Predominant expression of monoamine oxidase B isoform in rabbit renal proximal tubule: regulation by I2 imidazoline ligands in intact cells.

Previous studies have shown that a subpopulation of the catecholamine-degrading enzymes monoamine oxidase (MAO) A and B holds a previously unknown regulatory site, the I2-imidazoline binding site (I2BS). In the present work, we characterized the isoforms of monoamine oxidases expressed in the rabbit renal proximal tubule, defined their relationship with I2BS, and investigated the ability of I2BS ligands to inhibit enzyme activity in intact cells. Two findings indicate that MAO-B is the predominant isoform expressed in the renal proximal tubule cells: 1) Western blot performed with an anti-MAO-A/MAO-B polyclonal antiserum revealed a single 55-kDa band corresponding to MAO-B; 2) enzyme assays showed an elevated MAO-B activity ([14C]beta-phenylethylamine oxidation: Vmax = 1.31 +/- 0.41 nmol/min/mg protein), whereas MAO-A activity was only detectable ([14C]5-HT oxidation: Vmax = 80.3 +/- 19 pmol/min/mg protein). Photoaffinity labeling with the I2BS ligand [125I]2-(3-azido-4-iodophenoxy)-methylimidazoline revealed a single 55-kDa band, which indicates that MAO-B of the renal proximal tubule cells holds the I2 imidazoline binding site. [3H]Idazoxan binding studies and enzyme assays showed that, in intact cells, I2BS ligands bind to and inhibit MAO-B. Indeed, the increase in the accessibility of intracellular compartment by cell permeabilization did not enhance [3H]idazoxan binding, which indicates that, in intact cells, intracellular I2BS are fully occupied by imidazoline ligands. In addition, enzyme assays showed that incubation of proximal tubule cells with imidazoline ligands leads to a complete, dose-dependent inhibition of MAO activity. These data show the predominant expression of MAO-B in rabbit renal proximal tubule and its regulation by imidazoline ligands in intact cells.

I2-imidazoline binding sites and monoamine oxidase activity in human postmortem brain from patients with Parkinson's disease.

I2-imidazoline binding site (I2BS) has been identified with a regulatory site located on a sub-population of monoamine oxidase (MAO)-A and -B. Previous studies showed a modification of MAO and I2BS in the elderly and in neurodegenerative processes such as Alzheimer's disease. In the present study, we studied the potential modification of I2 binding sites and monoamine oxidases in Parkinson's disease. Putamen and cerebral cortex were collected from 17 normal subjects (79 +/- 12 yr) and 16 patients (76 +/- 9 yr) affected by Parkinson's disease. In mitochondrial preparations, radioligand binding studies with [3H]idazoxan showed that putamen and frontal cortex express equivalent amount of I2BS. The density and affinity of I2BS were similar in normal subjects (putamen: Bmax = 207 +/- 58 fmol/mg of protein, Kd = 10.1 +/- 3.4 nM; cerebral cortex: Bmax = 193 +/- 54 fmol/mg of protein, Kd = 12.8 +/- 6.8 nM) and Parkinson's disease patients (putamen: Bmax = 193 +/- 60 fmol/mg of protein, Kd = 9.8 +/- 4.6 nM; cerebral cortex: Bmax = 199 +/- 49 fmol/mg of protein, Kd = 15.9 +/- 8.1 nM). The activity of total monoamine oxidase and monoamine oxidase B, measured by [14C]tyramine and [14C]phenylethylamine oxidation, respectively, were higher in putamen than in cerebral cortex. No differences have been detected in the enzyme activity between normal and pathological subjects. These data suggest that, although MAO and I2BS may play a role in the development of Parkinson's disease, they are not altered in the chronic phase of this disease.

I2-imidazoline binding sites: relationship with different monoamine oxidase domains and identification of histidine residues mediating ligand binding regulation by H+1.

