Effects of zinc ion on type A monoamine oxidase in monkey brain mitochondria

The effects of ZnSO(4) on types A and B monoamine oxidase (MAO) isozymes in monkey brain mitochondria were investigated, in vitro. Type A MAO activity in monkey brain decreased to about 50% with 1 microM ZnSO(4) using serotonin as a substrate, and this inhibition was proportional to the concentration of ZnSO(4). ZnSO(4) had no effect, however, on type B MAO activity in monkey brain using beta-phenylethylamine as a substrate. The inhibition by ZnSO(4) of type A MAO activity was competitive and reversible. ZnSO(4) did not inhibit either type A or type B MAO activity in rat brain mitochondria. Almost similar results were also obtained when ZnCl(2) was used, in vitro. These results indicate that the inhibiting action of zinc ion differs depending on animal species and organ. Type A MAO in monkey brain mitochondria was highly sensitive to zinc ion, while type B activity was less sensitive.     

Styrene inhibits monoamine oxidase A, but not monoamine oxidase B in monkey brain mitochondria

The effects of styrene on mitochondrial monoamine oxidase (MAO) activity in rat and monkey brains were compared in vitro. After preincubation at 25 degrees C for 20 min with 1 mM styrene monomer MAO-A activity in monkey brain was inhibited potently using 5-HT (for MAO-A substrate), but MAO-B activity in monkey brain and platelets were slightly inhibited using beta-PEA (for MAO-B substrate). Styrene monomer also competitively inhibited MAO-A activity in a dose-dependent manner. MAO-A in monkey brain was inhibited by styrene in ascending order of potency: styrene trimer>styrene dimer>styrene monomer. In contrast styrene monomer slightly inhibited both MAO-A and MAO-B activities in rat brain mitochondria. In the present study styrene monomer potently inhibits MAO-A activity, but not MAO-B activity, in monkey brain mitochondria in vitro. These results indicate the inhibiting action of styrene differs depending on animal species and MAO isoforms.     

The inhibition of monoamine oxidase activity by various antidepressants: differences found in various mammalian species

The effects of the antidepressant drugs zimeldine, imipramine, maprotiline or nomifensine on mitochondrial monoamine oxidase (MAO) activity in mouse, rat, dog and monkey brains were compared in vitro. Mouse, rat, dog and monkey brain MAO-B activities were inhibited by zimeldine more potently than MAO-A activity. Imipramine inhibited MAO-B more potently than MAO-A activity in mouse and rat brains. When dog and monkey brains were investigated, MAO-A activity was inhibited more potently than MAO-B activity at high concentrations of imipramine, while at low concentrations, MAO-B activity was more potently inhibited. Maprotiline and nomifensine inhibited mouse and rat brain MAO-B activity more potently than MAO-A activity, while the inverse was true for dog and monkey brains. All four drugs are competitive inhibitors of MAO-A, but noncompetitive inhibitors of MAO-B in all animal brains. The respective Ki values of these reagents for monkey brain MAO-A and MAO-B were low compared to those of mouse, rat and dog. These results indicate that monkey brain MAOs are more sensitive to antidepressant drugs than those in rodent brain.     

Zinc benzoate, a contaminating environmental compound derived from polystyrene resin inhibits A-type monoamine oxidase

The contaminants in deionized and distilled water (DDI water) boiled with polystyrene resin inhibited A-type monoamine oxidase (MAO, MAO-A preferentially deaminates serotonin and norepinephrine and regulates these amines concentration) activity in monkey brain mitochondria. To identify these contaminants, we attempted measurements by HPLC, FT-IR and NMR. The compound inhibiting MAO-A activity was zinc benzoate. Although it potently inhibited MAO-A activity, zinc benzoate did not effect MAO-B in monkey brain mitochondria. It also reversibly and competitively inhibited MAO-A activity in a dose-dependent manner. Zinc benzoate, however, did not inhibit either MAO-A or -B activities in rat brain mitochondria. These results indicate that zinc benzoate, which inhibits MAO-A activity, is easily incorporated in DDI water by boiling polystyrene and also may be a contaminating environmental chemical compound that alters the levels of serotonin and norepinephrine in the central nervous system.     

