Liver MAO Research Articles

Inhibitory effects of Triton X-100 and some other detergents on a new type, distinct from type A and B, of monoamine oxidase in carp liver mitochondria.

We examined the inhibitory effects of Triton X-100 and some other detergents on a new type of MAO, distinct from both MAO-A and MAO-B, in carp liver mitochondria. SDS, octyl-β-D-glucopyranoside, sodium cholate and Triton X-100, at relatively low concentrations, caused strong inhibition of activity towards tyramine, but digitonin caused only weak inhibition. SDS, octyl-β-D-glucopyranoside and cholate caused almost complete inhibition of activity at the concentrations tested. The extent of inhibition by Triton X-100 was greater after preincubation at 37°C for 30 min than that without preincubation; but with or without preincubation, the inhibition was not substrate-selective and was not complete at 2% of detergent. Without preincubation, the mode of inhibition by Triton X-100 was competitive and reversible with respect to the oxidations of 5-HT, tyramine and β-phenylethylamine; but after preincubation, it became non-competitive and irreversible, depending on the concentration of detergent used. These findings support our previous conclusion on the homogeneity of carp liver MAO and suggest that Triton X-100 acts differently upon the enzyme, depending on both preincubation and the concentration of detergent used.

Monoamine oxidases A and B as components of a membrane complex.

The activities of monoamine oxidase A (MAO A) and monoamine oxidase B (MAO B) represent two independent types of substrate binding site, as indicated by experiments with selective inhibitors and also by substrate competition. We have tried to determine whether A and B active sites of human brain and liver MAO are located on physically separable enzyme forms or as subunits in large membrane-bound complexes. MAO was extracted from several sources by a procedure that was designed to give solubilized enzyme in high-speed supernatants, with ratios of MAO A/MAO B activities similar to those in initial crude homogenates. This solubilized enzyme gave gel filtration profiles that suggested the presence of large molecular complexes. Affinity binding experiments indicated that both MAO A and B activities may occur in the same complexes in tissues that initially contain both activities. These complexes were broken down to enzymatically active subunits by treatment with either low concentrations of sodium dodecyl sulfate, with phospholipase A2, or with a combination of both agents. Results of this study support a concept of MAO as part of a membrane unit in which A and B are two distinct enzymes embedded in a phospholipid structure. The enzymatic activity of MAO A is critically dependent on associated phospholipids, whereas that of MAO B is not.

Selective inhibition of type a monoamine oxidase by pyrazidol

Effect of a new antidepressant pyrazidol (1,10-trimethylene-8-methyl-1,2,3,4-tetrahydropyrazino /1,2-/ indole) on the liver and brain MAO activity of rats was studied in experiments in vivo and in vitro. Pyrazidol selectively blocks type A MAO (the substrates serotonin and noradrenaline) and does not virtually affect or has a far less action on type B MAO (the substrate 2-phenylethylamine).

Effects of L-DOPA administration upon monoamine oxidase activity in rat tissues

The effects of L-DOPA administration in various doses (250,500 and 1000 mg/kg for 7 days) upon type A and B MAO activities in rat tissues have been investigated using the substrates 5-HT, tyramine and benzylamine. The specific activities of MAO in heart, kidney and brain were significantly increased after L-DOPA, whereas liver and vas deferens MAO was unchanged. None of the observed changes was totally specific for either form of the enzyme, although some evidence for a slight selectivity on type A MAO of heart and type B MAO of kidney and brain was obtained. These results indicate that some tissues may respond to elevated intracellular catecholamine levels, resulting from L-DOPA administration, by increasing their capacity to deaminate those amines. However, at present, the exact mechanism by which these changes are brought about is unknown.

Monoamine Oxidase and Catechol‐O‐Methyl Transferase Activity in Tetrahymena*

Tetrahymena pyriformis strain HSM was found to have monomine oxidase (MAO) and a catechol-3-methyl transferase-like (COMT) activity. As in mammalian tissues, the MAO activity is predominantly localized in the mitochondrial pellet and COMT in the cytosol. The COMT-like activity was present in amounts comparable to several mouse tissues and was inhibited by tropolone. MAO activity was much lower than in any of the mouse tissues tested, and its activity varied greatly from preparation to preparation. The substrate preference of Tetrahymena MAO was tryptamine greater than serotonin greater than dopamine, and activity increased with increasing pH from pH 6.5 to pH 7.8, as does that of mouse liver MAO. Teh Km of Tetrahymena MAO for tryptamine was approximately 4 micrometer, an order of magnitude lower than that of mouse liver MAO. Sensitivity of inhibition by MAO inhibitors was variable. In some preparations, no inhibition was observed. In others clear inhibition was obtained, harmine and clorgyline being among the most potent inhibitors.

