Liver Mitochondrial Monoamine Oxidase Research Articles

Sensitivity of liver monoamine oxidase in the lamprey Lampetra fluviatilis to some tricyclic compounds

A comparative analysis of the effect of the five acridine, three phenothiazine and one xanten (pyronine G) derivatives on the activity of liver mitochondrial monoamine oxidase (MAO) in sexually mature male river lampreys Lampetra fluviatilis has been conducted. Tyramine, dopamine, serotonin, noradrenaline, benzylamine, β-phenylethylamine and N-methylhistamine have been used as substrates for analyzing the monoamine oxidase activity of heterocyclic compounds. The analyzed synthetic hexamerous tricyclic compounds exhibit irreversible inhibition of the enzyme but no specificity depending on the desaminated substrate. The number and identity of heteroatoms in the analyzed heterocyclic compounds have been established to influence their inhibitory efficiency. The data of substrate-inhibitory analysis obtained with the use of the specific substrates provide indirect evidence for the existence of a single MAO form in the lamprey liver.

Enzymologic characteristics of monoamine oxidase from liver of the skipjack tuna Katsuwonus pelamis

Substrate and inhibitory specificity of mitochondrial monoamine oxidase (MAO) from liver of skipjack tuna Katsuwonus pelamis was studied. The results of substrate—inhibitory analysis with application of chlorgilin and deprenyl might be indirect proofs of existence of one molecular MAO form in the tuna liver. Studied enzyme, as liver MAO of terrestrial mammals, deaminates tyramine, tryptamine, dopamine, serotonin, noradrenalin, benzylamine, β-phenylethylamine, N-methylhistamine and does not deaminate histamine, is not suppressed by 10 mM semicarbazide. Takrin, acriflavin, proflavin, acridine orange and pyronine G were established to be irreversible inhibitors of middle strength in respect to MAO of tuna liver. The specificity of inhibitors action upon deamination of various substrates was equal.

Catalytic characteristics of monoamine oxidase of whitefish Coregonus lavaretus ludoga

Study of substrate-inhibitory specificity of liver mitochondrial monoamine oxidase (MAO) of sexually mature individuals of the whitefish Coregonus lavaretus ludoga P. from the Ladoga Lake has revealed distinguished peculiarities of catalytical properties of this enzyme. The studied MAO, on one hand, like the classical enzyme of homoiothermal animals, is able to deaminate tyramine, serotonin, benzylamine, tryptamine, and beta-phenylalanine, but, on the other hand, to deaminate histamine, the classic substrate of diamine oxidase. The found equal activity and sorptional ability of the enzyme toward six studied substrates including histamine, as well as results of the substrate-inhibitory analysis with use of specific inhibitors--deprenyl and chlorgilin--indicate homogeneity of the enzyme. The detected for the first time among the fish MAO wide substrate specificity and an unusually low sensitivity to both studied acetylene inhibitors does not allow ascribing unanimously the studied enzyme to the known MAO forms of organs and tissues of homoiothermal organisms. Apparently, the revealed enzyme form of poikilothermal organism is not the true MAO, but performs a large amine oxidase function.

Selective inhibition of bovine plasma amine oxidase by homopropargylamine, a new inactivator motif.

Propargylic and activated allylic amines are known to inactivate the quinone-dependent plasma amine oxidases, possibly through active-site modification by the alpha,beta-unsaturated aldehyde turnover products. Although homopropargylamine (1-amino-3-butyne, 1) is a nonobvious candidate as a mechanism-based inhibitor, 1 was found to be an unusually potent time- and concentration-dependent irreversible inactivator of bovine plasma amine oxidase (BPAO), exhibiting a 30 min IC(50) of 2.9 microM at 30 degrees C ([BPAO] = 1.2 microM). Preserved cofactor redox activity of the denatured inactivated enzyme indicates that inactivation by 1 involves either a cofactor modification that reverses upon enzyme denaturation or a modification of an active-site residue. Because inactivation by 1 may involve enzyme alkylation by the reactive 2,3-butadienal (3) tautomer of the 3-butynal turnover product of 1, aldehyde 3 was prepared and was found to inactivate BPAO, but only at high concentration. In addition, whereas inhibition by 3 was blunted by the presence of mercaptoethanol, no such protection was observed against 1. The amine whose turnover should lead directly to 3 was prepared (1-amino-2,3-butadiene, 4) and was found to be an even more potent inactivator of BPAO than 1, exhibiting a 5 min IC(50) of 1.25 microM. Rat liver mitochondrial monoamine oxidase was also inactivated by 4, as expected, but only very weakly by 1. Potential mechanisms explaining the selective inhibition of BPAO by 1 are discussed.

