Molecular weight of mitochondrial type B MAO in various organs of guinea pig

Molecular weights of mitochondrial type B monoamine oxidase (MAO) in guinea pig brain, liver and kidney were estimated, and their identities and multiplicity were studied. We ascertained what concentration of 3H-pargyline bound to type B MAO specifically from the inhibition curve toward serotonin (5-HT) and β-phenylethylamine (β-PEA) by pargyline. Pargyline irreversibly binds to FAD in MAO at a one to one molecular ratio. 3H-pargyline bound to type B MAO specifically and irreversibly by incubation for 5 hr at 37°C, and SDS-disc electrophoresis was carried out using 3H-pargyline as a tracer. The molecular weight of MAO was estimated after specific binding of pargyline was corrected for non-specific binding. The molecular weight of type B MAO in every organ was found to be 60, 000, giving a single peak after solubilization with 6% SDS, but several peaks at higher molecular weight were found in each organ after solubilization with 2% SDS. In the brain, there appeared to be a peak of 100, 000, and it was suggested that the MAO existed as a dimer which was composed of a FAD containing subunit and a low molecular weight subunit containing no FAD. In the liver, there appeared to be peaks of 120, 000 and 240, 000, and it was suggested that the MAO existed as a dimer and tetramer. In the kidney, there appeared to be a peak of 180, 000, and MAO was suggested to exist as a trimer.