Species differences in the deamination of dopamine and other substrates for monoamine oxidase in brain

Cortex and caudate specimens from human, non-human primate and rodent brains were examined for their ability to deaminate dopamine and for their sensitivity to irreversible monoamine oxidase (MAO) inhibitors. Using inhibition curves obtained with clorgyline, deprenyl and pargyline to estimate the relative proportions of MAO-A and MAO-B activity, dopamine was found to be deaminated predominantly by MAO-A in rat cortex and caudate. In contrast, dopamine was primarily an MAO-B substrate in human and vervet cortex and caudate. When clorgyline inhibition curves with tyramine or dopamine as substrate were compared in human, vervet and rat cortex, more pronounced species differences were found with dopamine than with tyramine. In all three species caudate tended to be more sensitive to inhibition by low concentrations of clorgyline than was cortex, suggesting a higher proportion of MAO-A activity in caudate. Similar species differences were also found when MAO-A activities were estimated using serotonin (5-HT): -phenylethylamine (PEA) ratios (5-HT/5-HT + PEA). These ratios with selective substrates were highly correlated with clorgyline inhibition curves obtained with tyramine as substrate across 29 brain regions and tissues from different rodent and primate species (r=0.85, P<0.001). Data from both the substrate ratios and the clorgyline inhibition curves confirmed the relative predominance of MAO-B activity in primate brain regions (70–85%) as compared to rat brain regions (45%). Smaller species differences were observed in liver. Species differences in the proportion of brain MAO-A and B activities and in the deamination of dopamine and other substrates for MAO may have important implications in regard to the widespread use of rodent rather than primate models in the study of biogenic amine metabolism and of drugs affecting amine function.     

Enzymic and molecular characteristics of a new form of monoamine oxidase, distinct from form-A and form-B

The present study was undertaken to clarify the enzymic and molecular properties of monoamine oxidase (MAO) in carp brain. In particular, its sensitivities to selective MAO inhibitors, kinetic properties and molecular weight were compared with those of the enzyme in carp liver. The selective and potent MAO-A and MAO-B inhibitors FLA 788(+), FLA 336(+), MD 780236 and benzylcyanide caused dose-dependent inhibitions of MAO activity in both carp brain and liver; the inhibition curves were all single-sigmoidal, and the degrees of inhibition of the activities towards 5-hydroxytryptamine (5-HT, selective MAO-A substrate), tyramine (substrate for both forms of MAO) and beta-phenylethylamine (PEA, selective MAO-B substrate) were similar. This was also the case for inhibition of activity in carp brain by the irreversible and selective MAO-A and MAO-B inhibitors clorgyline and I-deprenyl, indicating the presence in both preparations of a single MAO which differs from either form of MAO. Studies on the substrate specificities and Km values for these three substrates and the inhibitory effects of some compounds suggested that the enzymic characters of MAO in carp preparations were similar and that these enzymes might be FAD-containing enzymes, like MAO in various mammals. By labelling the preparations with radioactive pargyline and then subjecting them to sodium dodecyl sulfate electrophoresis, the apparent molecular weights of carp brain and liver MAO were estimated as 60,000 daltons. The same value was also obtained for rat brain and liver mitochondrial MAO-B. These results indicate that by the present definitions of MAO-A and MAO-B, MAO in carp brain and liver is similar to, but distinct from, both these forms of MAO.     