Anti‐Dipsotropic Isoflavones: The Potential Therapeutic Agents for Alcohol Dependence

AbstractFor Abstract see ChemInform Abstract in Full Text.

Anti-dipsotropic isoflavones: the potential therapeutic agents for alcohol dependence.

Daidzin is the active principle of Radix puerariae (RP), an herbal remedy that has been used apparently safely and effectively for the treatment of "alcohol addiction" in China for more than a millennium. It has been shown to reduce alcohol consumption in all animal models tested to date. A link between daidzin's capacity to reduce alcohol consumption and its ability to increase liver mitochondrial monoamine oxidase (MAO): aldehyde dehydrogenase (ALDH-2) activity ratio has been established. Daidzin analogs that potently inhibit ALDH-2 but not MAO are the most anti-dipsotropic, whereas those that also inhibit MAO are not. On the basis of these findings, it was proposed that the liver mitochondrial MAO-ALDH-2 pathway is the primary site of action of daidzin and that a biogenic aldehyde derived from the action of MAO mediates its anti-dipsotropic action. Therefore, to design and synthesize more potent anti-dipsotropic analogs, structural features that would enhance ALDH-2 inhibition and/or decrease MAO inhibition needed to be evaluated. Structure-activity-relationship (SAR) studies have revealed that a sufficient set of criteria for a potent anti-dipsotropic analog is an isoflavone with a free 4'-OH function and a straight-chain alkyl at the 7 position that has a terminal polar function such as -OH, -COOH, or -NH2. The preferable chain lengths for the 7-O-omega-carboxy, 7-O-omega-hydroxy, and 7-O-omega-amino substituents are 5 < or = n < or = 10, 2 < or = n < or = 6, and n > or = 4, respectively. Analogs that meet these criteria have increased potency for ALDH-2 inhibition and/or decreased potency for MAO inhibition and are, therefore, likely to be potent anti-dipsotropic agents.

Nonselective inhibition of monoamine oxidases A and B by activators of soluble guanylate cyclase.

Several activators of soluble guanylate cyclase were investigated as potential inhibitors of rat liver mitochondrial monoamine oxidases (MAO) A and B. They all fitted into the previously designed "molds" of substrate-inhibitor binding sites of these enzymes. However, only two of them, NO donors (7-nitro-benzotetrazine-1,3-dioxide (7-NBTDO) and benzodifuroxan), caused nonselective inhibition of MAO A and MAO B with IC(50) values of 1.3-1.6 and 6.8-6.3 microM, respectively. The inhibitory effect on both MAO A and MAO B was reduced by mitochondria wash suggesting reversible mode of the enzyme inhibition. There was no correlation between potency of MAO inhibition and activation of human platelet soluble guanylate cyclase. The NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO) had no effect on the manifestation of MAO inhibition by benzodifuroxan and 7-NBTDO; however, at 50 microM concentration carboxy-PTIO caused potent inhibition of MAO A with minor effect on MAO B activity. The data suggest that nonselective inhibition of MAO A and MAO B by benzodifuroxan and 7-NBTDO can be attributed to the properties of the chemical structures of these compounds. The results of the present study demonstrate a real possibility for the development of a new generation of effective reversible nonselective MAO inhibitors exhibiting equal inhibitory activity with respect to both MAO A and MAO B.

