The effect of age and thyroid hormones upon the ability of the chick heart to deaminate monoamines.

The effect of age and thyroid hormones upon the ability of chick heart homogenates to metabolize monoamines has been investigated. 5-Hydroxytryptamine is entirely metabolized by a monoamine oxidase (MAO) with the characteristics of MAO-A, whereas some of the tyramine and all of the benzylamine are oxidatively deaminated by a clorgyline-resistant, but semicarbazide-sensitive enzyme, with a similar subcellular distribution to that of MAO. The remainder of the tyramine deamination is brought about by MAO-A and MAO-B. The specific activities of both clorgyline-sensitive and resistant enzymes are increased by the same proportion by increase in age or by treatment with (--)-thyroxine, and decreased by 2-thiouracil. The significance of these results is discussed.     

Selective influences of age and thyroid hormones on type A monoamine oxidase of the rat heart.

The specific actiivty of rat heart MAO, towards both tyramine and benzylamine as substrates, was found to increase with the age of the animal, and also after administration of (-)-thyroxine to young male rats. Conversely, enzyme activity was decreased in animals made hypothyroid by including 2-thiouracil in their diet. However, with both age and altered thyroid status, relatively greater changes in the deamination of tyramine rather than in that of benzylamine, were obtained. Clorgyline and deprenyl, used as inhibitors of rat heart MAO, indicated that tyramine is metabolized solely by MAO-A, whereas benzylamine is a substrate for both MAO-A and -B, and also a clorgyline- and deprenyl-resistant enzymic activity. The proportional contribution of MAO-A, -B and the clorgyline-resistant enzyme towards the total benzylamine deamination in the rat heart was found to vary with the age and with altered thyroid status of the animal in such a way that selective changes in the activity of MAO-A appear to be largely responsible for the overall changes in the specific activity of rat heart MAO which occur in response to these developmental factors.     

Selective influences of age and thyroid hormones on Type A monoamine oxidase of the rat heart

The specific activity of rat heart MAO, towards both tyramine and benzylamine as substrates, was found to increase with the age of the animal, and also after administration of (?)-thyroxine to young male rats. Conversely, enzyme activity was decreased in animals made hypothyroid by including 2-thiouracil in their diet. However, with both age and altered thyroid status, relatively greater changes in the deamination of tyramine rather than in that of benzylamine, were obtained. Clorgyline and deprenyl, used as inhibitors of rat heart MAO, indicated that tyramine is metabolized solely by MAO-A, whereas benzylamine is a substrate for both MAO-A and -B, and also a clorgyline- and deprenyl-resistant enzymic activity. The proportional contribution of MAO-A, -B and the clorgyline-resistant enzyme towards the total benzylamine deamination in the rat heart was found to vary with the age and with altered thyroid status of the animal in such a way that selective changes in the activity of MAO-A appear to be largely responsible for the overall changes in the specific activity of rat heart MAO which occur in response to these developmental factors.     

Tyramine infusions and selective monoamine oxidase inhibitor treatment

The relationship between changes in IV tyramine pressor sensitivity accompanying selective monoamine oxidase (MAO) inhibitor treatment and estimates of MAO-A and MAO-B inhibition in vivo were studied. Reductions in platelet MAO activity provided an index of MAO-B inhibition, while changes in plasma 3-methoxy-4-hydroxyphenethylene glycol (MHPG) were used as an hypothesized reflection of MAO-A inhibition. Chronic treatment with the MAO-A inhibitor clorgyline and the MAO-B inhibitor pargyline showed significant inhibition of the alternate MAO enzyme as well, although this crossover effect was greater for pargyline than clorgyline. The MAO-B inhibitor deprenyl appeared to maintain the greatest degree of MAO inhibition selectivity in vivo. Tyramine pressor sensitivity changes accompanying administration of the MAO inhibitors were highly correlated with decreases in plasma MHPG (r=0.92), supporting our previous data indicating the rank order of clorgyline > pargyline > deprenyl for enhancement of tyramine pressor sensitivity and, thus, suggesting that tyramin potentiation is primarily a function of MAO-A rather than MAO-B inhibition. Changes in plasma MHPG are suggested to provide a potentially useful clinical index of in vivo MAO-A inhibition.Presently with the Biological Psychiatry Branch, NIMH     

