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.     

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.     

Selectivity of clorgyline and pargyline as inhibitors of monoamine oxidases A and B in vivo in man

During 4 weeks of treatment with clorgyline, a selective MAO-A inhibitor, platelet monoamine oxidase (MAO) activity was unchanged. During a similar 4-week crossover treatment period with pargyline, a selective MAO-B inhibitor, platelet MAO activity was essentially completely inhibited in the same individuals. The differential effects of the two drugs on platelet MAO, which consists exclusively of the MAO-B form, suggests that the in vitro selectivity of clorgyline, and possibly of pargyline, on MAO-A and MAO-B may be maintained in vivo during long-term administration in man. Reductions in blood pressure, heart rate, and plasma amine oxidase activity were generally similar in magnitude during treatment with both drugs, however, suggesting that either these effects are nonspecific consequences of both MAO-A and MAO-B inhibition, or that pargyline also inhibited MAO-A activity.     

Substrate- and inhibitor-related characteristics of human platelet monoamine oxidase

Human platelet monoamine oxidase (MAO) preferentially deaminated benzylamine and phenylethylamine, two substrates relatively specific for type B MAO, in comparison to 5-hydroxytryptamine, a substrate specific for type A MAO. In studies comparing human platelet and rat brain MAO specific activities, benzylamine and 5-hydroxytryptamine deamination by platelets was approximately 90 and 2 per cent, respectively, that of brain, while platelet deamination of dopamine, tryptamine and tyramine was 20 per cent or less than that of brain. Among sixteen drugs studied, platelet MAO activity was selectively inhibited by low concentrations of the MAO-B inhibitors, deprenyl and pargyline, and was relatively insensitive to the MAO-A inhibitors, clorgyline and Lilly 51641. These observations, in addition to the simple sigmoid inhibition curves obtained with increasing concentrations of either clorgyline or deprenyl, suggest that platelet MAO consists of essentially one distinguishable form of MAO which most closely resembles the MAO type B found in other tissues.     

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.     

Monoamine oxidase in pancreatic islets,exocrine pancreas,and liver from rats. Characterization with clorgyline,deprenyl, pargyline,tranylcypromine, and amezinium

Monoamine oxidase (MAO) was characterized in tissue homogenates from pancreatic islets, exocrine pancreas, and liver from rats. Phenylethylamine was preferentially deaminated by pancreatic islet MAO while 5-hydroxytryptamine was preferentially deaminated by MAO from exocrine pancreas, and tyramine was a good substrate for both tissues. All three substrates were well deaminated by liver tissue. Clorgyline, a selective inhibitor of MAO-A, preferentially inhibited deamination of 5-hydroxytryptamine by all three tissue homogenates, while deprenyl, a selective inhibitor of MAO-B, preferentially inhibited deamination of phenylethylamine. In the case of pargyline, a less selective MAO-B inhibitor, the preference in favour of phenylethylamine was less pronounced. According to these results, MAO in pancreatic islets can be classified as predominantly type B enzyme species and MAO in exocrine pancreas as predominantly type A enzyme species while both types of the enzyme are present in the liver. Using the same three MAO substrates and compared with the effects of the selective enzyme inhibitors, clorgyline and deprenyl, tranylcypromine can be classified as a potent nonselective inhibitor of MAO in homogenates of all three tissues investigated with a slight preference in favour of the inhibition of the B-form of the enzyme, while in contrast amezinium can be classified as a weak nonselective inhibitor of MAO with a slight preference in favour of the inhibition of the A-form of the enzyme. All MAO inhibitors tested also inhibited insulin secretion by isolated incubated rat pancreatic islets, however only at IC50 which were two to three decimal powers higher than those necessary for the inhibition of the MAO activity, thus indicating that inhibition of MAO activity and inhibition of insulin secretion are apparently not closely related.     

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.     

Inhibition of MAO-A activity enhances behavioural activity of rats assessed using water maze and open arena tasks.

To determine if the inhibition of MAO-A and/or MAO-B activities can influence cognitive processes in adult rats, we analysed whether chronic treatment with clorgyline, 1-deprenyl and pargyline could modify the performance of adult rats in a modified version of the water maze task. The effects of these treatments on locomotor activity and enzyme activities were also assessed. Rats were treated for 24 days with clorgyline (0.2 mg/kg), 1-deprenyl (0.25 mg/kg) and pargyline (I or 10 mg/kg). The treatments were started two weeks before the water maze experiment and continued until the end of testing. The rats were trained to find a submerged platform (6 days: I trial/day; 7 th day: probe trial). Over the next three days, locomotor activity was assessed in an open arena. Treatments with clorgyline (MAO-A inhibitor), 1-deprenyl (MAO-B inhibitor) and pargyline (non-selective MAO inhibitor) did not improve the finding of the hidden platform, when compared to treatment with saline, but significantly increased the swimming speed of the rats. The different treatments, when compared to saline, failed to modify the distance covered and the number of groomings performed in the open arena. However, clorgyline and pargyline, 10 mg/kg, increased the number of faecal boli and clorgyline enhanced the number of rearings made when compared to saline, 1-deprenyl and pargyline, 10 mg/kg. These results indicate that near total inhibition of MAO-A by clorgyline and pargyline as assessed by MAO activity measurement induces an increase in locomotor activity but that inhibition of MAO-A or MAO-B, either alone or combined, does not facilitate spatial learning in adult rats.     

