Monoamine oxidase

Monoamine oxidases (MAO) () are a family of enzymes that catalyze the oxidation of monoamines, employing oxygen to clip off their amine group. They are found bound to the outer membrane of mitochondria in most cell types of the body. The first such enzyme was discovered in 1928 by Mary Bernheim in the liver and was named tyramine oxidase. The MAOs belong to the protein family of flavin-containing amine oxidoreductases. MAOs are important in the breakdown of monoamines ingested in food, and also serve to inactivate monoamine neurotransmitters. Because of the latter, they are involved in a number of psychiatric and neurological diseases, some of which can be treated with monoamine oxidase inhibitors (MAOIs) which block the action of MAOs. In humans there are two types of MAO: MAO-A and MAO-B. Both are found in neurons and astroglia. Outside the central nervous system: MAO-A is also found in the liver, pulmonary vascular endothelium, gastrointestinal tract, and placenta. MAO-B is mostly found in blood platelets. MAO-A appears at roughly 80% of adulthood levels at birth, increasing very slightly after the first 4 years of life, while MAO-B is almost non-detectable in the infant brain. Regional distribution of the monoamine oxidases is characterized by extremely high levels of both MAOs in the hypothalamus and hippocampal uncus, as well as a large amount of MAO-B with very little MAO-A in the striatum and globus pallidus. The cortex has relatively high levels of only MAO-A, with the exception of areas of the cingulate cortex, which contains a balance of both. Autopsied brains demonstrated the predicted increased concentration of MAO-A in regions dense in serotonergic neurotransmission, however MAO-B only correlated with norepinephrine. Other studies, in which the activities of MAO (not protein amounts) were examined in rat brain, revealed the highest MAO-B activity in the median eminence of hypothalamus. Dorsal raphe nucleus and medial preoptic area have relatively high MAO-B activity, but much lower than MAO-B activity in the median eminence.