Superoxide radical from xanthine oxidase acting upon lumazine

The univalent and divalent reductions of dioxygen were measured using lumazine as a low turnover substrate and both xanthine and acetaldehyde as high turnover substrates. These measurements were made in solutions equilibrated with air and with 100% O₂. The univalent route of dioxygen reduction predominated with the low turnover substrate and was increased by raising pO₂ and by lowering substrate concentration. These results support the view that electron egress from heavily reduced xanthine oxidase occurs by divalent transfers, while that from the partially reduced enzyme is by univalent transfers. Xanthine oxidase, acting as lumazine, is a convenient source of O₂^⨪.