Aldehyde oxidase catalyzes nitrite reduction: An important nitric oxide producing pathway during ischemia

Abstract

Aldehyde oxidase (AO) plays an important role in drug and xenobiotic metabolism. While AO has structural similarity to bacterial nitrite reductases, it is unknown if AO catalyzes nitrite reduction to NO. Furthermore, the mechanism, magnitude, and quantitative importance of AO‐mediated nitrite reduction have not been studied. To investigate this pathway and its quantitative importance, isolated enzymatic studies were performed. The kinetics and magnitude of AO dependent NO formation was characterized. Typical reducing substrates of AO such as aldehyde and NADH reduced nitrite to NO in the presence of AO. Reduced AO was the direct electron donor to nitrite with reduction occurring at its molybdenum site. Studies of NO generation at different pH values indicated that nitrite reduction occurs via an acid‐catalyzed mechanism. Kinetics of AO catalyzed nitrite reduction under anaerobic conditions followed Michaelis‐Menten kinetics with Vmax= 8.5 nmol/s.unit, Km= 2.7 mM with the flavin site electron donor NADH, and Vmax= 13.5 nmol/s.unit, Km= 3.3 mM with the molybdenum site electron donor cinnamaldehyde. pH values, oxygen pressure, and reducing substrate levels were important regulators of AO catalyzed NO generation. It was determined that during ischemia pulmonary or myocardial AO and nitrite levels are sufficient to result in NO generation comparable to maximal production by NOS.