Deamination of hordenine by monoamine oxidase and its action on vasa deferentia of the rat

The selectivity of the naturally occurring amine, N,N-dimethyltyramine (hordenine) for monoamine oxidase (MAO) and its action upon isolated vasa deferentia of the rat was investigated. Hordenine was deaminated by rat liver MAO with a Michaelis constant of 479 microM and maximum velocity of 128 nmol (mg protein)-1 h-1 compared with 144 microM and 482 nmol (mg protein)-1 h-1 for tyramine. Studies, with selective irreversible inhibitors of MAO, showed that hordenine was a highly selective substrate for MAO-B of liver and that it was not deaminated by the MAO-A of intestinal epithelium. In contrast to tyramine, hordenine did not produce contractions of isolated vasa deferentia. However, 25 microM hordenine potentiated contractile responses of vasa, from control animals, to submaximal doses of noradrenaline and inhibited responses to tyramine. It did not alter responses, to noradrenaline, of vasa denervated by chronic pretreatment of rats with guanethidine. Therefore, it appears that hordenine acted as an inhibitor of noradrenaline uptake, in isolated vasa deferentia. These results indicate that dietary-hordenine is unlikely to be deaminated by intestinal MAO as this is predominantly MAO-A. Consequently, it is likely to be absorbed and could affect the sympathetic nervous system, by virtue of its action as an inhibitor of noradrenaline uptake.     

The comparative influence of pyrazidol, inkazan and other antidepressant monoamine oxidase inhibitors on the pressor effect of tyramine

In experiments on conscious normotensive male Wistar rats the new antidepressants, reversible MAO-A inhibitors, pyrazidole and incazane, as well as moclobemid increased the pressor effect of orally administered tyramine. The drugs potentiated also the pressor effect of intravenous tyramine. More prolonged potentiation of tyramine action was produced by moclobemid, less prolonged by incazane. The potentiation by the studied MAO-A inhibitors of the pressor effect of tyramine reflects the inhibition of the activity of MAO-A and the first-pass metabolism of tyramine in the gut and liver, as well as the inhibition of intraneuronal MAO activity in noradrenergic nerve endings and the potentiation of sympathetic activity.     

Substrate and inhibitor selectivity of human heart monoamine oxidase.

The subcellular distribution, inhibitor sensitivity, thermostability and pH profiles of monoamine oxidase (MAO) from samples of human heart obtained at post mortem have been investigated with several substrates. A simple subcellular fractionation showed that, with either tyramine or benzylamine as substrate, about 50 per cent of the MAO activity was found in the mitochondrial fraction, with negligible quantities in the high speed supernatant. From the use of clorgyline, it appears that 5-HT is a substrate for MAO-A, benzylamine and β-phenethylamine are substrates for MAO-B, while tyramine and dopamine are substrates for both forms of the enzyme, d-Amphetamine was shown to be a selective competitive inhibitor of MAO-A, of similar potency to that observed with MAO from rat liver. No significant difference between the thermostability at 50° of the MAO activity towards 5-HT and benzylamine was observed. Preliminary results for the effect of pH on human heart MAO are presented. The results are discussed with respect to similar data obtained for MAO from other human and animal tissues.     

