Radical thoughts about the life of MAO

Monoamine oxidase (MAO)—a flavoenzyme that oxidatively deaminates a variety of biogenic and xenobiotic amines—plays an important role in the regulation of the intracellular concentrations of these monoamine neurotransmitters. Two different isozymes of MAO, known as MAO A and MAO B, have been identified; MAO A selectively oxidizes norepinephrine and serotonin and MAO B oxidizes phenylethylamine and benzylamine. Because of the importance of MAO in the treatment of neurological diseases, this chapter elucidates its catalytic mechanism so that this information can be used in the design of new classes of inactivators for this enzyme. Mechanism-based enzyme inactivators are unreactive compounds that bear a structural similarity to the substrate or product for a target enzyme. Once inside the active site, they are converted by the normal catalytic mechanism into species that inactivate the enzyme. These inactivators are important for the study of enzyme mechanisms because they are substrates for the target enzyme that are converted via the normal catalytic mechanism into products that inactivate the enzyme. From the inactivation mechanism, then, information about the catalytic mechanism can be gleaned. Much of the discussion in the chapter is based on this principle.