Monoamine oxidase-A (MAO-A) is a FAD-dependent mitochondrial enzyme, which, together with its isoenzyme MAO-B, catalyzes the oxidative deamination of structurally diverse amines Cesura and Pletscher (1992), Kalgutkar et al. (2001). MAO-A is mainly involved in the metabolism of monoamine neurotransmitters, such as dopamine, norepinephrine, and 5-hydroxytryptamine. Although there is no clear distinction between MAO-A and MAO-B selective substrates, evidence indicates that norepinephrine and 5-hydroxytryptamine are mainly substrates for MAO-A, with dopamine being deaminated by both isoforms, likely in a region-specific manner. The sympathomimetic amine tyramine shows similar Km values for MAO-A and MAO-B, but appears to be mainly metabolized by MAO-A in the gastrointestinal tract Hasan et al. (1988) and in sympathetic noradrenergic neurons.
Inhibitors of MAO-A are in clinical use for affective disorders. On a historical note, the serendipitous finding that the tubercolostatic agent iproniazide had antidepressant properties and inhibited MAO paved the way for modern pharmacopsychiatry. Iproniazide can be considered the first antidepressant introduced into clinical practice, Pletscher (1991).
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