Comparative lipophilicities of substrates of monoamine oxidase

Oil/water partition coefficients of various substrates of monoamine oxidase (MAO) and kinetic parameters of MAO-A and -B of rat liver at two pH values, pH 7 and pH 9, were investigated. Octanol, heptane or benzene were chosen for the oil phases. The deamination of the biogenic amines 5-hydroxytryptamine (5-HT), tyramine, 2-phenethylamine (PEA) and benzylamine was studied at pH 7 and pH 9. Results indicated all four substrates were very hydrophilic, and the oil/water partition coefficients of benzylamine and PEA were higher than those of 5-HT and tyramine. The changes in Km and Vmax values at pH 7 and pH 9 indicated that the affinities of MAO-A towards 5-HT and tyramine slightly increased at pH 9 and those of MAO-B towards tyramine and benzylamine also increased at pH 9, while uncharged amines at pH 9 amounted to about a hundred times of those at pH 7. It is concluded that the mitochondrial MAO bound to the membrane may metabolize charged molecules as well as uncharged counterparts.     

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.     

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.     

The effect of lipophilic compounds upon the activity of rat liver mitochondrial monoamine oxidase-A and -B

The effect of various lipophilic compounds on the activity of monoamine oxidase (MAO) was determined. The local anaesthetics procaine, procainamide, tetracaine and lignocaine were all MAO-A selective inhibitors, whereas benzyl alcohol, butan-l-ol, hexan-l-ol and octan-l-ol inhibited MAO-B selectively. Procaine was found to be a competitive inhibitor of the deamination of 5-hydroxy-tryptamine (5-HT), tyramine, β-phenethylamine and benzylamine. Benzyl alcohol was competitive towards β-phenethylamine and benzylamine, but a mixed-type inhibitor towards 5-HT and tyramine. The same patterns of inhibition for both drugs were found when the activity was assayed under atmospheres of either oxygen or air. The inhibition produced by both compounds was fully reversible. Triton X-100 appeared to inhibit the activity of MAO-A selectively when preincubated with the enzyme for 30 min at 30°. This selectivity was lost when the preincubation temperature was raised to 37°. The inhibition of MAO activity by Triton X-100 after preincubation at 37° was found to be irreversible. Sodium deoxycholate and SDS were also found to inhibit the activity of MAO after preincubation with the enzyme at 37°. The significance of these results is discussed.     

Modification of blood pressure and nictitating membrane response to sympathetic amines by selective monoamine oxidase inhibitors, types A and B, in the cat.

The selective monoamine oxidase (MAO) inhibitors clorgyline, selegiline and AGN 1135 did not cause a change in responses of the cat nictitating membrane to preganglionic sympathetic nerve stimulation at 5 Hz. Both selective MAO-A and MAO-B inhibitors markedly potentiated nictitating membrane contractions in response to beta-phenylethylamine (PEA). However, the responses to tyramine were unchanged. The pressor responses to tyramine were potentiated by the selective MAO-A inhibitor clorgyline (2 mg kg-1) but not by selegiline (1.0 mg kg-1) and AGN 1135 (1.5 mg kg-1), selective MAO-B inhibitors. At the doses used selegiline and AGN 1135 caused a near total selective inhibition of liver and brain MAO-B, while clorgyline inhibited MAO-A only in the brain. AGN 1135, like selegiline, could be a useful drug in potentiating the action of L-DOPA in Parkinson's disease.     

Cardiovascular sympathomimetic amine interactions in rats treated with monoamine oxidase inhibitors and the novel oxazolidinone antibiotic linezolid

