Inhibition of monoamine oxidase A by β-Carboline derivatives

β-Carbolines are endogenous inhibitors of monoamine oxidase (MAO). The interaction of nine β-carboline derivatives and four 3,4-dihydro forms with purified MAO A was investigated. All the compounds tested were reversible competitive inhibitors selective for MAO A, in agreement with previous studies on membrane preparations. The oxidation of kynuramine by MAO A in the presence of the more effective inhibitors showed a lag period before reaching the steady state. In general, the 1-methyl and 7-methoxy substituents increased the potency. Harmine, 2-methylharminium, 2,9-dimethylharminium, and harmaline were the most effective inhibitors of the purified MAO A, with low K(i) values of 5, 69, 15, and 48 nM, respectively. The inhibitors interacted with the covalently bound flavin to induce distinct spectral changes, the magnitude of which correlated with the efficacy of the inhibition. The more effective inhibitors could be in situ inhibitors of MAO A.</p

Redox properties of the flavin cofactor of monoamine oxidases A and B and their relationship to the kinetic mechanism

This chapter discusses the redox properties of the flavin cofactor of monoamine oxidases (MAO) A and B and their relationship to the kinetic mechanism that they follow. The purified enzyme preparations used in this study were the human liver MAO A expressed in yeast and bovine liver MAO B. The redox potentials in the absence of substrate were determined by the method developed by Massey. The redox potentials of cysteinyl-FAD in unliganded MAO A and B have been determined. The spectral changes were observed in a mixture of MAO B and a reference dye, indigo disulfonate. Reduction of the dye is characterized by the disappearance of the peak at 610 nm observed for the oxidized dye and the appearance of an equally prominent peak at 370 nm for the reduced dye. These changes are superimposed on the decrease at 456 nm observed for the reduction of cysteinyl-FAD from the oxidized form to a semiquinone. The redox potentials of the cysteinyl-FAD in unliganded MAO A and B are close to that for free flavin. It has been concluded that the redox potential in an enzyme-substrate complex is positively shifted towards the potential of an amine substrate. This shift is different for each substrate. The values for the redox potentials for MAO in the presence of physiological substrates remain to be determined. The rate of reduction of the flavin by substrate correlates with the redox potential—that is, an increase in the potential of the flavin favors electron transfer from amine to flavin.</p

Oxidation of Tetrahydrostilbazole by Monoamine Oxidase A Demonstrates the Effect of Alternate Pathways in the Kinetic Mechanism

The steady-state kinetics for the oxidation of 1-methyl-1,2,3,6-tetrahydrostilbazole (MTHS) by purified human liver monoamine oxidase A yielded biphasic double-reciprocal plots. Rate constants from stopped-flow studies were determined to show that the apparent stimulation at high substrate concentrations can be explained in terms of the alternate oxidative pathways available to monoamine oxidase A [Ramsay, R. R. (1991) Biochemistry 30, 4624–4629]. At low substrate concentrations, the slower reoxidation of the free enzyme (second-order rate constant was 4000 M−1 s−1) predominates, but at higher concentrations the faster reoxidation of the reduced enzyme-substrate complex (38 300 M−1 s−1) becomes significant. Computer simulation using this model predicts that similar biphasic curves could be obtained for the oxidation of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, but that nonlinearity would be obvious only at concentrations above 200Km.</p

Reactivation of NADH Dehydrogenase (Complex I) Inhibited by 1‐Methyl‐4‐(4′‐Alkylphenyl)pyridinium Analogues:A Clue to the Nature of the Inhibition Site

Abstract: Expression of the neurotoxicity of 1‐methyl‐4‐phenyl‐1.2,3,6‐tetrahydropyridine, following oxidation to l‐methyl‐4‐phenylpyridinium ion (MPP+), is believed to involve inhibition of mitochondrial electron transport from NADH dehydrogenase (complex l) to ubquinone. MPP+ and its analogues have been shown to Mock electron transport at or near the same site as two powerful inhibitors of mitochondrial respiration, rotenone and piericidin A. All three types of inhibitors combine at two sites on NADH dehydrogenase, a hydrophilic and hydrophobic one, and occupancy of both sites is required for complete inhibition. Tetraphenylboron anion (TPB−) in catalytic amounts is known to increase the effectiveness of positively charged MPP+ analogues in blodclng mitochondrial respiration. A part of this effect involves facitation of the entry of MPP+ oongeners into the hydrophobic site by ion pairing, as has been demonstrated in studies with submitochondrial particles (electron transport particles). This communication documents the fact that TPB−, when present in molar excess over the MPP+ analogues, reverses the inhibition. This seems to involve again strong ion pairing. removal of the inhibitory analogue from one to the two binding sites, and concentration of the inhibitor in the membrane, so that only the hydrophobic binding site remains occupied, resulting in lowering of the inhibiti to 30–40%.</p

Inhibition of NADH oxidation by 1-methyl-4-phenylpyridinium analogs as the basis for the prediction of the inhibitory potency of novel compounds

Inhibition of NADH dehydrogenase (Complex I) of the mitochondrial respiratory chain by 1-methyl-4-phenylpyridinium (MPP+) and its analogs results in dopaminergic cell death. In the present study, the inhibition of mitochondrial respiration and of NADH oxidation in inverted inner membrane preparations by the oxidation products of N-methyl-stilbazoles (N-methyl-styrylpyridiniums) are characterized. These nonflexible MPP+ analogs were found to be considerably more potent inhibitors than the corresponding MPP+ derivatives. The IC50 values for these compounds and previously published figures for MPP+ analogs were then used to select a computer model based on structural parameters to predict the inhibitory potency of other compounds that react at the "rotenone site" in Complex I. A series of 12 novel inhibitors different in structure from the basic set were used to test the predictive capacity of the models selected. Despite major structural differences between the novel test compounds and the MPP+ and styrylpyridinium analogs on which the models were based, substantial agreement was found between the predicted and experimentally determined IC50 values. The value of this technique lies in the potential for the prediction of the inhibitory potency of other drugs and toxins which block mitochondrial respiration by interacting at the rotenone sites.</p