Vol. 58, Issue 5, 1085-1090, November 2000

Analysis of the Pharmacological and Molecular Heterogeneity of I₂-Imidazoline-Binding Proteins using Monoamine Oxidase-Deficient Mouse Models

Anne Remaury, Rita Raddatz, Catherine Ordener, Sandra Savic, Jean C. Shih, Kevin Chen, Isabelle Seif, Edward De Maeyer, Stephen M. Lanier, and Angelo Parini

Institut National de la Sante et de la Recherche Medicale, Pharmacologie Moléculaire et Physiopathologie Rénale, Institut Louis Bugnard, Centre Hospitalier Universitaire Rangeuil, Toulouse, France (A.R., C.O., A.P.); Department of Pharmacology, Medical University of South Carolina, Charleston, South Carolina (R.R., S.S., S.M.L.); Department of Molecular Pharmacology and Toxicology, School of Pharmacy, and Department of Cell and Neurobiology, School of Medicine, University of Southern California, Los Angeles, California (J.C.S., K.C.); and Centre National de la Recherche Scientifique, Unite Mixte de Recherche, Institut Curie, Orsay, France (I.S., E.D.M.)

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 I₂ sites 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.