Hydrogen peroxide produced by mitochondrial monoamine oxidase catalysis: biological implications

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.