单胺氧化酶抑制剂在临床方面的应用

单胺氧化酶(monoamine oxidase,MAO)是人体内天然存在的一种酶,催化单胺类物质氧化脱氨反应的酶。人体内含有两种单胺氧化酶:单胺氧化酶A和单胺氧化酶B。单胺氧化酶A主要分布在儿茶酚胺能神经元中;单胺氧化酶B主要分布在5-羟色胺能神经元、组胺能神经元和神经胶质细胞中,这两种亚型都均可以使单胺类神经递质失活。而单胺氧化酶抑制剂则能够通过抑制单胺氧化酶的对单胺类物质的氧化活性,从而达到减轻或者消除由各种原因引起的单胺类物质减少或单胺氧化酶活性过高导致的疾病。本文主要总结了近几年单胺氧化酶抑制剂在临床上用于治疗帕金森病、抑郁症和幽门螺旋杆菌方面的最新进展。     

单胺氧化酶抑制剂在临床方面的应用

单胺氧化酶（monoamine oxidase, MAO）是人体内天然存在的一种酶,催化单胺类物质氧化脱氨反应的酶。人体内含有两种单胺氧化酶：单胺氧化酶A 和单胺氧化酶B。单胺氧化酶A 主要分布在儿茶酚胺能神经元中;单胺氧化酶B 主要分布在5- 羟色胺能神经元、组胺能神经元和神经胶质细胞中,这两种亚型都均可以使单胺类神经递质失活。而单胺氧化酶抑制剂则能够通过抑制单胺氧化酶的对单胺类物质的氧化活性,从而达到减轻或者消除由各种原因引起的单胺类物质减少或单胺氧化酶活性过高导致的疾病。本文主要总结了近几年单胺氧化酶抑制剂在临床上用于治疗帕金森病、抑郁症和幽门螺旋杆菌方面的最新进展。     

Enzyme Inhibitors of Marine Microbial Origin with Pharmaceutical Importance

Several enzyme inhibitors with various industrial uses were isolated from bacteria and actinomycetes living in the marine environment. These inhibitors are useful in medicine and agriculture. All the compounds, except the monoamine oxidase inhibitors, are novel, and their activities have been characterized.     

Proposed structural basis of interaction of piperine and related compounds with monoamine oxidases

Several studies have revealed piperine and a few related compounds as potent inhibitors of monoamine oxidases without delineating the underlying mechanism. Using in silico modelling, we propose a structural basis of such activity by showing that these compounds can successfully dock into the inhibitor binding pockets of human monoamine oxidase isoforms with predicted affinities comparable to some known inhibitors. The results therefore suggest that piperine can be a promising lead for developing novel monoamine oxidase inhibitors.     

Inhibition of amine oxidases activity by 1-acetyl-3,5-diphenyl-4,5-dihydro-(1H)-pyrazole derivatives

A novel series of 1-acetyl-3,5-diphenyl-4,5-dihydro-(1H)-pyrazole derivatives have been synthesised and investigated for the ability to inhibit selectively monoamine oxidases, swine kidney oxidase, and bovine serum amine oxidase. The newly synthesised compounds 1–6 proved to be reversible and non-competitive inhibitors of all types of the assayed amine oxidases. Compounds inhibit monoamine oxidases potently, displaying low I₅₀ values of particular interest. In particular 1-acetyl-3-(2,4-dihydroxyphenyl)-5-(3-methylphenyl)-4,5-dihydro-(1H)-pyrazole 6 showed to be a potent monoamine oxidase inhibitor with a K_i of about 10⁻⁸ M. Further insights in the theoretical evaluation of the possible interactions between the compounds and monoamine oxidase B have been developed through a computational approach.     

