On the formation and nature of the imidazoline I-2 binding site on human monoamine oxidase-B

An allosteric binding site with high affinity for imidazoline I-2 ligands has been proposed to exist on monoamine oxidase-B (MAO-B) However enzyme inhibition only occurs at ligand concentrations far higher than are required to saturate this site We here confirm previous reports that inactivation of recombinant human MAO-B with tranylcypromine results in the formation of a high affinity I-2 site on the enzyme measured as an increase in binding of [H-3]2-BFI Incubation of MAO-B with 2-phenylethylamine an endogenous trace amine and MAO-B substrate resulted in a progressive loss of enzyme activity increased enzyme mass distinct spectral changes and as was observed with tranylcypromine a parallel Increase in high affinity binding of [H-3]2-BFI Kinetic studies of the mechanism by which 2-BFI inhibits MAO-B activity suggested binding of 2-BFI at micromolar concentrations to a site distinct from the active site on at least two forms of the pure enzyme probably corresponding to oxidised and reduced enzyme states Studies with mutant enzymes revealed a pattern of changes consistent with binding of 2-BFI to the substrate entrance channel of human MAO-B Structural data confirm that high affinity binding of I-2 ligands occurs within the entrance channel of inactive enzyme while lower affinity binding at the same location in catalytically active enzyme results in mixed inhibition of MAO-B activity High affinity I-2 sites may form in vivo due to inactivation of a portion of MAO-B during amine oxidation while the low affinity I-2 site on active enzyme is a target for novel MAO-B inhibitor drugs (C) 2010 Elsevier Ltd All rights reserved