Monoamine oxidase: radiotracer development and human studies.

Monoamine oxidase (MAO) is an integral protein of outer mitochondrial membranes and occurs in neuronal and nonneuronal cells in the brain and in peripheral organs. It oxidizes amines from both endogenous and exogenous sources, thereby influencing the concentration of neurotransmitter amines as well as many xenobiotics. It occurs in two subtypes, MAO A and MAO B, which are different gene products and have different substrate and inhibitor specificities. Both MAO A and B can be imaged and quantified in the living human brain using positron emission tomography (PET) and radiotracers labeled with carbon-11. PET studies have been carried out to measure the effects of age, MAO inhibitor drugs, tobacco smoke exposure, and other factors on MAO activity in the human brain.