Immunological uniqueness of human monoamine oxidases A and B: new evidence from studies with monoclonal antibodies to human monoamine oxidase A.

Monoamine oxidase (EC 1.4.3.4; MAO) is the primary enzyme responsible for the intraneuronal degradation of biogenic amines in the central nervous system. An understanding of the physiological significance of the functional and regulatory differences between the two forms of the enzyme, MAOs A and B, would be facilitated by the availability of antibodies specific for the two forms of the enzyme. We previously isolated and characterized a monoclonal antibody (MAO B-1C2, previously designated MAO-1C2) which binds human MAO B but not A. We describe here four new monoclonal antibodies (designated MAO A-3C9, A-4F10, A-7B10, and A-7E10) which were elicited to highly purified MAO A from human placenta and which, in the presence of antimouse IgG and Staphylococcus aureus, immunoprecipitate greater than 90% of the catalytically active purified MAO A. MAO A-3C9 appears to have a lower affinity for purified MAO A than the other three antibodies and does not immunoprecipitate either MAO A or MAO B from human platelets or from Triton X-100 extracts of human placental and liver mitochondria. MAO A-4F10, A-7B10, and A-7E10 immunoprecipitate catalytically active MAO A from Triton X-100 extracts of human placental and liver mitochondria, but not catalytically active MAO B from either pletelets or from Triton X-100 extracts of human liver mitochondria. Collectively, these anti-MAO monoclonal antibodies reveal unique epitopes on human MAO A not shared by MAO B, and at least one epitope on MAO B not shared by MAO A. These immunochemical differences support the hypothesis that MAO A and MAO B are different proteins, presumably isozymes.