Characterization and quantitation of monoamine oxidases A and B in mitochondria from human placenta

Monoamine oxidases (MAOs) A and B, flavin-containing enzymes found in the outer mitochondrial membrane, oxidize many important biogenic and xenobiotic amines. The two enzymes are expressed in many tissues, with some tissues containing primarily one form and others containing both. Although MAO in placental mitochondria is widely reported to be type A, some investigators have reported low levels of MAO B activity as well. Because placenta is considered the preferred source for purification of type A MAO, we have reinvestigated placental MAO by immunoblotting with monoclonal antibodies and active site labeling with the MAO-specific ligand [3H]pargyline. We have confirmed that placental mitochondrial preparations contain MAO A and low but significant MAO B catalytic activity, as judged by accepted pharmacological criteria (deprenyl-sensitive beta-phenylethylamine and benzylamine oxidation). Immunoblotting revealed polypeptides of sizes expected for both MAO A and B subunits in preparations of placental mitochondria, as well as in preparations of MAO A purified extensively from placenta by partitioning between dextran and polyethylene glycol polymers and chromatography on DEAE-Sepharose CL-6B. Both MAO A and B active sites could be quantitated in placenta by labeling mitochondrial preparations with the MAO-specific affinity ligand [3H] pargyline, followed by immunoprecipitation with MAO A- and MAO B-specific monoclonal antibodies. These results indicate that MAO B activity and protein is consistently present in mitochondrial preparations of human placenta.