Monoamine oxidase (MAO; proteopedia link may be found here) – two enzymes, MAOA and MAOB, which catalyse the breakdown of the monoamine neurotransmitters. MAOA preferentially breaks down serotonin, melatonin, adrenaline and noradrenaline, whereas MAOB preferentially breaks down the trace amines, phenethylamine and benzylamine. Both degrade dopamine equally, as well as tyramine (which is a trace amine and sympathomimetic amine that is also found in certain foods like fermented foods like aged cheeses, certain alcoholic beverages, vegemite, marmite, etc.), although they are both expressed (i.e., found in appreciable quantities) in different tissues.
MAOA is preferentially expressed in the liver and digestive tract, whereas MAOB is preferentially expressed in the brain, although MAOA is also found in the CNS. Tyramine build-up is responsible for a fairly common reaction seen in those people that are on non-subtype-selective monoamine oxidase inhibitors (MAOIs) like phenelzine and tranylcypromine, both of which are used to treat depression (albeit rarely) when they eat tyramine-laced foods, which can be fatal if enough tyramine is ingested (usually via a hypertensive crisis). The reaction usually begins about an hour after ingestion of tyramine-laced foods. Both subtypes are expressed in the brain and liver, but only MAO-A is expressed in the intestinal wall, which means that MAO-A is responsible for much of the degradation of dietary tyramine before it makes its way to the liver for further metabolism (which occurs to all drugs/compounds that enter our bodies).