Studies on monoamine oxidase. (Report 37) Effects of oxygen concentration on rat liver and brain monoamine oxidase (author's transl)

MAO activity in rat brain mitochondria with tyramine as substrate at 100% oxygen concentration was three times as much as that at 20%. When serotonin served as substrate, difference in activities between the two oxygen concentrations was not significant. Similar results were obtained when rat liver MAO was used as the enzyme source. At 100% oxygen concentration, pargyline showed the most potent inhibition of MAO activity in liver mitochondria with tyramine as substrate, but inhibitions caused by pheniprazine and harmaline were not remarkable. At 100% oxygen concentration, harmaline showed the most potent inhibition of MAO activity in the liver when serotonin served as substrate, while inhibitions of the MAO activity by pargyline and pheniprazine were weak. At 20% oxygen concentration, harmaline showed the most potent inhibition of MAO activity in the brain when serotonin was used as substrate. These inhibitions were studied using Lineweaver-Burk plots. Pargyline revealed a noncompetitive inhibition to MAO activity in liver and brain with tyramine and serotonin as substrate, harmaline a competitive inhibition to MAO activity in liver and brain with tyramine as substrate, while noncompetitive inhibition to MAO activity in liver and brain was evident when serotonin was used as the substrate.

Studies on Monoamine Oxidase (Report XXIV) Effect of Harmine on Monoamine Oxidase

Using beef brain and liver mitochondria, the effects of several inhibitors on the oxidation of serotonin and tyramine were studied. Mitochondrial MAO of beef brain showed pH optima of 8.8 with serotonin and 8.0 with tyramine. Pheniprazine had similar effects on the oxidations of serotonin and tyramine by brain MAO at pH 7.4 and pH 8.5. Amphetamine inhibited serotonin oxidation much more than tyramine oxidation by brain MAO both at pH 7.4 and pH 8.5. Harmine selectively inhibited serotonin oxidation by brain MAO at pH 7.4. It inhibited serotonin oxidation by brain MAO competitively and tyramine oxidation uncompetitively. However, after heat treatment of mitochondrial MAO of brain at 60°C for 10 min, its effects on the oxidations of serotonin and tyramine were similar. The pI-activity curve of inhibition of oxidation of serotonin coincided with that of tyramine. The effect of harmine on MAO of brain mitochondria differed markedly before and after heat treatment: the pI-activity curve was biphasic before heat treatment but it became sigmoidal after heat treatment. These results suggest that there are at least two types of MAO in beef brain mitochondria: one heat labile, with low affinity for harmine, and the other rather heat stable, with high atnity for harmine. Harmine inhibited liver MAO only very slightly with either serotonin or tyramine as substrate.

Allotopic properties of human brain monoamine oxidase.

A NUMBER of preparations of monoamine oxidase (monoamine: O2 oxidoreductase (deaminating) E.C.1.4.3.4.) (MAO) have been shown to contain multiple forms of the enzyme which differ in their electrophoretic mobilities1, 2, inhibitor sensitivities3–5, thermal stabilities6 and, in some cases, immunological properties7–8. Little is known about the nature of these multiple forms although it has been suggested that they might be protein conformers9. The possibility that they might arise from the binding of different amounts or types of lipid material to a single enzyme species is, however, supported by the observation that the electrophoretically separable forms of rat liver MAO contain widely different amounts of phospholipid5, 10. In addition we have recently shown that treatment of a preparation of rat liver monoamine oxidase with the chaotropic agent sodium perchiorate seemed to render the enzyme homogeneous by a number of criteria without any significant loss of activity10. This treatment was shown to be accompanied by the release of lipid material from the enzyme preparation and, since chaotropic agents are known to weaken hydrophobic bonds between protein and lipid11, these results gave further support to the view that the multiple forms are caused by the association of lipid membrane material with a single enzyme.

Pig liver monoamine oxidase: Studies on the subunit structure

The molecular weight of pig liver MAO has previously been shown to be about 115,000 with 1 mole of covalently bound FAD per mole of enzyme. Gel filtration of purified enzyme on Sepharose 4B in 6 m guanidine and 0.1 m mercaptoethanol (MCE) and analytical ultracentrifugation in 0.1% sodium dodecyl sulfate (SDS) and 0.1% MCE yielded molecular weights of 55,000 and 63,000, respectively. By polyacrylamide electrophoresis in 0.1% SDS + MCE one band of 60,000 MW appeared. These results seem to imply that the enzyme is composed of two subunits of which one carries the active site. If MCE was omitted during the gel electrophoresis two equally large bands of about 60,000 MW were formed. By using enzyme inhibited by [ 14C]pargyline, a MAO-inhibitor blocking the active site of the enzyme in a 1:1 molar ratio, it was found, however, that both bands contained pargyline. Furthermore, amino acid analyses yielded the same amino acid composition of the two bands. The results are interpreted that the enzyme is composed of two subunits of identical molecular size (about 60,000) of which only one contains the active site and that the enzyme preparation contained two forms of the enzyme presumably differing in the number of disulfide bonds.