Naphthylisopropylamine and N-benzylamphetamine derivatives as monoamine oxidase inhibitors

A series of naphthylisopropylamine and N-benzyl-4-methylthioamphetamine derivatives were evaluated as monoamine oxidase inhibitors. Their potencies were compared with those of a series of amphetamine derivatives, to test if the increase of electron richness of the aromatic ring and overall size of the molecule might improve their potency as enzyme inhibitors. Molecular dockings were performed to gain insight regarding the binding mode of these inhibitors and rationalize their different potencies. In the case of naphthylisopropylamine derivatives, the increased electron-donating capacity and size of the aromatic moiety resulting from replacement of the phenyl ring of amphetamine derivatives by a naphthalene system resulted in more potent compounds. In the other case, extension of the arylisopropylamine molecule by N-benzylation of the amino group led to a decrease in potency as monoamine oxidase inhibitors.     

3D-QSAR method on indole and pyrrole inhibitors of monoamine oxidase type A

Monoamine oxidase A (MAO-A) converts norepinephrine and serotonin to an oxidative form. These monoamine neurotransmitters have important roles in depression. The MAO-A inhibitors have been discovered for neurodegenerative disease therapy. In order to design novel MAO-A inhibitors, in this study, the quantitative structure-activity relationships for the combined series of indoles and pyrroles were elucidated and the structural conditions to show good inhibitory effects on MAO-A were derived. This result can help us design new inhibitors irrespective of their specific moiety.     

Isolation and characterization of a monoamine oxidase B selective inhibitor from tobacco smoke

It is well established that tobacco smokers have reduced levels of monoamine oxidase activities both in the brain and peripheral organs. Furthermore, extensive evidence suggests that smokers are less prone to develop Parkinson's disease. These facts, plus the observation that inhibition of monoamine oxidase B protects against the parkinsonian inducing effects of the nigrostriatal neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, have prompted studies to identify monoamine oxidase inhibitors in the tobacco plant and tobacco cigarette smoke. Our previous efforts on cured tobacco leaf extracts have led to the characterization of 2,3,6-trimethyl-1,4-naphthoquinone, a non-selective monoamine oxidase inhibitor, and farnesylacetone, a selective monoamine oxidase B inhibitor. We now have extended these studies to tobacco smoke constituents. Fractionation of the smoke extracts has confirmed and extended the qualitative results of an earlier report [J. Korean Soc. Tob. Sci.1997, 19, 136] demonstrating the inhibitory activity of the terpene trans,trans-farnesol on rat brain MAO-B. In the present study, K(i) values for the inhibition of human, baboon, monkey, dog, rat, and mouse liver MAO-B have been determined. Noteworthy is the absence of inhibitory effects on human placental MAO-A and beef liver MAO-B. A limited structure-activity relationship study of analogs of trans,trans-farnesol is reported. Although the health hazards associated with the use of tobacco products preclude any therapeutic opportunities linked to smoking, these results suggest the possibility of identifying novel structures of compounds that could lead to the development of neuroprotective agents.     

Monoamine oxidase in adult Hymenolepis diminuta (Cestoda).

A membrane-bound monoamine oxidase (EC 1.4.3.4) was demonstrated in homogenates of Hymenolepis diminuta. The enzyme oxidized a variety of biologically active amines (in decreasing order: dopamine, adrenaline, noradrenaline, tryptamine, tyramine, octopamine), there was, however, no activity with 5-hydroxytryptamine or benzylamine. No diamine oxidase (EC 1.4.3.6.) could be detected in H. diminuta (using histamine, cadaverine or putrescine as substrates). The monoamine oxidase from H. diminuta was not inhibited by azide, hydroxylamine or semicarbazide, but was inhibited by cupferron, alpha-alpha dipyridyl and iodoacetamide, and by the specific monoamine oxidase inhibitors pargyline, nialamide and iproniazid. Several anthelmintics were also found to be inhibitors of monoamine oxidase. The possible roles of monoamine oxidase in H. diminuta are discussed.     

A kinetic evaluation of monoamine oxidase activity in rat liver mitochondrial outer membranes

1. A preparation of mitochondrial outer membranes from rat liver can be shown to contain two kinetically distinct monoamine oxidase activities. These activities are distinguishable by their different sensitivities to the irreversible inhibitor clorgyline, and by the effect of the reversible inhibitors benzyl cyanide and 4-cyanophenol. 2. The substrate specificities of the preparation and the two enzyme species have been elucidated.     

