Inhibition of monoamine oxidase A and B activities by imidazol(ine)/guanidine drugs, nature of the interaction and distinction from I₂-imidazoline receptors in rat liver

Abstract

1. I₂-Imidazoline sites ([³H]-idazoxan binding) have been identified on monoamine oxidase (MAO) and proposed to modulate the activity of the enzyme through an allosteric inhibitory mechanism (Tesson et al., 1995). The main aim of this study was to assess the inhibitory effects and nature of the inhibition of imidazol(ine)/guanidine drugs on rat liver MAO-A and MAO-B isoforms and to compare their inhibitory potencies with their affinities for the sites labelled by [³H]-clonidine in the same tissue.

2. Competition for [³H]-clonidine binding in rat liver mitochondrial fractions by imidazol(ine)/guanidine compounds revealed that the pharmacological profile of the interaction (2 - styryl - 2 - imidazoline, LSL 61112>idazoxan>2 - benzofuranyl - 2 - imidazoline, 2-BFI=cirazoline>guanabenz>oxymetazoline>>clonidine) was typical of that for I₂-sites.

3. Clonidine inhibited rat liver MAO-A and MAO-B activities with very low potency (IC₅₀s: 700 μM and 6 mM, respectively) and displayed the typical pattern of competitive enzyme inhibition (Lineweaver-Burk plots: increased K_m and unchanged V_max values). Other imidazol(ine)/guanidine drugs also were weak MAO inhibitors with the exception of guanabenz, 2-BFI and cirazoline on MAO-A (IC₅₀s: 4–11 μM) and 2-benzofuranyl-2-imidazol (LSL 60101) on MAO-B (IC₅₀: 16 μM). Idazoxan was a full inhibitor, although with rather low potency, on both MAO-A and MAO-B isoenzymes (IC₅₀s: 280 μM and 624 μM, respectively). Kinetic analyses of MAO-A inhibition by these drugs revealed that the interactions were competitive. For the same drugs acting on MAO-B the interactions were of the mixed type inhibition (increased K_m and decreased V_max values), although the greater inhibitory effects on the apparent value of V_max/K_m than on the V_max value indicated that the competitive element of the MAO-B inhibition predominated.

4. Competition for [³H]-Ro 41-1049 binding to MAO-A or [³H]-Ro 19-6327 binding to MAO-B in rat liver mitochondrial fractions by imidazol(ine)/guanidine compounds revealed that the drug inhibition constants (K_i values) were similar to the IC₅₀ values displayed for the inhibition of MAO-A or MAO-B activities. In fact, very good correlations were obtained when the affinities of drugs at MAO-A or MAO-B catalytic sites were correlated with their potencies in inhibiting MAO-A (r=0.92) or MAO-B (r=0.99) activity. This further suggested a direct drug interaction with the catalytic sites of MAO-A and MAO-B isoforms.

5. No significant correlations were found when the potencies of imidazol(ine)/guanidine drugs at the high affinity site (pK_iH, nanomolar range) or the low-affinity site (pK_iL, micromolar range) of I₂-imidazoline receptors labelled with [³H]-clonidine were correlated with the pIC₅₀ values of the same drugs for inhibition of MAO-A or MAO-B activity. These discrepancies indicated that I₂-imidazoline receptors are not directly related to the site of action of these drugs on MAO activity in rat liver mitochondrial fractions.

6. Although these studies cannot exclude the presence of additional binding sites on MAO that do not affect the activity of the enzyme, they would suggest that I₂-imidazoline receptors represent molecular species that are distinct from MAO.

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