Superoxide generation by photomediated redox cycling of anthraquinones

Anthraquinones (AQs) comprise one important class of secondary metabolites predominantly produced by fungi and higher plants but also produced by a variety of other organisms. Humans orally ingest AQs from environmental sources as well as through direct use as nonprescription laxatives, and some AQ derivatives are used as topically applied antipsoritic agents. Some AQs are mutagenic. We present evidence that aqueous solutions of several AQs in the presence of an appropriate reducing agent and dissolved oxygen generate superoxide when they are illuminated with broad-spectrum light. Redox cycling of AQs could be responsible for some aspects of their toxicity in biological systems.