Reactivity of semiquinone radicals and its relation to the biochemical role of superoxide

Abstract

Semiquinone radicals derived from 9, 10-anthraquinone-2-sodium sulphonate (AQS), menadione (MD), duroquine (DQ) and 2,5-dimethylbenzoquinone (DMBQ) by pulse radiolysis of aqueous solutions containing propan-2-ol and quinone are shown to reduce methaemoglobin and cytochrome c at rates which greatly exceed those of the corresponding reductions by superoxide. Reasons for this are suggested. Rate constants are reported for these and related reactions, including dismutation of the semiquinones.

From studies of the γ radiolysis of similar quinone–haem solutions containing oxygen, it is shown that the equilibrium O^–₂+ Q ⇄ O₂+ Q^–(4), is maintained in these systems, and in similar ones in which superoxide is generated by enzymes, thereby converting superoxide to the more reactive radical, semiquinone. Addition of superoxide dismutase to these systems suppresses reactions caused by semiquinone, a conclusion of current biochemical significance. Measurements of the equilibrium constant K₄, and hence of redox properties of the semiquinones, have been obtained from studies of this suppression, and agree with previous measurements by other methods.