract

In the present study the inability of 1,4-benzoquinone to support NADPH-cytochrome reductase catalysed redox cycling was investigated. The results 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 reduced semiquinone form does not pass its electron on to molecular oxygen, i.e. giving rise to redox cycling, but is reduced by a second electron, either by NADPH-cytochrome reductase upon protonation of the semiquinone or through disproportionation, both giving rise to the two-electron reduced hydroquinone. At pH values below the p K a of the hydroquinone, the electrons of the hydroquinone are also not passed on to molecular oxygen due to efficient protonation. However, at pH values around or above the p K a (9.85) of the two-electron reduced hydroquinone form, significant redox cycling activity is observed in a 1,4-benzoquinone containing incubation. Further experiments demonstrate a similarity in both the concentration and pH dependence of 1,4-benzoquinone or 1,4-hydroquinone supported NADPH-cytochrome reductase catalysed redox cycling. From these observations it is concluded that 1,4-benzoquinone is able to redox cycle from its deprotonated two-electron reduced hydroquinone form, but only at relatively high pH values. Together the data provide an insight into why the NADPH-cytochrome reductase catalysed redox cycling of 1,4-benzoquinone is inhibited at physiological conditions, but initiated at increased pH values.