Human monoamine oxidases (MAOs) are flavin-containing enzymes that are present on the outer mitochondrial membrane and act on primary, secondary and tertiary amines. In contrast to the P450s which have a large number of isozymes, MAOs number only two isozymes, MAO-A and MAO-B. These gene products share over 70% sequence identity, are approximately 59KDa in size and have overlapping substrates (for example dopamine, tryamine and tryptamine) but each form also has distinct substrate specificities. MAO-A (primary type in fibroblasts) preferentially oxidises serotonin (5-Hydroxytryptamine) whereas MAO-B (primary type in platelets) prefers phenylethylamine. MAOs are of particular clinical interest because of the use of MAO inhibitors (MAOI) as antidepressants or in the treatment of neurodegenerative diseases Benedetti 2001, Beedham 1997).