Also oxidizes hypoxanthine, some other purines and pterins, and aldehydes (i.e. possesses the activity of EC 1.2.3.1); probably acts on the hydrated derivatives of these substrates. Under some conditions the product is mainly superoxide rather than peroxide: R-H + H₂O + 2 O₂ ROH + 2 H⁺ + 2 O₂^-.

The enzyme from animal tissues can be interconverted to EC 1.1.1.204. that from liver exists in vivo mainly as the dehydrogenase form, but can be converted into the oxidase form by storage at -20C, by treatment with proteolytic enzymes or with organic solvents, or by thiol reagents such as Cu(2+), N-ethylmaleimide or 4-mercuribenzoate. The effect of thiol reagents can be reversed by thiols such as 1,4-dithioerythritol. EC 1.1.1.204 can also be converted into EC 1.1.3.22 by EC 1.8.4.7 in the presence of oxidized glutathione. The Micrococcus enzyme can use ferredoxin as acceptor.

Faz parte do grupo de molibdopterina oxidoredutases de eucariontes.