MOLECULAR CHARACTERIZATION AND ISOLATION OF A 45-KILODALTON IMIDAZOLINE RECEPTOR PROTEIN FROM THE RAT-BRAIN

Imidazoli(di)nes bind to molecular entities different from alpha(2)-adrenoceptors: the so-called imidazoline receptors (IRs). Two main types of IRs have been described, the clonidine- and the idazoxan-preferring types, as well as other IRs whose pharmacological properties do not fit either type, but little is known about the molecular features of these receptors. In this study, IR proteins have been solubilized from the rat brain, using the zwitterionic detergent CHAPS, and analyzed by pharmacological and immunological means two of the four peaks discriminated by gel filtration chromatography using [H-3]idazoxan binding and a specific antibody. The IR eluted in the first peak accounted for 80% of the specific binding of [H-3]idazoxan to solubilized brain membranes, and its pharmacological features corresponded to the non-adrenoceptor component of [H-3]idazoxan binding in rat brain native membranes. The elution volume of this peak corresponded to a 130-140-kDa protein, but immunoblot analysis with a specific anti-IR antiserum showed the presence of a similar to 45-kDa IR protein, suggesting that this receptor is either an oligomeric protein complex or that it is associated with other proteins. This result was in agreement with the isolation and immunodetection of a 45-kDa peptide by affinity chromatography, which supported the relationship between this protein and a rat brain imidazoline binding site. The second peak, accounting for 15% of the specific binding of [H-3]idazoxan to solubilized membranes, had a Mr of similar to 65-70,000, as determined by gel filtration chromatography and immunoblotting. The rank order of potency of drugs against [H-3]idazoxan, but not [H-3]clonidine, binding (cirazoline > guanabenz > amiloride >> efaroxan > agmatine) indicated that both entities are most probably idazoxan-preferring IRs.