Monoamine oxidase activities in lymphocytes and granulocytes taken from pig blood

The characterisation of monoamine oxidase activities in lymphocytes and granulocytes was studied using cells prepared from pig blood. The specific activities against beta-phenylethylamine, benzylamine, tyramine and 5-hydroxytryptamine as substrates in granulocytes (G) were approximately twice those found in lymphocytes (L). The absence of the semicarbazide-sensitive amine oxidase (SSAO) was confirmed by insensitivity of the latter to semicarbazide as inhibitor with benzylamine as substrate. MAO activity present in (G) and (L) was selectively inhibited by low deprenyl concentrations; this fact, in addition to the simple sigmoid inhibition curves obtained with increasing concentrations of clorgyline with tyramine as substrate, suggests that the MAO activity present both in (G) and (L) is predominantly of the MAO-B form. The absence of any contamination with plasma amine oxidase (EC 1.4.3.6) was confirmed by the fact that activity towards benzylamine (Bz) was insensitive to KCN-induced inhibition. Kinetic constants were determined for each fraction towards beta-phenylethylamine (PEA) and Bz as substrates. MAO-B was titrated with unlabelled pargyline, deprenyl and [3H]-pargyline; the corresponding Kcat values, turnover number and the active concentrations were then determined. The molecular weight of MAO-B present in both cellular fractions was calculated by SDS-electrophoresis and fluorography, after reaction with [3H]-pargyline. Some of these results are compared with those obtained with human blood leucocytes.     

Deamination of released 3H-noradrenaline in the canine saphenous vein

Summary Experiments were designed to determine the effect of monoamine oxidase (MAO) inhibitors on the release and the metabolism of noradrenaline in the canine saphenous vein. Helical strips were incubated with ³H-noradrenaline and mounted for superfusion and measurement of the efflux of labelled transmitter and its metabolites; in certain experiments the tissue content of ³H-noradrenaline and its metabolites was also determined. The MAO-A inhibitor clorgyline, and the non-specific inhibitor pargyline, but not the MAO-B inhibitor deprenyl decreased the appearance of deaminated and O-methylated deaminated metabolites under basal conditions and during electrical stimulation. The MAO-A and the non-specific MAO inhibitor did not decrease the efflux of VMA to the same extent as that of the other deaminated metabolites. During superfusion with etidocaine, an agent causing increased leakage of stored transmitter, clorgyline abolished the appearance of DOPEG. Addition of semicarbazide in preparations treated with pargyline did not affect the efflux of deaminated and O-methylated deaminated metabolites. From the measurement of tissue VMA, it appeared that the efflux of VMA poorly reflects quick changes in the rate of its formation but that formation is abolished by pretreatment with pargyline. These experiments indicate that in the canine saphenous vein: (1) DOPEG is formed mainly in intraneuronal sites, while DOMA, MOPEG and VMA are formed extraneuronally; (2) VMA is retained in the tissue after its formation; and (3) the only subtype of MAO involved in the metabolism of ³H-noradrenaline released from adrenergic nerve endings can be classified as MAO-A.Supported in part by grant HL-05883     

The effect of various MAO-B inhibitors on rabbit arterial strip response to tyramine

Studies on the response to tyramine of the rabbit pulmonal artery strip have revealed that several specific MAO-B inhibitors (TZ-650, J-580, AGN-1135, U-1424 MDL-72165 and Ro 16-6491) potentiated the responses in a dose-dependent manner, similarly as a semi-selective MAO-B inhibitor pargyline and a nonselective MAO inhibitor tranylcypromine did. Only (-)deprenyl inhibited the response. Thus the inhibition of noradrenaline-releasing effect of tyramine, a property favorable from the point of view of safety in clinical practice, is a specific feature of (-)deprenyl and not a more general characteristics of MAO-B inhibitors.     

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.     

The inhibition of A and B forms of MAO in the production of a characteristic behavioural syndrome in rats after l-tryptophan loading

l-Tryptophan was administered to rats pretreated with selective inhibitors of the A and B forms of MAO. deprenyl, a selective inhibitor of MAO-B, produced only minor changes in behaviour and in the concentrations of apparent 5-HT and 5-HIAA in brain. High doses of clorgyline, a selective inhibitor of MAO-A, produced a characteristic stereotyped syndrome of hypermotility and tremor as well as an increase in apparent 5-HT and a decrease in apparent 5-HIAA in brain. Small doses of deprenyl and clorgyline in combination, but not singly, produced maximal effects on behaviour as well as on the concentrations of apparent 5-HT and 5-HIAA in brain. Maximum behavioural and biochemical effects were also produced when either deprenyl or clorgyline was administered 18 hrs before the other.It is concluded that the syndrome may be dependent on the formation of an N-substituted derivative of 5-HT which is at least partly deaminated by MAO-B. Alternatively, the syndrome may be dependent on a sufficiently high concentration of 5-HT in a special compartment where it is partly deaminated by MAO-B.     

