3 Molybdenum Hydroxylases: Biological Distribution and Substrate-Inhibitor Specificity

Abstract

This chapter discusses the biological distribution and substrate-inhibitor specificity of molybdenum hydroxylases. Aldehyde oxidase and xanthine oxidase are molybdenum-containing enzymes that catalyze the oxidation of many aldehydes and nitrogen heterocycles. They have also been shown to catalyze, under in vitro conditions, a number of reductions. Although aldehyde oxidase is known to oxidize physiological compounds, such as N1-methylnicotinamide and pyridoxal, there does not yet appear to be a specific endogenous role for this enzyme. Xanthine oxidase is involved in endogenous purine catabolism, catalyzing the oxidation of hypoxanthine and xanthine to uric acid. This enzyme also has wide substrate specificity, and different functions have been assigned to it. One of the most plausible proposals is that both molybdenum hydroxylases may provide a biochemical barrier against ingested purines and pyrimidines, rendering them harmless. This is similar to the view put forward in a recent review on the role of the molybdenum hydroxylases as drug-metabolizing enzymes, except that the substrate specificity is not restricted to purines and pyrimidines.