Inhibition of rat brain monoamine oxidase activities by psoralen and isopsoralen: implications for the treatment of affective disorders

Psoralen and isopsoralen, furocoumarins isolated from the plant Psoralea corylifolia L., were demonstrated to exhibit in vitro inhibitory actions on monoamine oxidase (MAO) activities in rat brain mitochondria, preferentially inhibiting MAO-A activity over MAO-B activity. This inhibition of enzyme activities was found to be dose-dependent and reversible. For MAO-A, the IC50 values are 15.2 +/- 1.3 microM psoralen and 9.0 +/- 0.6 microM isopsoralen. For MAO-B, the IC50 values are 61.8 +/- 4.3 microM psoralen and 12.8 +/- 0.5 microM isopsoralen. Lineweaver-Burk transformation of the inhibition data indicates that inhibition by both psoralen and isopsoralen is non-competitive for MAO-A. The Ki values were calculated to be 14.0 microM for psoralen and 6.5 microM for isopsoralen. On the other hand, inhibition by both psoralen and isopsoralen is competitive for MAO-B. The Ki values were calculated to be 58.1 microM for psoralen and 10.8 microM for isopsoralen. These inhibitory actions of psoralen and isopsoralen on rat brain mitochondrial MAO activities are discussed in relation to their toxicities and their potential applications to treat affective disorders.     

Some factors influencing the metabolism of benzylamine by type A and B monoamine oxidase in rat heart and liver

The ability of MAO-A and MAO-B to metabolize benzylamine in vitro has been investigated in mitochondrial preparations from rat liver and heart. Although under normal circumstances benzylamine appeared to be metabolized exclusively by MAO-B in the rat liver, a contribution by both MAO-A and a clorgyline-resistant enzyme component was revealed when the MAO-B activity was much reduced by pretreatment of the mitochondria with appropriate concentrations of deprenyl. These three enzyme activities also contributed to benzylamine deamination in rat heart mitochondria. However, binding studies with [3H]pargyline, which provided an estimate of the respective concentrations of MAO-A and MAO-B active centres in heart mitochondria, indicated a ratio between MAO-A and MAO-B, markedly different from that shown by plots of inhibition of benzylamine metabolism by various concentrations of clorgyline. The interpretation of these clorgyline plots is discussed in terms of the kinetic constants of both MAO-A and MAO-B, and the relative amounts of each enzyme. It is proposed that although the turnover rate constant for benzylamine metabolism by MAO-A is much smaller than that shown by MAO-B, in those tissues containing a large ratio of MAO-A:MAO-B content, the metabolism of benzylamine by MAO-A can be detected.     

Some factors influencing the metabolism of benzylamine by type A and B monoamine oxidase in rat heart and liver

The ability of MAO-A and MAO-B to metabolize benzylamine in vitro has been investigated in mitochondrial preparations from rat liver and heart. Although under normal circumstances benzylamine appeared to be metabolized exclusively by MAO-B in the rat liver, a contribution by both MAO-A and a clorgyline-resistant enzyme component was revealed when the MAO-B activity was much reduced by pretreatment of the mitochondria with appropriate concentrations of deprenyl. These three enzyme activities also contributed to benzylamine deamination in rat heart mitochondria. However, binding studies with [³ H]pargyline, which provided an estimate of the respective concentrations of MAO-A and MAO-B active centres in heart mitochondria, indicated a ratio between MAO-A and MAO-B, markedly different from that shown by plots of inhibition of benzylamine metabolism by various concentrations of clorgyline. The interpretation of these clorgyline plots is discussed in terms of the kinetic constants of both MAO-A and MAO-B, and the relative amounts of each enzyme. It is proposed that although the turnover rate constant for benzylamine metabolism by MAO-A is much smaller than that shown by MAO-B, in those tissues containing a large ratio of MAO-A:MAO-B content, the metabolism of benzylamine by MAO-A can be detected.     

