Analysis of the pharmacological and molecular heterogeneity of I-2-imidazoline-binding proteins using monoamine oxidase-deficient mouse models

The I-2 subgroup of imidazoline-binding sites was identified as monoamine o xidases (MAOs), but it is unclear whether there are I-2-binding sites locat ed 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-d eficient mice. I-2-binding sites were identified using [H-3]idazoxan and th e photoaffinity adduct 2-[3-azido-4-[I-125]iodophenoxyl]methylimidazoline ( [I-125]AZIPI). [H-3]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 [H-3]idazoxan binding were not altered in MAO A kno ckout (KO) mice. In contrast, [H-3] idazoxan binding was completely abolish ed in both liver and brain of MAO B KO mice. In wild-type mice, [I-125]AZIP I photolabeled three proteins with apparent molecular masses of similar to 28 (liver), similar to 61 (brain), and similar to 55 kDa (liver and brain). The photolabeling of each protein was blocked by the imidazoline cirazolin e (10 mu M). Photolabeling of the similar to 61- and similar to 55-kDa prot eins was not observed in MAO A and B KO mice, respectively. In contrast, ph otolabeling of the liver similar to 28-kDa protein was still observed in MA O-deficient mice, indicating that this protein is unrelated to MAOs. These data indicate that I-2 imidazoline- binding sites identified by [H-3]idazox an reside solely on MAO B. The binding sites on MAO A and the liver similar to 28-kDa protein may represent additional subtypes of the family of the i midazoline-binding sites.