Xanthone derivatives as inhibitors for monoamine oxidase

Various xanthone derivatives were tested for in vitro inhibition of monoamine oxidase (MAO) in the mitochondrial fraction from rat brain and mouse liver. 1,6-Dihydroxyxanthone and 1,3,6-trihydroxyxanthone exhibited much potent inhibitory activity toward rat brain mitochondrial MAO (type A). On the other hand, 1,3,7-trihydroxyxanthone showed great potency for inhibition of mouse liver mitochondrial MAO (type B). These results indicate that 1,3,6-trihydroxyxanthone and 1,3,7-trihydroxyxanthone were rather specific inhibitors of type A MAO and type B MAO, respectively. 1,3-Dihydroxy-6-alkoxyxanthones containing alkoxy residues with carbon number 1 to 8 were synthesized and compared for their inhibitory activity toward rat brain mitochondrial MAO. Among them, 6-ethoxy-, 6-propyloxy-, and 6-butyloxy-derivatives were potent inhibitors, and especially, 1,3-dihydroxy-6-propyloxyxanthone was the most inhibitory. Lineweaver–Burk plot analysis demonstrates that these xanthone derivatives inhibited MAO in a competitive and reversible manner. Spectroscopic examinations suggest that complex formation between the flavin moiety and a xanthone derivative could be involved at least partly in their inhibitory action.

INFLUENCE OF ETHANOL ADMINISTRATION ON THE ACTIVITY AND COMPARTMENTATION OF RAT LIVER MONOAMINE OXIDASES

Single-dose ethanol administration to rats caused inhibition of liver mitochondrial monoamine oxidases (MAO) A and B, and an increase in susceptibility of MAO A (but not MAO B) to limited proteolysis. Chronic ethanol feeding resulted in a less distinct alteration in catalytic activity and susceptibility to proteolysis of mitochondrial MAO, but increased the amount of soluble MAO. The sensitivity of membrane-bound MAO to inhibitors (imipramine and chlorpromazine), action of which depends on their lipophilicity and/or hydrophobicity, remained unchanged, compared with controls. Increased amounts of soluble MAO seen after chronic ethanol feeding probably reflect an impairment of insertion of newly synthesized enzyme molecules into the outer mitochondrial membrane, rather than solubilization of MAO from it.

Inhibition of rat liver mitochondrial monoamine oxidase by hydrazine-thiazole derivatives: structure-activity relationships

The purpose of this research is to study the relationship between chemical structure and inhibitory activity of some hydrazine-thiazole derivatives on rat liver mitochondria monoamine oxidase (MAO). Forty-five compounds belonging to three series of hydrazine-thiazole derivatives, with either alkylic or arylic substituents in the thiazole ring, were tested. The highest inhibitory activity was observed with piperonyl derivatives 25 and 40, which contain a 4-methyl group in the thiazole nucleus. The structure-activity relationship of MAO inhibitors was established in relation to hydrophobic, electronic and steric hindrance parameters. A mechanism of enzyme inhibition was proposed based on the calculation of HOMO energies.

Irreversible inhibition of rat liver mitochondrial MAO A and MAO B by enantiomers of deprenyl and alpha-methylpargyline.

Using rat liver mitochondrial monoamine oxidase (MAO) A and MAO B, the possible influence of stereochemical factors upon the irreversible inhibition by propargylamine derivatives has been studied using the enantiomers of deprenyl and of alpha-methylpargyline. Whether studying the inhibition of MAO A or MAO B, little difference was found among enantiomeric pairs in the first-order rate constant (k2) for formation of the enzyme inhibitor adduct. Similarly, and with the exception of (S)-D-(+)-deprenyl (k2 = 0 or an extremely low value at MAO A), the computed value of k2 for the individual enantiomers showed little variation between MAO A and MAO B. These results suggest that inhibitor selectivity towards a particular form of the enzyme is determined predominantly at the competitive phase of the inhibition.

Aminium cation radical mechanism proposed for monoamine oxidase B catalysis: are there alternatives?