Inhibition of rat brain monoamine oxidase activity by cerebral anti-ischemic agent, ifenprodil

Ifenprodil, which is clinically used as a cerebral vasodilator, inhibited rat brain type A (MAO-A) and type B (MAO-B) monoamine oxidase activity. It did not, however, affect rat lung semicarbazide-sensitive amine oxidase. The degree of inhibition of either form of MAO was not changed by 30 min preincubation of the enzyme preparations at 37 degrees C with ifenprodil. Modes of inhibition of MAO-A and MAO-B by ifenprodil were competitive towards oxidation of their respective substrates, 5-hydroxytryptamine and benzylamine, with Ki values of 75 microM for inhibition of MAO-A and 110 microM for inhibition of MAO-B.     

Effect of lignocaine on monoamine oxidase activity of brain and liver

In vivo administration of a single dose (100-150 mg/kg, i.p.) of lignocaine produces no change in MAO activity, while long-term treatment (50 mg/kg/day for 15 and 30 consecutive days, i.p.) produces a slight but appreciable inhibition of MAO activity with tyramine or serotonin but not with benzylamine as substrate in both rat brain and liver mitochondria. Lignocaine (2-20 mM) inhibits (in vitro) both brain and liver mitochondrial MAO activity, using tyramine, serotonin and benzylamine as substrates, in a concentration-dependent manner. Furthermore, lignocaine produces a marked in vitro inhibition of serotonin and tyramine oxidation in MAO-A and not in MAO-B preparation of rat brain. Ackermann-Potter plots of MAO indicate that lignocaine-induced inhibition of MAO activity is reversible in nature. Lineweaver-Burk plots show that lignocaine (2-10 mM) produces a significant increase in Km and decrease in Vmax of MAO for tyramine and serotonin in both brain and liver. Similarly Km and Vmax values are changed using benzylamine as substrate in the presence of relatively higher concentrations of lignocaine (5-20 mM). These results suggest that lignocaine-induced inhibition of mitochondrial membrane-bound MAO activity of both neuronal and non-neuronal tissues is associated with its conformational change.     

Effect of diethylnitrosamine on monoamine oxidase in rat liver

The effect of diethylnitrosamine (DEN), a well-known experimental carcinogen, toward MAO-A and MAO-B activity of rat liver was investigated. The oxidations of both beta-PEA (MAO-B) and 5-HT (MAO-A) were inhibited by DEN. The K1 values of DEN in the inhibition of rat liver MAO-A and MAO-B activity were determined. The kinetic data show that DEN is a competitive, MAO-B selective inhibitor and its inhibitory effect on MAO-B is about 4-fold more potent than that on MAO-A. DEN might change the proportions of the multiple forms of MAO activity in tumor cells.     

Inhibition of brain mitochondrial monoamine oxidases by the endogenous compound 5-hydroxyoxindole

5-Hydroxyoxindole is a recently identified endogenous compound. Its physiological role remains unclear but certain evidence exists, that it may share some regulatory properties with isatin, a known endogenous inhibitor of monoamine oxidase (MAO) type B (MAO-B). In this study several oxidized indoles were tested for their in vitro inhibition of MAO type A (MAO-A) and B of rat brain non-synaptic mitochondria. 5-Hydroxyoxindole was less potent MAO-A inhibitor (IC50 56.8 microM) than isatin (31.8 microM) and especially 5-hydroxyisatin (6.5 microM), but it was the only highly selective MAO-A inhibitor among the all compounds studied (IC50 MAO-A:IC50 MAO-B = 0.044). Thus, the in vitro data suggest that MAO-A may represent potential target for 5-hydroxyoxindole.     

Interaction of psychotropic drugs with monoamine oxidase in rat brain

Literature observations indicate that some psychotropic drugs may have inhibitory activity towards monoamine oxidase (MAO). This study was undertaken to assess the potency, isozyme selectivity and mechanism of inhibition of representative first- and second-generation antidepressant drugs towards rat brain MAO-A and MAO-B. Five tricyclic antidepressants (imipramine, trimipramine, clomipramine, amitriptyline and doxepine) and three selective serotonin reuptake inhibitors (fluoxetine, fluvoxamine and citalopram) were examined. They showed inhibitory activity towards MAO-A and MAO-B, with clear selectivity for MAO-B (Ki in the micromolar range). Their mechanism of inhibition was competitive towards MAO-B and of a mixed competitive type towards MAO-A. The results suggest that some of the drugs examined might also contribute an MAO inhibitory effect in chronically treated patients.     