Linezolid (PNU-100766) is a new gram-positive oxazolidinone antibiotic that is effective at in vitro concentrations < or =4 microg/ml and in vivo doses < or =10 mg/kg. Because linezolid also competitively inhibits human monoamine oxidase-A (MAO-A; Ki = 55 microM), we monitored its effects on the cardiovascular responses to tyramine and amine cold remedies in comparison with standard MAO inhibitors. In anesthetized rats, the pressor response to 16 microg i.v. tyramine was potentiated by the MAO-A inhibitors clorgyline (0.1-1.0 mg/kg i.v.) and moclobemide (5.0-50 mg/kg p.o.), but not by the MAO-B inhibitor selegiline (0.15-15 mg/kg p.o.). Fifteen milligrams per kilogram intravenous linezolid weakly potentiated i.v. tyramine independent of changes in alpha-adrenoceptor reactivity, but this effect was not enhanced chronically (90-100 mg/kg/day). In conscious rats, 30 mg/kg/day oral linezolid (8 microg/ml plasma concentration) minimally affected the pressor response to 20 mg/kg oral tyramine, whereas 100 mg/kg/day linezolid (20 microg/ml plasma concentration) moderately potentiated this response similar to 3 mg/kg per day moclobemide. Linezolid's tyramine potentiation was reversible, attenuated by food, and independent of pseudoephedrine, phenylpropanolamine, and dextromethorphan interactions. These studies demonstrate that high-dose linezolid only moderately potentiates the cardiovascular effects of tyramine and validate these models for evaluating such MAO inhibitory interactions.     

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.     

The kinetics of monoamine oxidase inhibition by three 2-indolylmethylamine derivatives

The inhibition of bovine brain mitochondrial MAO-A and MAO-B by three acetylenic and non-acetylenic derivatives of 2-indolylmethylamine, chosen among more than 100 new compounds, were studied. The non-acetylenic derivative N-methyl-2(5-hydroxy-1-methylindolyl)methylamine (1) was a weak non-selective inhibitor which was shown to act in a reversible and competitive manner towards the deamination of tyramine. The two acetylenic derivatives N-methyl-N-(2-propynyl)-2-(5-benzyloxy-1-methylindolyl)methylamine (2) and N-methyl-N-(2-propynyl)-2-(5-hydroxy-1-methylindolyl)methylamine (3) were potent MAO inhibitors, one of them non-selective (compound 2) and the other MAO-A selective inhibitor (compound 3). Both of them were irreversible and competitive inhibitors, compound 2 towards the deamination of tyramine and compound 3 towards the deamination of serotonin and beta-phenylethylamine. A mechanism for the inhibition of the enzyme by both irreversible inhibitors is proposed and the inhibition parameters are determined.     

Biochemical and pharmacologic properties of 2614W94, a reversible,competitive inhibitor of monoamine oxidase-A

2614W94 [3-(1-trifluoromethyl)ethoxyphenoxathiin 10,10-dioxide] is a selective, reversible inhibitor of monoamine oxidase-A with a competitive mechanism of inhibition and a K_i value of 1.6 nM with serotonin as substrate. In pretreated rats, the ED₅₀ value after single oral dosing was 1.7 mg/kg, similar to an ED₅₀ value of 1.1 mg/kg estimated in the 5-hydroxytryptophan potentiation test. Maximal inhibition of monoamine oxidase-A (MAO-A) was observed by 0.5 h after dosing, suggesting rapid transport to brain. Inhibition in brain was maintained for several hours, followed by a gradual reversal with a half-time of 7.2 h. Brain levels of parent compound were higher than plasma levels at all times after dosing. No significant inhibition of MAO-B was observed. After preincubation of MAO with 2614W94 at 37°C, the inhibition was reversed by dialysis. Concentrations of serotonin, norepinephrine, and dopamine were clearly elevated in brains of rats after single oral doses, whereas levels of MAO metabolites were decreased. In a rat model designed to show blood pressure elevations in response to a threshold dose of orally administered tyramine, 2614W94 compared well with moclobemide, an MAO-A selective inhibitor that has not been associated with problems relating to dietary tyramine. The two stereoisomers of 2614W94 were both potent MAO-A inhibitors. In vitro and in vivo properties of 2614W94 suggest that this compound and its close analogs are among the most potent MAO-A inhibitors known and that they may have therapeutic potential as safe new antidepressant/anxiolytic agents. Drug Dev. Res. 45:1–9, 1998. © 1998 Wiley-Liss, Inc.     

