Inhibition of monoamine oxidase A and B activities by imidazol(ine)/guanidine drugs, nature of the interaction and distinction from I<inf>2</inf>-imidazoline receptors in rat liver

Andrés Ozaita, Gabriel Olmos, M. Assumpció Boronat, José Miguel Lizcano, Mercedes Unzeta, Jesús A. García-Sevilla

Research output: Contribution to journalArticleResearchpeer-review

81 Citations (Scopus)

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. 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.
Original languageEnglish
Pages (from-to)901-912
JournalBritish Journal of Pharmacology
Volume121
DOIs
Publication statusPublished - 15 Jul 1997

Keywords

  • Competitive inhibition
  • Guanidine drugs
  • I -imidazoline receptors 2
  • Imidazol(ine)
  • MAO-A and MAO-B isoforms
  • Mixed inhibition
  • [ H]-clonidine 3

Fingerprint

Dive into the research topics of 'Inhibition of monoamine oxidase A and B activities by imidazol(ine)/guanidine drugs, nature of the interaction and distinction from I<inf>2</inf>-imidazoline receptors in rat liver'. Together they form a unique fingerprint.

Cite this