1. The interaction of bovine liver mitochondrial monoamine oxidase B (MAO B) with a series of benzylamine analogues was investigated to provide mechanistic information relative to the proposed cation radical mechanism and to provide information on the structural requirements of the substrate binding site. 2. Steady-state kinetic analysis of MAO B with 11 ring-substituted benzylamine analogues showed substitution does not alter the reaction pathway. All amine analogues tested exhibit sizeable deuterium kinetic isotope effects. 3. Anaerobic stopped-flow kinetic studies showed (1) C-H bond cleavage is rate-limiting in enzyme-bound flavin reduction and (2) that no specially detectable flavin radicals are observed. 4. The binding affinity of para-substituted benzylamine analogues to MAO B increased as the hydrophobicity of the substituent increased. In contrast, meta-substitution of the ring showed reduced affinity with an increase in the van der Waals volume of the substituent. 5. The rate of enzyme reduction by para-substitution exhibited a strong negative dependence with the Taft (Es) steric value of the substituent. In contrast, the rate of enzyme reduction by meta-substituted benzylamines is independent of the nature of the substituent. 6. para-Substituted N,N-dimethylbenzylamine analogues are not substrates for MAO B but are competitive inhibitors of benzylamine oxidation with a weaker affinity with increasing van der Waals volume of the substituent. In contrast, meta-substituted N,N-dimethyl benzylamine analogues are weak substrates for MAO B with oxidation occurring exclusively at the benzyl carbon. 7. The consequences of these results on the possible mechanisms (aminium cation radical, H abstraction, and nucleophilic mechanism) for C-H bond cleavage proposed for MAO B are discussed.

Structure-activity relationships in the oxidation of benzylamine analogues by bovine liver mitochondrial monoamine oxidase B.

The influence of para and meta substitution of benzylamine on its interaction with bovine liver mitochondrial monoamine oxidase B (MAO B) has been investigated by steady-state and reductive half-reaction anaerobic stopped-flow kinetic approaches. Steady-state kinetic properties of each benzylamine analogue suggest that para or meta substitution does not alter the mechanistic pathway of catalysis [Husain, M., et al. (1982) Biochemistry 21, 595-600]. All analogues tested exhibited Dkcat values ranging from 5.5 to 8.9 and D[kcat/Km(amine)] values ranging from 3.3 to 8.1 D[kcat/Km(O2)] values of approximately 1 are observed for all substrate analogues. Values for Kd were calculated from steady-state isotope effect data [Klinman, J.P., & Matthews, R.G. (1985) J. Am. Chem. Soc. 107, 1058-1060] and are in good agreement with Ks values determined from analysis of the rate of MAO B reduction as a function of benzylamine analogue concentration in reductive half-reaction experiments. A linear correlation of benzylamine analogue Kd values with the hydrophobicity parameter (phi) is observed for the para-substituted analogues where the binding affinity increases with increasing hydrophobicity of the substituent. Statistical treatment of the correlation shows a small negative contribution to binding by the van der Waals volume (VW) of the para substituent. meta-Substituted benzylamine analogues show a decreased binding affinity with the VW of the substituent and no correlation with the hydrophobicity value of the substituents tested. No spectral evidence was found for any flavin radical intermediates during the time course of MAO B flavin reduction in anaerobic reductive half-reduction stopped-flow experiments with any of the alpha,alpha-diprotio- or alpha,alpha-dideuteriobenzylamine analogues tested. The limiting rates of enzyme reduction exhibit large Dk values (6.5-14.1) for all of the analogues tested. para-Substituted benzylamine analogues reduce MAO B with limiting rates that correlate with the steric influence (Es value) of the substituent. Statistical analysis shows the rate of MAO B reduction by para-substituted analogues to be retarded by increased values of Es and, with a smaller contribution, by the hydrophobicity value of the substituent. The rate of MAO B reduction by meta-substituted benzylamine analogues is essentially independent of the nature of the substituent. No evidence was found for any electronic contribution to the rate of MAO B flavin reduction by any of the analogues tested. These data demonstrate the steric orientation of the substrate to be important in the rate of amine oxidation by MAO B and that ring meta substituents favor this orientation.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipid peroxidation affects catalytic properties of rat liver mitochondrial monoamine oxidases and their sensitivity to proteolysis

1.1. Lipid peroxidation (LPO) in rat liver mitochondria decreased the activity of monoamine oxidase (MAO) with physiological substrates serotonin and 2-phenylethylamine (by 15–30%) and induced deamination of glucosamine, which was highly sensitive to selective MAO A inhibitor pirlindole.2.2. The LPO-induced changes in catalytic properties of MAOs are accompanied by their increased susceptibility to trypsinolysis, however sensitivity to inhibition by imipramine, chlorpromazine and spermine are insignificantly changed.3.3. It is suggested that these results reflect LPO-induced conformational changes of enzyme molecules in membrane rather than their membrane topography.

