Aldehyde Oxidase Functions as a Superoxide Generating NADH Oxidase: An Important Redox Regulated Pathway of Cellular Oxygen Radical Formation

The enzyme aldehyde oxidase (AO) is a member of the molybdenum hydroxylase family that includes xanthine oxidoreductase (XOR); however, its physiological substrates and functions remain unclear. Moreover, little is known about its role in cellular redox stress. Utilizing electron paramagnetic resonance spin trapping, we measured the role of AO in the generation of reactive oxygen species (ROS) through the oxidation of NADH and the effects of inhibitors of AO on NADH-mediated superoxide (O-2(center dot-)) generation. NADH was found to be a good substrate for AO with apparent K-m and V-max values of 29 mu M and 12 nmol min(-1) mg(-1), respectively. From O-2(center dot-) generation measurements by cytochrome c reduction the apparent K-m and V-max values of NADH for AO were 11 mu M and 15 nmol min(-1) mg(-1), respectively. With NADH oxidation by AO, >= 65% of the total electron flux led to O-2(center dot-) generation. Diphenyleneiodonium completely inhibited AO-mediated O-2(center dot-) production, confirming that this occurs at the FAD site. Inhibitors of this NADH-derived O-2(center dot-) generation were studied with amidone the most potent exerting complete inhibition at 100 mu M concentration, while 150 mu M menadione, raloxifene, or beta-estradiol led to 81%, 46%, or 26% inhibition, respectively. From the kinetic data, and the levels of AO and NADH, O-2(center dot-) production was estimated to be similar to 89 and similar to 4 nM/s in liver and heart, respectively, much higher than that estimated for XOR under similar conditions. Owing to the ubiquitous distribution of NADH, aldehydes, and other endogenous AO substrates, AO is predicted to have an important role in cellular redox stress and related disease pathogenesis.