Comparative aspects of monoamine oxidase

Abstract

This short review attempts to summarize a small fraction of the comparative aspects of the flavoprotein enzyme, monoamine oxidase (MAO), which is found mainly in the outer mitochondrial membranes of a wide variety of animal species. It deaminates both aromatic and aliphatic monoamines in the form

to their corresponding aldehydes and hydrogen peroxide. Some of the evidence is described which shows that MAO can be divided into two catalytically different forms, called MAO-A and MAO-B. This evidence derives from the fact that the substrate specificity of MAO depends not only on the species but also on the particular tissue studied. Some inhibitors of MAO including the irreversible inhibitors, clorgyline and deprenyl show considerable selectivity towards one or other form of the enzyme. Under appropriate conditions clorgyline is about 1000 times more active against MAO-A while deprenyl is more active against MAO-B. Use of these inhibitors will measure the proportions of the two forms of activity as they deaminate a fixed concentration of a chosen substrate. Such experiments may also reveal the presence of another deaminating enzyme that is not FAD-dependent and is not inhibited by clorgyline. With appropriate radioactive ligands it is possible to measure the number of active centres of each form and thus allow the relative turnover rate constants for MAO-A and MAO-B to be calculated for each substrate. As an example, the determination of MAO-A:B ratio is illustrated for rat and giraffe liver MAO. In addition a summary is given of the type of MAO activity responsible for the deamination of 5-hydroxytryptamine, tyramine, β-phenethylamine and benzylamine in heart, liver and brain of a few animal species.