The biological roles of mitochondrial-produced reactive oxygen species continue to receive intensive investigation since one
of the products (H2O2) has important cellular signaling roles as well as contributing to apoptotic responses. In general, the source of mitochondrial
reactive oxygen species is thought to be the superoxide anion produced from Complex I and Complex III components of the
electron transport chain. Superoxide anion readily dismutates to H2O2 with subsequent transformation to the hydroxyl radical by Fenton
chemistry. An overlooked source of H2O2 in the mitochondrion is its production as a catalytic reaction product from the outer membrane
enzymes: monoamine oxidases A and B. The literature is reviewed to document identified degenerative reactions attributed to H2O2 produced
by MAO A and by MAO B catalysis. Available information on the topologies of these enzymes in the mitochondrial outer membrane
is also discussed with relevance to H2O2 production and involvement in cell signaling functions as well as degenerative effects.
Keywords: Monoamine oxidases A and B, amine oxidation, mitochondrial outer membrane, mitochondrial intermembrane space, membrane
topology, reactive oxygen species, hydrogen peroxide, hydroxyl radical.
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