Oxidative metabolism of some hydrazine derivatives by rat liver and lung tissue fractions

The enzyme systems in rat liver and lung responsible for the oxidative metabolism of hydrazine derivatives were studied to determine whether these enzymes, cytochrome P-450 and monoamine oxidase, were responsible for metabolically activating hydrazines to carcinogenic/toxic metabolites. Cytochrome P-450 preferentially oxidized the nitrogen to nitrogen bond of 1,2-disubstituted hydrazines and hydrazides, while monoamine oxidase oxidized the nitrogen to nitrogen bond of all the classes of hydrazine derivatives that were tested. Oxidation of the nitrogen to nitrogen bond led to the formation of stable azo intermediates in the case of 1,2-disubstituted hydrazines and to unstable monoazo (diazene) metabolites in the case of monosubstituted hydrazines and hydrazides. In addition, cytochrome P-450 preferentially oxidized the carbon to nitrogen bond of monoalkylhydrazines; this reaction resulted in the formation of aldehyde metabolites (via hydrazone intermediates). Monosubstituted hydrazines were shown to be potent, irreversible inhibitors of mitochondrial monoamine oxidase. In contrast, the 1,2-disubstituted hydrazines appeared to be good substrates for the monoamine oxidase and served as competitive inhibitors at high concentrations. There did not appear to be any monoamine oxidase isozyme (form A or B) specificity in the metabolism of either the 1,2-disubstituted hydrazines or the monoalkylhydrazines, ethyl- and n-propylhydrazine.     

Synthesis and inhibitory effect of piperine derivates on monoamine oxidase

A series of piperine derivates (1-19) have been designed, synthesized and evaluated in vitro for their monoamine oxidase (MAO) A and B inhibitory activity and selectivity. It is worth noting that most of the small amine moieties substituted on the piperidine ring proved to be potent and selective inhibitors of MAO-B rather than of MAO-A. 5-(3,4-methylenedioxyphenyl)-2E,4E-pentadienoic acid n-propyl amide (3) showed the greatest MAO-B inhibitory activity (IC(50)(MAO-B)=0.045 μM) and good selectivity (IC(50)(MAO-A)=3.66 μM). The conjugated double bond and carbonyl group of piperine are proved to be an essential feature for piperine and related alkylamides to exhibit MAO-inhibitory activity. Binding mode of the titled compounds was predicted using FlexX algorithm. The design and optimization of novel small molecule monoamine oxidase inhibitors will be guided by the results of this report.     

Multiple amine oxidases in cucumber seedlings

Cell-free extracts of cucumber (Cucumis sativus L. cv. National Pickling) seedlings were found to have amine oxidase activity when assayed with tryptamine as a substrate. Studies of the effect of lowered pH on the extract indicated that this activity was heterogeneous, and three amine oxidases could be separated by ion exchange chromatography. The partially purified enzymes were tested for their activities with several substrates and for their sensitivities to various amine oxidase inhibitors. One of the enzymes may be a monoamine oxidase, although it is inhibited by some diamine oxidase inhibitors. The other two enzymes have properties more characteristic of the diamine oxidases. The possible relationship of the amine oxidases to indoleacetic acid biosynthesis in cucumber seedlings is discussed.     

Substrates and inhibitors of hepatic amine oxidase inhibit dimethylnitrosamine-induced mutagenesis in Salmonella typhimurium

The mutagenic effect of dimethylnitrosamine in Salmonella typhimurium TA100, in the presence of a rat-liver homogenate derived from animals treated with Aroclor 1254, was inhibited by substrates and inhibitors of monoamine oxidase. Substrates of diamine oxidase did not inhibit dimethylnitrosamine mutagenesis and, furthermore, monoamine oxidase inhibitors had no effects on mutagenesis by benzo[a]pyrene or aflatoxin B₁. The results suggest that monoamine oxidase participates in the activation of dimethylnitrosamine to a mustagen.     

