Superoxide is responsible for the vanadate stimulation of NAD(P)H oxidation by biological membranes.

Vandate augments the oxidation of NAD(P)H, but not of NMNH, by rat liver microsomes. Paraquat increases the vanadate effect on NADPH, but not on NADH, oxidation. Substoichiometric levels of NADPH caused the co-oxidation of NADH or NMNH and SOD inhibited in all cases. The ratio of NADH or NMNH co-oxidized per NADPH added allowed estimation of average chain length, which increased as the pH was lowered from 8.0 to 7.1. The initial rate of this co-oxidation of NMNH was a saturating function of the concentration of microsomes, reflecting a decrease in chain length with an increase in number of concomitant reaction chains, and due to increasing radical-radical termination reactions. Mitochondrial outer membranes behaved like the microsomal membranes, but mitochondrial inner membranes catalyzed a rapid oxidation of NADH which could be augmented by vanadate, whose action was enhanced by paraquat and inhibited by antimycin or rotenone. These and related observations support the view that vanadate stimulates NAD(P)H oxidation by biological membranes, not by virtue of interacting with enzymes, but rather by interacting with O-2.