Monoamine Oxidases A and B as Components of a Membrane Complex

Abstract

The activities of monoamine oxidase A (MAO A) and monoamine oxidase B (MAO B) represent 2 independent types of substrate binding site, as indicated by experiments with selective inhibitors and also by substrate competition. This study determines whether A and B active sites of human brain and liver MAO are located on physically separable enzyme forms or as subunits in large membrane-bound complexes. MAO was extracted from several sources by a procedure that was designed to give solubilized enzyme in high-speed supernatants, with ratios of MAO A/MAO B activities similar to those in initial crude homogenates. This solubilized enzyme gave gel filtration profiles that suggested that presence of large molecular complexes. Affinity binding experiments indicated that both MAO A and B activities may occur on the same complexes in tissues that initially contain both activities. These complexes were broken down to enzymatically active subunits by treatment with either low concentrations of sodium dodecyl sulfate, with phospholipase A2, or with a combination of both agents. MAO is part of a membrane unit in which A and B are 2 distinct enzymes embedded in a phospholipid structure. The enzymatic activity of MAO A is critically dependent on associated phospholipids, whereas that of MAO B is not.