Cough and Cold Remedies: a Potential Danger to Patients on Monoamine Oxidase Inhibitors

Experiments have been carried out in healthy volunteers to study the effects of phenylpropanolamine on the blood pressure and possible interactions with monamine oxidase inhibitors.In three subjects 50 mg. of phenylpropanolamine taken orally produced a modest rise of systolic pressure. Two proprietary preparations containing this dose in a slow-release form had no significant effect on the blood pressure. In all three subjects 100 mg. of phenylpropanolamine taken orally caused a more pronounced rise of systolic pressure and a rise of diastolic pressure.In contrast, 50 mg. of phenylpropanolamine orally caused a rapid and potentially dangerous rise of blood pressure in a subject taking the monamine oxidase inhibitor tranylcypromine, and a similar acute rise of blood pressure occurred in this subject when given a proprietary cough linctus containing the same dose of phenylpropanolamine. These and other results suggest that severe hypertensive episodes are more likely to occur when preparations containing phenylpropanolamine in a free form, rather than in slow-release form, are taken by patients being treated with monoamine oxidase inhibitors.The acute rise of blood pressure due to the interaction of phenylpropanolamine and monoamine oxidase inhibitors was reversed by intramuscular injection of phentolamine.     

Isolation and characterization of phenylethylamine and phenylethanolamine from human brain

The Presence of endogenous 2-phenylethylamine in mammalian tissues has long been suspected, in view of the fact that L-phenylanine, a substrate for L-aromatic amino acid decarboxylase (Lovenberg , Weissbach and Udenfriend , 1962), is found in substantial amounts in many neural and non-neural tissues. It has been difficult to demonstrate the presence of phenylethylamine in tissues of untreated animals because this amine is an excellent substrate for monoamine oxidase (Mantegazza and Riva , 1963). Using paper chromatography and electrophoresis, Nakajima , Kakimoto and Sano (1964) tentatively identified phenylethylamine in many organs of animals pretreated with monoamine oxidase inhibitors. Phenylethylamine exerts, in animals pretreated with such inhibitors, behavioural stimulant effects similar to those induced by amphetamine (Mantegazza and Riva , 1963). These effects may in part be attributable to catecholamine release (Fuxe , Grobecker and Jonsson , 1967) and partly to a direct effect exerted by phenylethylamine itself (Fischer , Ludmer and Sabelli , 1967; Giardina , Pedemonte and Sabelli , 1972). The brain content of phenylethylamine in mice (Mosnaim and Sabelli , 1971), rabbits (Sabelli , Giardina , Mosnaim and Inwang , 1972) and rats (Fischer , Spatz , Heller and Reggiani , 1972) is increased by antidepressive treatments (imipramine, monoamine oxidase inhibitors, electroshock) and reduced by reserpine. The urinary excretion of phenylethylamine is decreased in depressed patients (Fischer , Heller and Miró , 1968; Boulton and Milward , 1971; Inwang , Sugerman , Mosnaim and Sabelli , 1972; Fischer et al., 1972). However, the presence of phenylethylamine in brain has not yet been conclusively demonstrated because the analytical procedures used in the above-mentioned investigations were not sufficiently specific. In the present study we isolated and identified, by a number of analytical procedures, phenylethylamine and its metabolite 2-hydroxy-2-phenylethylamine (phenylethanolamine) from human brain. Molinoff , Landsberg and Axelrod (1969) have shown by enzymatic methods the formation of phenylethanolamine following the administration of phenylethylamine.     

Differences in substrate specificities of monoamine oxidase A from human liver and placenta.

The substrate specificities of monoamine oxidase (MAO) A isolated from human placenta and of human liver expressed in yeast have been compared in homogeneous preparations with respect to Vmax and Km values for natural and synthetic substrates and Ki values for competitive inhibitors. MAO A from these two sources is known to differ in at least 5 amino acid residues. While the Km and Ki values were found to be nearly identical in the enzymes from these two sources, the Vmax differed significantly on bulky synthetic substrates.     