Effect of monoamine oxidase inhibitors on qualitative alterations in enzymatic properties of mitochondrial monoamine oxidases

Preincubation of highly purified ox liver MAO with specific MAOI pargyline, iproniazid or tranylcypromine prevents qualitative alteration (“transformation”) in enzymatic properties of MAO after treatment of the enzyme with oxidized oleic acid (OOA). Pretreatment with pargyline or tranylcypromine of highly purified rat liver MAO prevents qualitative alteration in its enzymatic properties after incubation with Cu 2+ in aerobic conditions. Pretreatment of rats with iproniazid prevents appearance in liver mitochondria of histamine-, putrescine- and l-lysine-deaminating activity after parenteral administration of OOA into the rats. It is possible that MAOI may find new fields of application based on their property to prevent qualitative alteration in enzymatic properties of monoamine oxidases in some pathological states (e.g. those accompanied by accumulation of lipid peroxides in tissues).

The effect of different substrates on the inhibition of rat brain and liver monoamine oxidase by arylalkylhydrazines

The structure-dependent action of arylalkylhydrazines on brain and liver MAO using benzylamine, tyramine, dopamine, tryptamine and serotonin as substrates was studied. For all the substrates used the inhibitory power of unsubstituted arylalkylhydrazines with an alkyl chain ranging from —CH 2— to (—CH 2—) 5 decreased with increasing carbon chain length up to four methylene groups. The substance with five carbon alkyl chain exerted approximately the same action as the compound with three carbon alkyl chain. All analogues of these substances methylated on the carbon adjacent to the hydrazine group showed potent inhibitory properties. The deamination of dopamine, serotonin and tyramine was affected more strongly by the majority of MAO inhibitors, than that of benzylamine and tryptamine.

Mechanistic studies of beef liver mitochondrial amine oxidase: XVIII. Amine oxidase

A possible mechanism for the action of beef liver MAO on benzylamine at pH 7.4 and 25 ° is presented. The pros and cons of the proposed formal mechanism are discussed. The mechanism was derived from the chemical, kinetic, and product inhibition data. If all the data reported by the various investigators to date are correct, and the different substrates used do not account for the observed differences in their results, it appears that the MAO isolated from different species proceed by different ping pong mechanisms.

Synthesis et activites biologiques de certains derives thyroxiniens et adrenergiques—VII Influence de divers derives phenoliques iodes sur la monoamine oxydase hepatique de cobaye et sur la monoamine oxydase cerebrale de rat

The action on hepatic and cerebral monoamine oxidase of several iodinated phenols derivated from 3:5-diiodotyramine and thyroid hormones and some of their structural analogues was studied. The enzymatic activity was tested by a variety of methods and in addition a new technique was developed using polarographic measurement of dissolved oxygen. Iproniazid was used as standard inhibitor. 3:5-Diiodotyramine is a potent inhibitor of guinea-pig liver MAO but no effect was obtained on rat cerebral MAO. Iodothyroacetic acids [3:5-diiodothyroacetic acid (TA 2) and 3:5:3′-triiodothyroacetic acid (TA 3)] and 3:5-diidothyroadrenaline- (TAd 2) were inhibitors of both liver and cerebral MAO. L-thyroxine (T 4) and 3:5:3′- triiodo- L- thyronine (T 3) no activity on liver MAO, whereas triiodothyronine has a relative effect on the cerebral enzyme.

Studies on serotonin metabolism in liver disease (II) serotonin concentration and mao activity in the organs of liver damaged rat

Studies on serotonin metabolism in liver disease (II) serotonin concentration and mao activity in the organs of liver damaged rat

Selective brain monoamine oxidase inhibition resulting from an interaction between reversible and irreversible inhibitors

Administration of BW 392C60 to rats before treatment with pheniprazine protects liver MAO but not brain MAO from pheniprazine. By proper selection of doses of the two drugs, it is possible to inhibit completely MAO activity in brain while maintaining high MAO activity in liver and certain other peripheral organs. Of the other long acting inhibitors tested, pargyline alone was effectively antagonized by BW 392C60, while tranylcypromine and iproniazid were only slightly affected.