The effects of peptide and nonpeptide angiotensin II-receptor ligands on the activity of brain monoamine oxidase in water-repleted rats

The effects of i.c.v. administered peptide and nonpeptide ANG II-receptor ligands (losartan, EXP 3174, saralasin and sarmesin) on monoamine oxidase A (MAO-A) and MAO-B activities in the frontal cortex, striatum, hypothalamus and hippocampus of water-repleted rats were investigated. Alterations in MAO-A and MAO-B activities were found in different rat brain regions after ANG II which depended on the isoenzyme type and brain structure. MAO-A activity significantly increased in the frontal cortex and hypothalamus, brain regions containing AT1 receptors, mainly. MAO-A and MAO-B activities were affected differently by all studied ANG II-receptor ligands, which in most cases antagonized the effect of ANG II (losartan, an AT1-nonpeptide receptor antagonist being the most effective). There was no clearcut relationship between the inhibition of ANG II-induced water intake and the changes of MAO-A and MAO-B activities under the effect of the ANG II-receptor antagonists studied.     

Rasagiline [N-propargyl-1R(+)-aminoindan], a selective and potent inhibitor of mitochondrial monoamine oxidase B

1. Rasagiline [N-propargyl-1R(+)-aminoindan], was examined for its monoamine oxidase (MAO) A and B inhibitor activities in rats together with its S(-)-enantiomer (TVP 1022) and the racemic compound (AGN-1135) and compared to selegiline (1-deprenyl). The tissues that were studied for MAO inhibition were the brain, liver and small intestine. 2. While rasagiline and AGN1135 are highly potent selective irreversible inhibitors of MAO in vitro and in vivo, the S(-) enantiomer is relatively inactive in the tissues examined. 3. The in vitro IC(50) values for inhibition of rat brain MAO activity by rasagiline are 4.43+/-0.92 nM (type B), and 412+/-123 nM (type A). The ED(50) values for ex vivo inhibition of MAO in the brain and liver by a single dose of rasagiline are 0.1+/-0.01, 0.042+/-0.0045 mg kg(-1) respectively for MAO-B, and 6.48+/-0.81, 2.38+/-0.35 mg kg(-1) respectively for MAO-A. 4. Selective MAO-B inhibition in the liver and brain was maintained on chronic (21 days) oral dosage with ED(50) values of 0.014+/-0.002 and 0.013+/-0.001 mg kg(-1) respectively. 5. The degree of selectivity of rasagiline for inhibition of MAO-B as opposed to MAO-A was similar to that of selegiline. Rasagiline was three to 15 times more potent than selegiline for inhibition of MAO-B in rat brain and liver in vivo on acute and chronic administration, but had similar potency in vitro. 6. These data together with lack of tyramine sympathomimetic potentiation by rasagiline, at selective MAO-B inhibitory dosage, indicate that this inhibitor like selegiline may be a useful agent in the treatment of Parkinson's disease in either symptomatic or L-DOPA adjunct therapy, but lack of amphetamine-like metabolites could present a therapeutic advantage for rasagiline.     