Modeling of substrate-binding region of the active site of monoamine oxidase A

The mold of the substrate-binding region of the active site of monoamine oxidase A (MAO A) was designed using data of the enzyme interaction with reversible competitive inhibitors and the analysis of their three-dimensional structures. The superposition of ligands in biologically active conformations allowed determination of the shape and dimension of the active site cavity accommodating these compounds. The correctness of this approach was validated by the analysis of HIV protease interaction with its inhibitors using three-dimensional structures of HIV protease-inhibitor complexes. The mold of the substrate/inhibitor-binding site can be used for searching for new ligands in molecular databases and the development of a new generation of MAO inhibitors using lead structures that have not been employed for this purpose yet.     

MULTIPLE FORMS OF MONOAMINE OXIDASE IN DEVELOPING BRAIN: TISSUE AND SUBSTRATE SPECIFICITIES

Brains, hearts and livers from newborn and adult rats were assayed for monoamine oxidase activity using gel electrophoretic techniques. The results suggest that each of the tissues possesses multiple forms (isoenzymes) of monoamine oxidase and that these forms are different for the various tissues. Further, the forms of monoamine oxidase in the neonatal tissues differ from those in the corresponding adult tissue. These different forms of monoamine oxidase have different substrate specificities. Using 5-hydroxy[¹⁴C]tryptamine as substrate, we have demonstrated that the monoamine oxidase patterns appearing on the gel do indeed possess monoamine oxidase activity.     

Significance of multiple forms of brain monoamine oxidase in situ as probed by electron spin resonance.

Spin-labeled hydroxyamphetamine, a competitive reversible inhibitor of brain monoamine oxidase, has been shown to be useful as an electron spin resonance (ESR) probe of the microenvironment of the active sites of the possible monoamine oxidase multiple forms. The ESR spectrum of spin-labeled hydroxyamphetamine was strongly quenched upon binding to the enzyme. The conformation of the active site of rat brain monoamine oxidase existing in various physical states, i.e. monoamine oxidase in situ (intact brain mitochondria), crude solubilized monoamine oxidase (MAOS) and isolated monoamine oxidase fractions (MAOa and MAOb) were critically and systematically examined. Nonlinear least squares regression analyses have been used to fit the binding data (obtained at room temperature with varying spin-labeled hydroxyamphetamine concentrations) to three groups of independent noninteracting ligand-binding models. A Gibbs-Helmholtz relationship was applied to the interpretation of the measured apparent association constant K as a function of temperature ranging from 4-50 degrees with increments of 2 degreesmfrom the extracted intensive parameters, k (intrinsic association constant) and deltaF (intrinsic free energy), as well as the apparent heat, deltaH, it was clear that the microenvironment of the binding sites existing in the more purified enzyme fractions MAOa and MAOb were similar to those found in the crude solubilized enzyme. More importantly, they correlated well with the conformation of the sites characterized in situ. The data suggested that the microenvironment of this multienzyme system was unperturbed in spite of the treatment due to the isolation process. In terms of the composition of binding sites, MAOa appeared to be heterogeneous while MAOb appeared to be more homogeneous. Since the isolated fractions MAOa and MAOb possessed marked different substrate specificities, these observations directly implied that monoamine oxidase multiple forms do exist in situ. The extracted extensive parameters, n (specific binding activity, nanomoles/mg of protein), as well as the measured characteristic transition temperatures, indicated that the relative abundance of the sites which directly affected substrate specificities was indeed altered. The consistency of the characteristic transition temperatures of 21 degrees and 38 degrees for the case of intact membrane preparations was particularly significant. A tenable hypothesis is that the manipulation in the composition of the monoamine oxidase binding forms through intimate lipid-protein interactions, which has been amply demonstrated in many biomembrane systems to be functionally important might be the underlying regulatory mechanism in vivo.