The effect of monoamine oxidase inhibitors on ‘first-pass’ metabolism of tyramine in dog intestine

The role of the intestine in metabolic inactivation of tyramine (TYR) has been studied in an isolated intestinal loop preparation in anaesthetized dogs. In control animals there was extensive metabolism of tyramine on passage through the intestinal wall and p-hydroxyphenylacetic acid (p-OHPA) was the only metabolite found in venous plasma from the loop (mean ratio p-OHPA/TYR = 5). Oral or intravenous pretreatment with the monoamine oxidase (MAO) inhibitors tranylcypromine or MD780515 significantly lowered the ratio of p-OHPA/TYR (range = 0.2–1.1) measured 3hr after the last dose. Twenty-four hours after the last dose of MAO inhibitor p-OHPA/TYR ratios in dogs pretreated orally with MD780515 had returned to control levels while ratios in dogs pretreated orally with tranylcypromine remained low (mean = 1.6). In vitro rates of deamination of the substrates 5-hydroxytryptamine (selective for the A form of MAO) and β-phenylethylamine (selective for the B form of MAO) in homogenates of intestine paralleled the in vivo findings in most cases. Tranylcypromine produced a nonselective irreversible inhibition of both MAO-A and MAO-B whereas MD780515 was found to be a selective inhibitor of MAO-A and also appeared to be reversible.     

Substrate-selective activation of rat liver mitochondrial mono amine oxidase by oxygen

The activation by oxygen of the activity of monoamine oxidase (MAO) in preparations of rat liver mitochondrial membrane vesicles has been studied. The increase of activity with the substrates 5-hydroxytryptamine, tyramine, β-phenethylamine and benzylamine were in all cases uncompetitive. However, the degree of activation depended upon the particular substrate used. An attempt has been made to explain these results by use of an assumption that MAO activity can be divided into two forms unrelated to MAO-A and MAO-B. These two forms, tentatively called MAO-1 and MAO-2, differ in their Michaelis constants for oxygen.     

Tyramine pressor sensitivity changes during deprenyl treatment

Deprenyl has previously been reported to be a selective monoamine oxidase (MAO) type B inhibitor, which is associated with little or no enhancement of the pressor effects of tyramine. Employing an intravenous steady-state tyramine infusion technique, the effects of different doses of deprenyl and, for comparison, the mixed inhibitor tranlcypromine on the pressor response to tyramine were studied in 11 depressed patients. After 3 weeks of treatment, deprenyl produced dose-proportionate increases in tyramine sensitivity at all three doses (10, 30, and 60 mg/day) when compared to placebo baseline tyramine responses. While only a modest (3.7-fold) increase in tyramine sensitivity was found with the 10 mg/day deprenyl dose, the increase in tyramine sensitivity at the 60 mg/day dose of deprenyl (22-fold) approached that found with tranylcypromine. Reductions in plasma 3-methoxy,4-hydroxyphenylglycol (MHPG), used as a possible index of in vivo MAO-A inhibition, were highly correlated with increases in tyramine pressor sensitivity (r=0.82). The data suggest that deprenyl acts as a relatively selective MAO-B inhibitor at low doses, but that this selectivity is lost at higher doses, resulting in a significant crossover inhibition of MAO-A and increased tyramine pressor sensitivity.     

Mechanism of monoamine oxidase-A inhibition by BW 1370U87

BW 1370U87 is a potent, selective inhibitor of rat and human MAO-A with a competitive mechanism of action. K_i = 0.01 μM with either serotonin or tyramine as substrate. After preincubation of BW 1370U87 with mitochondrial MAO, full enzyme activity was restored by dialysis. Following oral administration to rats, BW 1370U87 inhibited brain MAO-A in a dose-dependent manner, with a duration greater than 6 hr, but less than 24 hr. No significant inhibition of MAO-B by BW 1370U87 was observed either in vitro or ex vivo. The selectivity, reversibility, and competitive kinetics of the inhibition by BW 1370U87 may contribute to an improved safety profile with this novel MAO-A inhibitor.