We have shown that I2-imidazoline binding sites (I2BSs) are located on both monoamine oxidases A (MAO-A) and B (MAO-B) and are selectively regulated by H+ and K+ in vitro. In the present study we used chemical modifying agents to investigate the localization of I2BSs with respect to different MAO domains and the mechanisms of ligand binding regulation by K+ and H+. In mitochondrial or solubilized preparations from rabbit kidney and liver, modification of cysteine residues, which are critical for MAO activity, did not affect [3H]idazoxan binding, indicating that I2BS is not associated to the cysteine-containing flavin adenine dinucleotide (FAD) prosthetic group or to the catalytic site of MAOs. Among various chemical modifying agents, only diethylpyrocarbonate and 4-bromophenacyl bromide, two histidine modifying agents, inhibited [3H]idazoxan binding to I2BS. The pH profile of diethylpyrocarbonate effect was consistent with the specific modification of histidine residues. In protection experiments, the effect of diethylpyrocarbonate was not prevented in the presence of saturating concentrations of amiloride, guanabenz or KCl, suggesting that these residues are not located within the ligand or K+ binding sites. In contrast, histidine residues appear to be within a MAO domain involved in regulation of [3H]idazoxan binding by H+. Indeed, the pH-dependent increase in [3H]idazoxan binding was fully abolished after treatment of solubilized material with diethylpyrocarbonate. In conclusion, our results show that MAO I2BSs are not located within the flavin adenine dinucleotide prosthetic group or the catalytic site. Histidine(s) residue(s) involved in the regulation of ligand binding to I2BS by H+ also has been identified.

Selectivity of rilmenidine for I1-imidazoline-binding sites in rabbit proximal tubule cells.

Imidazoline and imidazoline-like compounds may elicit their pharmacological activities through the interaction with three membrane proteins: alpha 2-adrenergic receptors (alpha 2-AR), I1-binding sites (I1BS), and I2-binding sites (I2BS). We have recently shown that these three proteins are co-expressed in the renal proximal tubule cells, where they could mediate the renal effects of imidazoline and structurally related antihypertensive drugs such as clonidine and rilmenidine. To identify the receptor involved in regulation of the tubular effects of clonidine and rilmenidine, we performed binding studies on isolated cells from rabbit kidney proximal tubule using [3H]idazoxan, an I1/I2 ligand, and [3H]rauwolscine, a selective alpha 2-adrenergic antagonist. Competition studies of [3H]idazoxan binding showed that rilmenidine and clonidine interact with both I1BS and I2BS. The comparison of inhibition constants (rilmenidine: Ki for I1BS. 7.1 +/- 3.5 nM; Ki for I2BS, 5,189 +/- 1,816 nM; clonidine: Ki for I1BS, 58.2 +/- 17.3 nM; Ki for I2BS, 4,179 +/- 2,633 nM) demonstrated that rilmenidine and clonidine are 731-and 72-fold, respectively, more selective for I1BS than for I2BS. In addition, in competition experiments with [3H]-rauwolscine, rilmenidine poorly interacted with alpha 2-AR (Ki 2,440 +/- 322 nM), whereas clonidine displayed an affinity (Ki, 32 +/- 12 nM) close to that observed for I1BS. Taken together, these data show that although clonidine is not able to discriminate alpha 2-AR from I1BS, rilmenidine selectively binds only to I1BS. This suggests that the renal effects of rilmenidine are related to its selective interaction with this class of binding site.

[Imidazoline-guanidine site: a subtype of imidazoline receptors]. / Le site imidazolinique-guanidinique: un sous-type de récepteurs imidazoliniques.

Since the demonstration that imidazoline and guanidinium alpha-2 adrenergic agonists induce some of their functional effects by a "nonadrenergic" mechanism, many efforts have been done to identify an imidazoline receptor. Binding studies have allowed to characterize two classes of potential imidazoline receptors: the "(p-amino)clonidine" and the "idazoxan" binding sites. These last, that we named "imidazoline-guanidinium receptive sites" (IGRS) on the basis of their ligand-recognition properties, have been identified, for the first time, in the proximal tubule from rabbit and human kidney. In the present report we will summarize the studies that led us to the characterization of IGRS.