Abstract

The I₂ subgroup of imidazoline-binding sites was identified as monoamine oxidases (MAOs), but it is unclear whether there are I₂-binding sites located on proteins distinct from MAOs. To address this issue, we characterized I₂-binding proteins in liver and brain of wild-type and MAO A- and MAO B-deficient mice. I₂-binding sites were identified using [³H]idazoxan and the photoaffinity adduct 2-[3-azido-4-[¹²⁵I]iodophenoxyl]methylimidazoline ([¹²⁵I]AZIPI). [³H]Idazoxan labeled binding sites with ligand recognition properties typical of I₂sites in both brain and liver of wild-type mice. High-affinity, specific [³H]idazoxan binding were not altered in MAO A knockout (KO) mice. In contrast, [³H]idazoxan binding was completely abolished in both liver and brain of MAO B KO mice. In wild-type mice, [¹²⁵I]AZIPI photolabeled three proteins with apparent molecular masses of ∼28 (liver), ∼61 (brain), and ∼55 kDa (liver and brain). The photolabeling of each protein was blocked by the imidazoline cirazoline (10 μM). Photolabeling of the ∼61- and ∼55-kDa proteins was not observed in MAO A and B KO mice, respectively. In contrast, photolabeling of the liver ∼28-kDa protein was still observed in MAO-deficient mice, indicating that this protein is unrelated to MAOs. These data indicate that I₂ imidazoline-binding sites identified by [³H]idazoxan reside solely on MAO B. The binding sites on MAO A and the liver ∼28-kDa protein may represent additional subtypes of the family of the imidazoline-binding sites.