Identification of the amino acid bound to the labile adduct formed during inactivation of monoamine oxidase by 1-phenylcyclopropylamine

Activity-dependent Regulation of Histone Lysine Demethylase KDM1A by a Putative Thiol/Disulfide Switch

Lysine demethylation of proteins such as histones is catalyzed by several classes of enzymes, including the FAD-dependent amine oxidases KDM1A/B. The KDM1 family is homologous to the mitochondrial monoamine oxidases MAO-A/B and produces hydrogen peroxide in the nucleus as a byproduct of demethylation. Here, we show KDM1A is highly thiol-reactive in vitro and in cellular models. Enzyme activity is potently and reversibly inhibited by the drug disulfiram and by hydrogen peroxide. Hydrogen peroxide produced by KDM1A catalysis reduces thiol labeling and inactivates demethylase activity over time. MALDI-TOF mass spectrometry indicates that hydrogen peroxide blocks labeling of cysteine 600, which we propose forms an intramolecular disulfide with cysteine 618 to negatively regulate the catalytic activity of KDM1A. This activity-dependent regulation is unique among histone-modifying enzymes but consistent with redox sensitivity of epigenetic regulators.

An overview of phenylcyclopropylamine derivatives: biochemical and biological significance and recent developments

trans-2-Phencylcyclopropylamine (2-PCPA), a potent, clinically used antidepressant, affects monoamine neurotransmitter levels by inhibiting the main metabolizing enzymes, monoamine oxidases (MAOs). However, the antidepressant action of this compound was not fully explained by its effects on MAOs due to its wide variety of biological effects. 2-PCPA also affects depression-associated pathophysiological pathways, and linked with increased levels of trace amines in brain, upregulation of GABAB receptors (where GABA is gamma amino butyric acid), modulation of phospholipid metabolism, and interference with various cytochrome P450 (CYP) enzymes. Consequently, despite its adverse effects and limited clinical applicability, 2-PCPA has attracted interest as a structural scaffold for the development of mechanism-based inhibitors of various enzymes, including lysine-specific demethylase 1 (LSD1), which is a possible target for cancer chemotherapy. In the recent years, many reports have appeared in the literature based on 2-PCPA scaffold and their potential medicinal implications. This review mainly focuses on the medicinal chemistry aspects including drug design, structure-activity relationships (SAR), biological and biochemical properties, and mechanism of actions of 2-PCPA and its derivatives. Furthermore, we also highlight recent advance in this area and discuss their future applications for beneficial therapeutic effects.

Inactivation of mitochondrial monoamine oxidase B by methylthio-substituted benzylamines

Mitochondrial monoamine oxidase was inactivated by o-mercaptobenzylamine (1) and o- (2) and p-methylthiobenzylamine (5). Experiments were carried out to provide evidence for possible mechanisms of inactivation. The corresponding o- (3) and p-hydroxybenzylamine (4) are not inactivators. Four radiolabeled analogues of 2 and 5, having radioactivity at either the methyl or benzyl groups, were synthesized, and all were shown to incorporate multiple equivalents of radioactivity into the enzyme. Inactivation in the presence of an electrophile scavenger decreased the number of molecules incorporated, but still multiple molecules became incorporated; catalase did not further reduce the number of inactivator molecules bound. Two inactivation mechanisms are proposed, one involving a nucleophilic aromatic substitution (SNAr) mechanism and the other a dealkylation mechanism. Evidence for both mechanisms is that inactivation leads to reduction of the flavin (oxidation of the inactivator), but upon denaturation the flavin is reoxidized, indicating that attachment is not at the flavin. A cysteine titration indicates the loss of four cysteines after inactivation and denaturation. Support for the SNAr mechanism was obtained by showing that o- and p-chlorobenzylamine also inactivate MAO. Chemical model studies were carried out that also support both SNAr and dealkylation mechanisms.

Inactivation of monoamine oxidase B by 1-phenylcyclopropylamine: mass spectral evidence for the flavin adduct

Incubation of 1-phenylcyclopropylamine with bovine liver MAO (MAO B), followed by complete enzymatic digestion to single amino acid residues and subsequent analysis by on-line liquid chromatography-electrospray ionization mass spectrometry, was used to investigate the resulting flavin adduct structure.

