Monoamine oxidase (MAO) A and B isoenzymes located at the mitochondrial outer membrane, catalyze the oxidative deamination of monoamine neurotransmitters in both the brain and peripheral tissues, and produce hydrogen peroxide (H2O2) as one of the products. The role of MAO in the homeostasis of neurotransmitters critical for synaptic transmission has long been recognized.  The deduced amino acid sequence showed that MAO A and B share 70% identity in primary sequence.  Both MAO A and MAO B, the domain responsible for membrane binding is the C-terminal helix. To confirm these two clones were indeed MAO A and B, each cDNA clone was expressed in mammalian cells. Similar to the endogenous enzymes, the cDNA-expressed MAO A preferred 5-HT as a substrate and was sensitive to clorgyline inhibition whereas the cDNA-expressed MAO B preferred phenylethylamine as a substrate and was sensitive to deprenyl inhibition. Both enzymes (independent of the species of origin) are flavoenzymes containing a single covalent FAD cofactor per monomer. In the case of MAO B, the covalent attachment is via a thioether linkage to Cys397 and the 8a–methylene of the isoalloxazine ring of the flavin. In the case of MAO A, a similar thioether covalent linkage of the flavin ring to Cys406 is found.


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