Vol. 292, Issue 3, 1135-1145, March 2000

Imidazoline-Binding Domains on Monoamine Oxidase B and Subpopulations of Enzyme¹

Rita Raddatz, Sandra L. Savic, Venkatesalu Bakthavachalam, John Lesnick, Jeffrey R. Jasper, Christopher R. McGrath, Angelo Parini and Stephen M. Lanier

Research Biochemicals International, Natick, Massachusetts (V.B., C.R.M.); Roche Bioscience, Palo Alto, California (J.L., J.R.J.); Institut National de la Santé et de la Recherche Médicale, Pharmacologie Moleculaire et Physiopathologie Renale, Institut Louis Bugnard, Toulouse, France (A.P.); and Department of Pharmacology, Medical University of South Carolina, Charleston, South Carolina (R.R., S.L.S., S.M.L.)

A series of phenoxy-substituted methylimidazoline derivatives were synthesized and used to define the ligand recognition properties of the imidazoline-binding domain (IBD) on monoamine oxidase (MAO)-B and its role in substrate processing. The rank order of potency for selected compounds in competitive binding studies with the imidazoline [³H]idazoxan was different from that in enzyme activity assays, suggesting that the IBD and the site involved in enzyme inhibition are distinct. IC₅₀ values for inhibition of MAO-B activity by imidazoline/guanidinium ligands were one to two orders of magnitude greater than ligand concentrations that probably saturate the IBD, but were equal to the K_d values of these ligands in competitive binding assays with the reversible MAO-B inhibitor [³H]Ro 19-6327. In addition, the degree of enzyme inhibition by these ligands was similar in platelet and liver, tissues exhibiting 10-fold differences in the amount of the IBD-accessible enzyme subpopulation. These data suggested that the inhibitory effect of these compounds on MAO-B activity involved a secondary interaction with the enzyme domain recognizing the inhibitor Ro 19-6327 and does not involve interaction with the IBD. Subsequent radioligand-binding studies indicated that human liver MAO-B actually existed as two distinct populations that differed in the accessibility of their IBD. The relatively small amounts of MAO-B possessing an accessible IBD (~5% in human liver) precludes determination of the functional consequences of ligand binding to the IBD. This subpopulation of MAO-B may be selectively regulated or generated in different individuals or tissues and targeted by pharmacologically active compounds in a cell type-specific manner.