Abstract

Highly purified monoamine oxidase from beef and rat liver has been compared with regard to the presence of A and B type enzymes. The beef enzyme oxidizes B and A-B type substrates rapidly and A type substrates slowly in both membrane ahd soluble preparations. Deprenyl, a selective inhibitor of the B type enzyme, inhibits it at very low concentrations and the I₅₀ for inhibition is about the same with A, B, and A-B type substrates. Clorgyline, a selective inhibitor of the A type enzyme, inhibits only at high concentrations, and the I₅₀ value is again independent of the substrate used. This shows that only the B type enzyme occurs in beef liver. Rat liver, in contrast, contains both A and B type of monoamine oxidase. Deprenyl inhibits the oxidation of B substrates at low concentrations, and clorgyline inhibits oxidation of A substrates at low concentrations, while biphasic inhibition is seen with A-B substrates upon titration with either inhibitor. Deprenyl and pargyline are stoichiometrically bound to the beef and rat liver enzymes, but clorgyline is bound to the beef enzyme in amounts in considerable stoichiometric excess indicating nonspecific binding. The enzyme from both sources binds these acetylenic suicide inhibitors at N-5 of the covalently bound flavin in a flavocyanine linkage. No conversion of the A to the B type of enzyme on extraction of the enzyme from the membrane and removal of lipids has been noted.