The activation by oxygen of the activity of monoamine oxidase (MAO) in preparations of rat liver mitochondrial membrane vesicles has been studied. The increase of activity with the substrates 5-hydroxytryptamine, tyramine, β-phenethylamine and benzylamine were in all cases uncompetitive. However, the degree of activation depended upon the particular substrate used. An attempt has been made to explain these results by use of an assumption that MAO activity can be divided into two forms unrelated to MAO-A and MAO-B. These two forms, tentatively called MAO-1 and MAO-2, differ in their Michaelis constants for oxygen.
