Localization of the imidazoline binding domain on monoamine oxidase B

Monoamine oxidase B (MAO-B) was recently identified as a member of the family of imidazoline binding proteins. To localize the imidazoline binding domain on MAO-B, we labeled the domain with the imidazoline photoaffinity adduct [125I]2-(3-azido-4-iodophenoxy)methylimidazoline in rat and human liver and visualized labeled peptides by autoradiography/sodium dodecyl sulfate-polyacrylamide gel electrophoresis after CNBr cleavage of the labeled protein. Based on species-specific fragmentation patterns and immunoprecipitation of labeled peptides, the imidazoline binding domain was localized to residues K149 to M222 of human MAO-B. The imidazoline binding domain is encompassed within a region that influences substrate processing but is distinct from primary sites of interaction for the enzyme inhibitors pargyline and lazabemide (Ro 19-6327). Radioligand binding assays and photoaffinity labeling also indicated that the various classes of compounds did not cross-compete at the different enzyme domains. Identification of an imidazoline binding domain on MAO-B provides a new opportunity for the potential pharmacological development of imidazoline/guanidinium compounds and also presents additional avenues for structure/function analysis of the monoamine oxidase enzymes.