Monoamine oxidase (MAO) is an enzyme responsible for the breakdown of monoamines, which are neurotransmitters such as serotonin, dopamine, and norepinephrine, playing a crucial role in regulating mood and emotional behaviors. Inhibitors of MAO are used as treatments for various psychiatric and neurological disorders, including depression and Parkinson's disease, by preventing the breakdown of these neurotransmitters and thereby increasing their levels in the brain.