Studies of monoamine oxidase and semicarbazide-sensitive amine oxidase. I. Inhibition by a selective monoamine oxidase-B inhibitor, MD 780236

In vitro studies of the effect of MD 780236, a selective monoamine oxidase (MAO)-B inhibitor, on a semicarbazide-sensitive amine oxidase (SSAO) in rat testis and lung showed that this compound dose-dependently inhibited SSAO activity. The extents of inhibition of MAO-A, -B and SSAO in these two rat tissues by this compound after 30 min of preincubation were found to be MAO-B greater than MAO-A greater than SSAO. This selectivity was also evident in preparations without preincubation. Degree of inhibition of SSAO was not significantly influenced by pretreatment with either 10(-3) M clorgyline, I-deprenyl or 10(-4) M SKF 525A. Inhibition of SSAO was not enhanced by varying the time of preincubation of the enzyme and the compound, indicating direct action on and reversible inhibition of SSAO. The inhibition of SSAO by MD 780236 was non-competitive with or without preincubation, with a K1 value of 110 muM. Although MD 780236 is a selective and "suicide substrate" inhibitor of MAO-B, these present results indicate that this compound may also inhibit SSAO activity, but by a mechanism different from that for MAO-B. These findings confirm an earlier hypothesis that compounds that inhibit both MAO and SSAO have totally different modes of action on these two different amine oxidases.     

Drug-induced changes in motor activity after selective MAO inhibition

The increase in motor activity produced in mice by phenylethylamine (PEA), L-DOPA and amphetamine was evaluated after selective inhibition of MAO Type A (by clorgyline) or Type B (by low doses of pargyline). PEA-induced motor stimulation was intensified in the presence of MAO-B inhibition, but not when MAO-A was inhibited. This was paralleled by higher concentrations of brain and plasma PEA (after injection) in mice in which there was inhibition of MAO-B compared with control or MAO-A inhibition. Conversely, L-DOPA produced significant stimulation only when MAO-A was inhibited. The clorgyline pretreatment resulted in larger increases in brain dopamine concentrations (in the striatum, olfactory tubercles and in the area containing the substantia nigra) than did MAO-B inhibition. This effect occurred both in mice receiving L-DOPA + inhibitor and in mice receiving the inhibitor alone. Amphetamine-induced stimulation was increased following the inhibition of either form of MAO, and this was not the result of changes in the distribution or metabolism of amphetamine. These results support the concept that MAO-A and MAO-B deaminate different substrates in the rodent CNS and that amphetamine may utilize either dopamine or PEA in producing its stimulant effects.     

Synthesis,Toxicological, and Pharmacological Assessment of Derivatives of 2-Aryl-4-(3-arylpropyl)morpholines

The synthesis of nine original morpholine derivatives, i.e. 2-aryl-4-(3-arylpropyl)morpholines, is described. The structure of all synthesised derivatives was proved by IR and ¹H-NMR, and some of them by ¹³C-NMR. Acute toxicity studies of the compounds were performed on mice. A comparative pharmacological study of the in vivo effects on the central nervous system was undertaken using the screening tests: hexobarbital induced sleeping time; locomotor activity; and behaviour despair (for antidepressive activity). The most active compound 4-(2-benzoylethyl)-2-phenyl-3-methyl) morpholine 4e was studied for MAO-A and MAO-B inhibition in vitro in rat brain mitochondria preparations.     

Studies of monoamine oxidases: Effect of Triton X-100 and bile salts on monoamine oxidase in brain mitochondria

Triton X-100 and the bile salts, cholate and deoxycholate, detergents often used in the solubilization of monoamine oxidase (MAO) from mitochondria, have been found to cause an inhibition of the enzyme activity. With beef brain mitochondria, it was found that there was a differential effect of Triton X-100 on the putative MAO types A and B, with MAO-A being more susceptible to inhibition by Triton X-100. This was indicated by the greater loss of serotonin-deaminating than of phenyl ethylamine-deaminating activity in the presence of Triton X-100. Although the bile salts also caused substantial inactivation at concentrations above 0.1%, no differentiation between MAO types could be made. Kinetic studies of the inhibition by Triton X-100 indicated two different mechanisms were occurring with the two MAO types. The inhibition was competitive for MAO-A, but uncompetitive for MAO-B. Removal of Triton X-100 by co-polymer beads restored some, but not all of the activity for both MAO-A and MAO-B types. This suggests that the activity loss may have been due in part to inactivation when the enzyme was separated from the mitochondrial membrane.