Monoamine oxidase (MAO) is an enzyme of the mitochondrial outer membrane
and catalyzes the oxidative deamination of biogenic amines throughout the
body. MAO is critical in the neuronal metabolism of catecholamine and indolamine
transmitters. Cultured skin fibroblasts show both MAO-A and MAO-B
and both MAOs differ in molecular structure. MAO-A, the primary type in fibroblasts,
preferentially degrades serotonin and norepinephrine. Only MAO-B is
present in platelets and only MAO-A is present in trophoblasts. MAO-B, the
primary type found not only in platelets but also in the brain of man and other
primates, preferentially degrades phenylethylamine and benzylamine. MAO
has been of particular interest to psychiatry and genetics because of the
suggestion that low activity is a ‘genetic marker’ for schizophrenia. The genes
which encode MAO-A and MAO-B map to human chromosome Xp11.23.