Characterization of [ [formula omitted]]idazoxan binding proteins in solubilized membranes from rabbit and human liver

Abstract

Several studies have shown that I 2 imidazoline binding sites are localized on monoamine oxidases. Recent results obtained after solubilization of rat brain membranes and analysis by size-exclusion chromatography suggested the existence of additional I 2 imidazoline binding sites located on proteins distinct from monoamine oxidases. In order to define whether such binding sites are expressed in human and rabbit liver, we solubilized I 2 imidazoline binding sites and monoamine oxidases and compared their elution profile by size-exclusion chromatography. I 2 binding sites were labeled using [ 3 H ]idazoxan. Monoamine oxidases were identified by the measure of [ 14 C ]tyramine oxidation and Western blot analysis using an anti-MAO-A/MAO-B polyclonal antiserum. After solubilization of rabbit or human liver using 1% digitonin, 90% of [ 3 H ]idazoxan binding eluted in a major peak corresponding to a M r of ∼175 000 Da. A minor peak, ( M r∼100 000 Da) representing 10% of the recovered [ 3 H ]idazoxan binding, was also observed. [ 14 C ]tyramine oxidation as well as immunoreactive bands corresponding to MAOs were exclusively detected in fractions containing [ 3 H ]idazoxan binding. These results show that solubilized I 2 imidazoline binding sites distinct from monoamine oxidases are not detectable in rabbit and human liver.