Thiazole derivatives as inhibitors of purified bovine liver mitochondrial monoamine oxidase-B: structure-activity relationships and theoretical study

Structure-activity relationships were performed on a new series of thiazole derivatives which selectively inactivate monoamine oxidase-B (MAO-B), purified from mitochondrial beef liver. All of the synthesized and tested compounds showed non-competitive inhibition, suggesting the formation of a stable adduct between the tertiary amine function, linked to the thiazolyl derivatives and the active site of the enzyme. The mechanism of MAO-B inhibition is discussed in terms of the Ionization Potential of the amine nitrogen atom and the conformational flexibility of the inhibitors.

Syntheses of amino nitrones. Potential intramolecular traps for radical intermediates in monoamine oxidase-catalyzed reactions

Monoamine oxidase (MAO) is a flavin-dependent enzyme that catalyzes the oxidative deamination of a variety of amine neurotransmitters and toxic amines. Although there have been several studies that support the intermediacy of an amine radical cation and an alpha-radical during enzyme catalysis, there is no direct, i.e. EPR, evidence for these species as they are formed. Amino nitrones have been designed which, upon radical formation would produce an intermediate that is a resonance structure of the corresponding nitroxyl radical, which should be observable by EPR spectroscopy. Syntheses of seven different amino nitrones, three acyclic, and four cyclic analogues were attempted. The protected amino nitrones were stable, but all three of the acyclic amino nitrones were unstable. One of the cyclic analogues was very stable (39), one was stable only in organic solvents (40), one was stable only in aqueous medium below pH 6.5 (41), and the other (42) was stable for just a short time at room temperature, decomposing to a stable free radical. None of these analogues produced a MAO-catalyzed radical, yet 41 is a poor substrate (Km=0.2mM; k(cat) = 0.034 min-1) and 39 is a mixed inhibitor (Ki = 26.5 mM). Although this approach does not appear to be applicable to amino nitrones, it should be a valuable approach for other enzymes where radical intermediates are suspected and nonamine nitrones can be utilized.

Inactivation of monoamine oxidase B by cis- and trans-5-aminomethyl-3-(4-methoxyphenyl)dihydrofuran-2(3H)-ones

Monoamine oxidase B was previously shown to be inactivated by cis- (3) and trans-5-(aminomethyl)-3-(4-methoxyphenyl)dihydrofuran-2(3H)-one hydrochloride (4) in a time-dependent manner (Ding, Z.; Silverman, R. B. J. Med. Chem. 1992, 35, 885) and to catalyze its oxidative decarboxylation (Silverman, R. B.; Zhou, J.J.P.; Ding, C. Z.; Lu, X. J. Am. Chem. Soc. 1995, 117, 12895). By [14C]-labeling of the aryl methoxyl groups of these two inactivators, it is shown that this is not a mechanism-based inactivation and that multiple enzyme residues are labeled.

Monoamine oxidase contains a redox-active disulfide

Mitochondrial monoamine oxidases A and B (MAO A and MAO B) are ubiquitous homodimeric FAD-containing oxidases that catalyze the oxidation of biogenic amines. Both enzymes play a vital role in the regulation of neurotransmitter levels in brain and are of interest as drug targets. However, little is known about the amino acid residues involved in the catalysis. The experiments reported here show that both MAO A and MAO B contain a redox-active disulfide at the catalytic center. The results imply that MAO may be a novel type of disulfide oxidoreductase and open the way to characterizing the catalytic and chemical mechanism of the enzyme.

The new generation of monoamine oxidase inhibitors

Irreversible and unspecific inhibitors of MAO were the first modern antidepressants, but after an initial success they fell into discredit due to adverse side effects. In the past two decades interest in MAO inhibitors has been renewed because of progress in basic research, a milestone being the finding that there are two subtypes of MAO, MAO-A and MAO-B. These are distinct proteins with high amino acid homology, coded by separate genes both located on the short arm of the human chromosome X. The enzyme subforms show different substrate specificities in vitro and different distributions within the central nervous system and in peripheral organs. In the central nervous system of man MAO-A seems to be mainly involved in the metabolism of 5 HT and noradrenaline, whereas 2-phenylethylamine and probably dopamine are predominantly deaminated by MAO-B. In the intestinal tract tyramine is mainly metabolized by MAO-A. These characteristics indicate distinct physiological functions of the two MAO-subforms. Several irreversible and reversible non-hydrazine inhibitors with relative selectivities for one of the MAO-subforms have been developed. They belong to various chemical classes with different modes of enzyme inhibition. These range from covalent mechanism based interaction (e.g. by propargyl- and allylamine derivatives) to pseudosubstrate inhibition (e.g. by 2-aminoethyl-carboxamides) and non-covalent interaction (e.g. by brofaromine, toloxatone and possibly moclobemide). The most important pharmacological effects of the new types of MAO inhibitors are those observed in neuropsychiatric disorders. The inhibitors of MAO-A show a favorable action in various forms of mental depression. The drugs seem to have about the same activity as other types of antidepressants, including tricyclic and related compounds as well as classical MAO inhibitors. The onset of action of the MAO-A inhibitors is claimed to be relatively fast. Other possible indications of these drugs include disorders with cognitive impairment, e.g. dementia of the Alzheimer type. In subjects with Parkinson's disease the MAO-B inhibitor L-deprenyl exerts a L-dopa-sparing effect, prolongs L-dopa action and seems to have a favorable influence regarding on-off disabilities. The action is in general transitory (months to several years). In addition L-deprenyl has been shown to delay the necessity for L-dopa treatment in patients with early parkinsonism. Whether the drug influence the progression of the disease is still a matter of debate. L-deprenyl also appears to have some antidepressant effect (especially in higher doses) and to exert a beneficial influence in other disorders, e.g. dementia of the Alzheimer type.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism-based inactivation of monoamine oxidases A and B by tetrahydropyridines and dihydropyridines