Deuterium isotope effect of phenelzine on the inhibition of rat liver mitochondrial mono amine oxidase activity

Phenelzine is a suicide monoamine oxidase (MAO) inhibitor with antidepressant properties. The present study compares the inhibition of rat liver mitochondrial MAO by phenelzine and 1,1dideuterated phenelzine and the metabolism of these drugs by that enzyme. Phenylacetaldehyde, which was measured by a high performance liquid Chromatographic procedure, was found to be the major metabolite of phenelzine after incubation with MAO. The time-courses of aldehyde formation were non-linear due to the time-dependent inhibition of MAO. The reaction rate was reduced substantially when the hydrogen atom in the 1-carbon position was replaced by deuterium. The V H V D value was 3.1, indicating a primary isotope effect. Such a substitution of deuterium in the phenelzine molecule did not affect significantly the initial reversible inhibition of MAO, which was determined by comparison of their K i values. The irreversible inhibition, as estimated from ic 50 values, however, was potentiated substantially by deuteration. These results support the notion that the irreversible inhibition of MAO activity by phenelzine proceeds via a phenylethyldiazene intermediate, which reacts with the enzyme to form a covalent adduct. An alternative pathway involving hydrogen abstraction from carbon-1 of phenelzine or via rearrangement of the diazine on the enzyme surface could occur to form a phenylethylidene hydrazine intermediate which would subsequently be hydrolyzed to phenylacetaldehyde. The reduction in the rate of phenylethylidene hydrazine formation due to the isotope effect could lead to the accumulation of phenylethyldiazene intermediate and thus potentiate the inhibition of MAO activity.

Leakage of monoamine oxidase from rat liver after hepatic ischemia or administration of the hepatotoxin allyl formate.

Monoamine oxidase (MAO) activities in rat liver perfusate obtained by perfusion with sucrose buffer and lipid peroxide and superoxide dismutase (SOD) activity levels in liver tissue were investigated after pretreatment of rats with total hepatic ischemia or the perilobular hepatotoxin allyl formate. MAO activities in rat liver perfusate assessed with β-phenylethylamine and 5-hydroxytryptamine as substrates and liver tissue lipid peroxide of these rats were increased, but SOD activity was decreased. The accumulation of lipid peroxides was partly attributed to the decreased activity of SOD as a consequence of membrane disorders. When allyl formate was administered to the rats, amine oxidase activities in plasma were not completely inhibited by MAO inhibitors (clorgyline or deprenyl), and the activity measured with 1μM benzylamine as substrate was elevated. However, the complete inhibition of amine oxidase activities in the perfusates was obtainable and the activity toward 1μM benzylamine was not observed. The Km values of these amine oxidases for β-phenylethylamine and 5-hydroxytryptamine were the same as those of liver mitochondrial MAO. These results suggest that MAO in liver mitochondria was released into the blood as a consequence of membrane disorder but that two or more other distinct amine oxidase activities that were detected in the plasma were not released from the liver.

MPTP and MPTP analogs induced cell death in cultured rat hepatocytes involving the formation of pyridinium metabolites

MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) which has been shown to produce a Parkinson-like syndrome in humans and monkeys also causes cell death in cultures of rat hepatocytes. Treatment of cells with MPTP or its metabolite MPP + (1-methyl-4-phenyl pyridinium ion), resulted in leakage of lactic acid dehydrogenase and 14C-labeled adenine nucleotides, as well as marked depletion of ATP and glutathione. Deprenyl, a specific inhibitor of monoamine oxidase-B, the enzyme catalyzing the oxidation of MPTP into MPP +, blocked the lethal effect of MPTP, but gave no protection from MPP +-induced cell death. The 4′-fluoro and 4′-chloro analogs of MPTP evoked toxicities similar to that of the parent compound, while N-butyl-PTP, 4′-amino-MPTP, and 2′-methyl-MPTP were relatively less toxic. N-Acetylamino-MPTP was found virtually nontoxic. The cell death produced by these analogs was also associated with leakage of [ 14C]adenine nucleotides, which is an indicator of loss of ATP from cells. All these compounds except the N-acetylamino analog were converted to corresponding pyridinium metabolites by liver cells when analyzed by high-pressure liquid chromatography and plasma desorption mass spectrometry. MPTP and its analogs also served as substrates for rat liver mitochondrial monoamine oxidase to varying degrees. Toxicity of various analogs, with the noticeable exception of 2′-methyl-MPTP, was inhibited by deprenyl. These findings indicate that the conversion of MPTP and its analogs to corresponding pyridinium metabolites is essential for the expression of toxicity.

Inactivation of monoamine oxidase A by the monoamine oxidase B inactivators 1-phenylcyclopropylamine, 1-benzylcyclopropylamine, and N-cyclopropyl-alpha-methylbenzylamine.

Three known mechanism-based inactivators of beef liver mitochondrial monoamine oxidase (MAO) B are tested as inactivators of human placental mitochondrial MAO A. 1-Phenylcyclopropylamine (1-PCPA), 1-benzylcyclopropylamine (1-BCPA), and N-cyclopropyl-alpha-methylbenzylamine (N-C alpha MBA) are time-dependent irreversible inactivators of MAO A. The KI values for 1-PCPA and N-C alpha MBA, analogues of the MAO B substrate benzylamine, are much higher with MAO A than with MAO B. Evidence is presented to show that 1-PCPA inactivates MAO A by attachment to the flavin cofactor, unlike the reaction with MAO B in which 1-PCPA can attach to both a cysteine residue and the flavin [Silverman, R.B., & Zieske, P.A. (1985) Biochemistry 24, 2128-2138]. The reaction of 1-BCPA with MAO A was too slow to study in detail. N-C alpha MBA exhibits the same properties toward inactivation of MAO A that it does for inactivation of MAO B. Attachment in both cases is shown to be to one cysteine residue per enzyme molecule. The results with 1-PCPA indicate that the active site topographies of MAO A and MAO B are different. The ability of N-C alpha MBA to undergo attachment to a cysteine residue in both MAO A and MAO B may lead the way toward peptide mapping of the two isozymes in order to determine differences in their primary structures.

Electrophoretic properties of mitochondrial monoamine oxidase in monkey liver

Monkey liver monoamine oxidase (MAO) was predominantly the B-form enzyme from the observed differences in substrate specificities and differences in sensitivities to MAO inhibitors. It is known that a MAO inhibitor, pargyline, binds to MAO irreversibly in the molar ratio of 1:1. 3H-pargyline was used as a marker to determine the existence of MAO. The molecular weight of MAO in monkey liver mitochondria was investigated by SDS-polyacrylamide gel electrophoresis after solubilization of 3H-pargyline binding mitochondria with 6% sodium dodecyl sulphate (SDS). The subunit molecular weight was found to be 60,000. The molecular weight determined from the electrophoretic mobility on several concentrations of gels by disc gel electrophoresis in the absence of SDS was found to be 120,000. These results indicate that monkey liver mitochondrial MAO exists as a dimer. Isoelectric focusing of the enzyme after solubilization with 0.1% Triton X-100 and 0.75% Triton X-100 and 0.75% Lubrol showed that it had a pI value near 6.5. Similar pI values were obtained for enzyme preparations solubilized with 0.75% Triton X-100 after treatment with phospholipase A or methylethylketone. These results suggest that the pI value of MAO in monkey liver mitochondria does not depend on the properties of detergents used to solubilize the enzyme preparation.

Guanabenz as inhibitor of copper-containing amine oxidases.

The effects of guanabenz on some copper containing amine oxidases are described. Guanabenz 'in vitro' inhibits pig plasma benzylamine oxidase with a IC50 M 5.1 +/- 0.8 X 10(-6) M. It also inhibits pig kidney diamine oxidase and rat liver mitochondrial monoamine oxidase at higher concentrations. The significance of this property of guanabenz is discussed.