An enzyme that is closely
related to aldehyde oxidase (EC 1.2.3.1). Both are metalloflavoproteins of about 300,000 daltons. They consist of two subunits of equal size and contain molybdenum, FAD, and iron
(as Fe/S) in a ratio of 1:1:4 per subunit. These enzymes are
widely distributed and catalyze a reaction in which the substrate is hydroxylated by an oxygen atom derived from water
and electrons from the substrate are transferred to a variety of
acceptors. These two enzymes have a broad overlapping substrate specificity including many purines, pyrimidines, and
pteridines. However, xanthine, the best known substrate for
xanthine oxidase, is not a substrate for aldehyde oxidase
whereas the reverse is true for quaternary pyridinium compounds, such as N0
-methylnicotinamide. The role of oxygen as
an electron acceptor with its production of hydrogen peroxide
and the intermediate superoxide anion (both potential toxicants) may not be important in vivo since there is evidence
that these enzymes are NAD-dependent dehydrogenases
in vivo and become oxidases as a result of modification during
purification. However, they are probably important in two
types of detoxication. The first of these is the hydroxylation of
exogenous aldehydes, purines, pyrimidines and other heterocyclic compounds, and the second involves the utilization of
exogenous compounds as electron acceptors. Examples of the
latter include the conversion of organic nitro compounds to
hydroxyamino derivatives and the reduction of N-oxides to the
free base.
