PGH synthase and lipoxygenase generate superoxide in the presence of NADH or NADPH.

Purified PGH synthase when acting on arachidonic acid in the presence of reduced nicotinamide-adenine dinucleotide or reduced nicotinamide-adenine dinucleotide 3'-phosphate generated superoxide in burst-like fashion. In eight experiments using different batches of enzyme, the mean +/- SE rate of superoxide generation from 100 U of enzyme measured as the superoxide dismutase-inhibitable reduction of cytochrome c was 5.06 +/- 0.19 nmol/min in the first minute and 0.35 +/- 0.03 nmol/min subsequently. Optimum rates of superoxide were seen at concentrations of reduced nicotinamide-adenine dinucleotide in excess of 80 microM and reduced nicotinamide-adenine dinucleotide 3'-phosphate in excess of 100 microM. Using prostaglandin G2 or linoleic acid as substrate rather than arachidonate also resulted in superoxide generation. When prostaglandin H2 was used as substrate, no superoxide was generated. The rate of superoxide generation was markedly inhibited by cyclooxygenase inhibitors. Superoxide generation was also observed during the action of lipoxygenase on either linoleic or arachidonic acid in the presence of reduced nicotinamide-adenine dinucleotide or reduced nicotinamide-adenine dinucleotide 3'-phosphate but not in their absence. Indomethacin had no effect on superoxide generation from lipoxygenase. We conclude that PGH synthase and lipoxygenase produce superoxide via a side-chain reaction dependent on the presence of suitable reducing cosubstrate. This mechanism is analogous to that described for peroxidases in general.