Enzymes for catalyzing the oxidative deamination of monoamines
Monoamine oxidases (MAOs) belong to the family of flavin-containing amine oxidoreductases and are integral proteins of outer mitochondrial membranes. Isoenzymes MAO-A and MAO-B
share roughly 70% of their structure. They occur in various cells in peripheral organs and in the CNS in both neuronal and non-neuronal cells, where they oxidatively deaminate biogenic and xenobiotic amines. They play an important role in the CNS in metabolic inactivation of released monoamine transmitters (catecholamines, serotonin and trace amines) and in detoxification of xenobiotic amines.
In rodent brain MAO-B metabolizes the trace amines phenylethylamine, methylhistamine, tyramine as well as octopamine. In human brain dopamine is also a substrate. Recent results identified MAO-B as a synthesizing enzyme of glial GABA, which is released to mediate tonic inhibition in the cerebellum and striatum. MAO-B is expressed in the CNS in serotonergic as well as in histaminergic neurons and in some astrocytes. The highest levels of MAO-B in peripheral organs are found in the liver and in the islets of Langerhans.
MAO inhibitors have provided therapies for both psychiatric as well as neurologic disorders. MAO-B has been reported to increase with aging and in several regions of the CNS in association with neurodegenerative disorders such as Parkinson's, Alzheimer's and Huntington's diseases. Plaque-associated astrocytes in Alzheimer's disease cortices have been shown to contain an increased amount of MAO-B activity.