Substrate specificity of monoamine oxidase in pig liver mitochondria.

In pig liver both the A and the B form of monoamine oxidase (MAO) were found to be responsible for the oxidation of 5-hydroxytryptamine (5HT), a substrate oxidised by the A form alone in most other tissues. With increasing concentrations of this substrate, the percentage of the substrate oxidised by the B form increased. The Km value of the A and the B form of MAO for 5HT was 200 microns and 2.2 mM, respectively. It is suggested that the division of the monoamines into A and B form substrates should be done on the basis of the molecular turnover numbers rather than on their activities, and that the substrate specificities of the two forms of MAO should be determined over a large range of substrate concentrations.