Localization of I2-Imidazoline Binding Sites on Monoamine Oxidases

Abstract

Imidazoline binding sites (IBS) were proposed to be responsible for some of the pharmacological and therapeutic activities of imidazoline and related compounds and have been classified into two subtypes, I1BS and I2BS. Convergent studies attribute a role in central blood pressure regulation to the I1BS. In contrast, the function of I2BS remains unknown. In the present study, by combining biochemical and molecular biology approaches, we show that 1) microsequencing of I2BS purified from rabbit kidney mitochondria allowed the recovery of four peptide sequence stretches displaying up to 85.7% similarity with human, rat, and bovine monoamine oxidases (MAO)-A and -B; 2) I2BS and MAO displayed identical biophysical characteristics as their activities, measured by [3H]idazoxan binding and [14C]tyramine oxidation, respectively, could not be separated using various chromatographic procedures; and 3) heterologous expression of human placenta MAO-A and human liver MAO-B in yeast, inherently devoid of I2BS and MAO activities, led to the coexpression of [3H]idazoxan binding sites displaying ligand-recognition properties typical of I2BS. These results show definitely that I2BS is located on both MAO-A and -B. The fact that I2BS ligands inhibited MAO activity independently of the interaction with the catalytic region suggests that I2BS might be a previously unknown MAO regulatory site.

Stimulation of locus coeruleus neurons by non‐I1/I2‐type imidazoline receptors: an in vivo and in vitro electrophysiological study

“…Several studies have demonstrated that the I 2 -imidazoline site is a MAO regulatory site (Tesson et al, 1995;Tesson & Parini, 1991). The results obtained with microdialysis techniques suggest that 2-BFI may act as a MAO inhibitor .…”

“…This protocol for clorgyline treatment downregulates non-adrenoceptor [ 3 H]-idazoxan binding sites in the rat brain (Olmos et al, 1993). Since several authors have suggested that I 2 -imidazoline sites are MAO regulatory sites (Tesson et al, 1995;Tesson & Parini, 1991) we also evaluated in vitro the eect of clorgyline in addition to an imidazoline, on the LC.…”

Autoradiographic Comparison of [3H]-Clonidine Binding to Non-Adrenergic Sites and α2-Adrenergic Receptors in Human Brain

“…Therefore, it is uncertain if the cloned I 1 site is actually a true receptor (Piletz et al 2000a). I 2 sites are somewhat better understood, and have been physically linked to monoamine oxidases (Tesson et al 1995). In addition, there are at least two subtypes of I 2 sites that exhibit biphasic binding curves (Eglen et al 1998).…”

Imidazoline/Guanidinium Binding Domains on Monoamine Oxidases

“…125 I]AZIPI and do not inhibit [ 3 H]idazoxan binding to the heterologously expressed enzymes (9). Thus the binding of these compounds likely involves a site on the enzymes distinct from the substrate recognition site.…”

“…Partial amino acid sequencing of a purified rabbit kidney imidazoline binding protein revealed high sequence similarity to monoamine oxidases of other species whose sequences are known, and heterologous expression of human MAO-A or MAO-B indicates that both isoforms recognize the imidazoline, [ 3 H]idazoxan, a ligand commonly used to identify imidazoline binding sites in various tissues (9). However, the affinity of the enzymes for [ 3 H]idazoxan following expression in Saccharomyces cerevisiae is 10 -50-fold lower than that expected for the imidazoline binding site (9), and some members of the imidazoline binding protein family do not recognize this radioligand when evaluated in their natural environment within the cell (10).…”