Analysis of the pharmacological and molecular heterogeneity of I(2)-imidazoline-binding proteins using monoamine...

The I(2) subgroup of imidazoline-binding sites was identified as monoamine oxidases (MAOs), but it is unclear whether there are I(2)-binding sites located on proteins distinct from MAOs. To address this issue, we characterized I(2)-binding proteins in liver and brain of wild-type and MAO A- and MAO B-deficient mice. I(2)-binding sites were identified using [(3)H]idazoxan and the photoaffinity adduct 2-[3-azido-4-[(125)I]iodophenoxyl]methylimidazoline ([(125)I]AZIPI). [(3)H]Idazoxan labeled binding sites with ligand recognition properties typical of I(2) sites in both brain and liver of wild-type mice. High-affinity, specific [(3)H]idazoxan binding were not altered in MAO A knockout (KO) mice. In contrast, [(3)H]idazoxan binding was completely abolished in both liver and brain of MAO B KO mice. In wild-type mice, [(125)I]AZIPI photolabeled three proteins with apparent molecular masses of approximately 28 (liver), approximately 61 (brain), and approximately 55 kDa (liver and brain). The photolabeling of each protein was blocked by the imidazoline cirazoline (10 microM). Photolabeling of the approximately 61- and approximately 55-kDa proteins was not observed in MAO A and B KO mice, respectively. In contrast, photolabeling of the liver approximately 28-kDa protein was still observed in MAO-deficient mice, indicating that this protein is unrelated to MAOs. These data indicate that I(2) imidazoline-binding sites identified by [(3)H]idazoxan reside solely on MAO B. The binding sites on MAO A and the liver approximately 28-kDa protein may represent additional subtypes of the family of the imidazoline-binding sites.