Imidazoline-binding domains on monoamine oxidase B and subpopulations of enzyme

A series of phenoxy-substituted methylimidazoline derivatives were synthesi zed and used to define the ligand recognition properties of the imidazoline -binding domain (IBD) on monoamine oxidase (MAO)-B and its role in substrat e processing. The rank order of potency for selected compounds in competiti ve binding studies with the imidazoline [H-3]idazoxan was different from th at in enzyme activity assays, suggesting that the IBD and the site involved in enzyme inhibition are distinct. IC50 values for inhibition of MAO-B act ivity by imidazoline/ guanidinium ligands were one to two orders of magnitu de greater than ligand concentrations that probably saturate the IBD, but w ere equal to the Kd values of these ligands in competitive binding assays w ith the reversible MAO-B inhibitor [H-3]Ro 19-6327. In addition, the degree of enzyme inhibition by these ligands was similar in platelet and liver, t issues exhibiting 10-fold differences in the amount of the IBD-accessible e nzyme subpopulation. These data suggested that the inhibitory effect of the se compounds on MAO-B activity involved a secondary interaction with the en zyme domain recognizing the inhibitor Ro 19-6327 and does not involve inter action with the IBD. Subsequent radioligand-binding studies indicated that human liver MAO-B actually existed as two distinct populations that differe d in the accessibility of their IBD. The relatively small amounts of MAO-B possessing an accessible IBD (similar to 5% in human liver) precludes deter mination of the functional consequences of ligand binding to the IBD. This subpopulation of MAO-B may be selectively regulated or generated in differe nt individuals or tissues and targeted by pharmacologically active compound s in a cell type-specific manner.