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its primary oxidation product, 1-methyl-4-phenyl-2,3-dihydropyridinium (MPDP+), are mechanism-based inhibitors of monoamine oxidases A and B. The pseudo-first-order rate constants for inactivation were determined for various analogues of MPTP and MPDP+ and the concentrations in all redox states were measured throughout the reaction. Disproportionation was observed for all the dihydropyridiniums, but non-enzymic oxidation was insignificant. The dihydropyridiniums were poor substrates for monoamine oxidase A and, consequently, inactivated the enzyme only slowly, despite partition coefficients lower than those for the tetrahydropyridines. For monoamine oxidase B, the dihydropyridiniums were more effective inactivators than the tetrahydropyridines. Substitutions in the aromatic ring had no major effect on the inactivation of monoamine oxidase B, but the 2'-ethyl- and 3'-chloro-substituted compounds were very poor mechanism-based inactivators of monoamine oxidase A. It is clear that both oxidation steps can generate the reactive species responsible for inactivation.

Biochemistry and genetics of monoamine oxidase

This chapter reviews the two mitochondrial flavin containing isozymes of monoamine oxidase. Section 1, "Biochemistry" discusses assays, substrates and inhibitors, phylogenic and tissue distribution, interactions with lipids, nutritional studies, protein structure, kinetic and chemical mechanistic proposals, and biosynthesis. Section 2, "Inheritance" discusses possible genes involved in expression, genetic studies of platelet MAO-B and fibroblast MAO-A, and chromosomal location. Section 3, "Molecular Genetics" reviews the cloning of their cDNAs, their intra- and interspecies homology and structural inferences made from deduced amino acid sequences. Section 4, "Regulation" gives an overview of levels in development and aging, and effect of drugs. The final section 5, "Role in Human Disease" discusses physiological function and effects of altered levels in humans and animal models including complete absence due to a submicroscopic chromosomal deletion in several human patients.

The potential use of mechanism-based enzyme inactivators in medicine

Mechanism-based enzyme inactivator, alanine racemase, S-adenosylhomocysteine hydrolase, D-amino acid aminotransferase, gamma-aminobutyric acid aminotransferase, arginine decarboxylase, aromatase, L-aromatic amino acid decarboxylase, dihydrofolate reductase, dihydroorotate dehydrogenase DNA polymerase I, dopamine beta-hydroxylase, histidine decarboxylase, beta-lactamase, monoamine oxidase, ornithine decarboxylase, serine proteases, testosterone 5 alpha-reductase, thymidylate synthetase, xanthine oxidase.

Overview of the present state of MAO inhibitors

In this paper an overview of the present state of monoamine oxidase inhibitors (MAOIs) is presented. The irreversible inhibitors are firstly considered. They have been divided into four chemical types: substituted hydrazine, cyclopropylamine, propargylamine and allylamine derivatives. Moreover, a tetrahydropyridine derivative (MPTP), recently described as an irreversible inhibitor of MAO-B, has been included among the irreversible MAOIs. The reversible inhibitors such as tetrahydro-beta-carbolines and salsolinol, phenylalkylamines: amphetamine, amiflamine and 2,3-dichloro-alpha-methyl-benzylamine. Among the short acting or reversible inhibitors the 4-(2-benzofuranyl) piperidine series and the morpholinoethylamino derivatives are discussed. Finally the oxazolidinone series is presented separately, as in this series reversible or irreversible inhibitors of the A or B form of MAO have been obtained.