Cardiovascular changes in response to selective monoamine oxidase inhibition in the rat

Chronic (21 days) treatment with the selective monoamine oxidase (MAO)-A inhibitor clorgyline, but not with the MAO-B inhibitor deprenyl in pithed rats leads to increased blood pressure responses to sympathetic stimulation and intravenous tyramine, and to elevated unstimulated heart rates. No significant changes are observed in plasma catecholamine responses to sympathetic stimulation, nor in β-adrenoreceptor numbers in heart ventricles. These findings suggest that the hypotensive effects of MAO inhibitors result from central nervous system rather than peripheral nervous system alterations.     

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.     

Combined inhibition of serotonin uptake and oxidative deamination attenuates audiogenic seizures in DBA/2J mice

We previously reported that 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THBC) attenuated audiogenic seizures (AGS) in 21-day-old DBA/2J mice and also inhibited brain monoamine oxidase-A (MAO-A) and serotonin (5-hydroxytryptamine, 5-HT) uptake leading to increased brain 5-HT concentration. In this study, the sensitivity of AGS to 5-HT manipulation was evaluated by utilizing drug combinations which paralleled the actions of 6-MeO-THBC and which also have been associated with the production of a serotonergic motor syndrome in rats. Combination of a specific 5-HT uptake inhibitor (fluoxetine or citalopram) with the MAO-A inhibitor clorgyline inhibited AGS more effectively than the individual drugs but combination with the MAO-B inhibitor deprenyl did not. Combined administration of clorgyline plus deprenyl also suppressed AGS. Inhibition of AGS by tryptophan was potentiated by combination with either of the mixed MAO inhibitors nialamide or tranylcypromine. The effects of these drugs individually and in combination on brain MAO-A and MAO-B activity and 5-HT uptake were also determined ex vivo and were consistent with expected mechanisms of action. These results suggest, first of all, that the inhibition of AGS produced by 6-MeO-THBC is a consequence of its combined MAO-A and 5-HT uptake inhibition properties. Secondly, the similarity of results of pharmacological manipulations of the 5-HT system which produce the rat motor syndrome and which inhibit AGS in the mouse suggests that AGS in 21-day-old DBA/2J mice may be a useful system for assessing functional consequences of these serotonergic manipulations.     

Inhibition of MAO‐A Activity Enhances Behavioural Activity of Rats Assessed Using Water Maze and Open Arena Tasks

Abstract: To determine if the inhibition of MAO‐A and/or MAO‐B activities can influence cognitive processes in adult rats, we analysed whether chronic treatment with clorgyline, l‐deprenyl and pargyline could modify the performance of adult rats in a modified version of the water maze task. The effects of these treatments on locomotor activity and enzyme activities were also assessed. Rats were treated for 24 days with clorgyline (0.2 mg/kg), l‐deprenyl (0.25 mg/kg) and pargyline (1 or 10 mg/kg). The treatments were started two weeks before the water maze experiment and continued until the end of testing. The rats were trained to find a submerged platform (6 days:1 trial/day; 7 th day: probe trial). Over the next three days, locomotor activity was assessed in an open arena. Treatments with clorgyline (MAO‐A inhibitor), l‐deprenyl (MAO‐B inhibitor) and pargyline (non‐selective MAO inhibitor) did not improve the finding of the hidden platform, when compared to treatment with saline, but significantly increased the swimming speed of the rats. The different treatments, when compared to saline, failed to modify the distance covered and the number of groomings performed in the open arena. However, clorgyline and pargyline, 10 mg/kg, increased the number of faecal boli and clorgyline enhanced the number of rearings made when compared to saline, l‐deprenyl and pargyline, 10 mg/kg. These results indicate that near total inhibition of MAO‐A by clorgyline and pargyline as assessed by MAO activity measurement induces an increase in locomotor activity but that inhibition of MAO‐A or MAO‐B, either alone or combined, does not facilitate spatial learning in adult rats.     

The effects of selective and non-selective monoamine oxidase (MAO) inhibitors on conflict behavior in the rat

Conflict behavior in rats was examined over the course of several weeks of chronic treatment with selective and non-selective monoamine oxidase inhibitors (MAOIs). In daily 10min sessions, rats were trained to drink from a tube which was occasionally electrified (0.5mA). Electrification was signalled by the presence of a tone. Within 3-4 weeks, control (i.e. non-drug) conflict behavior had stabilized (30-40 shocks and 8-12ml water/session) and drug testing began. Chronic administration (two injections/day for 8 weeks) with a non-selective (i.e. MAO-A and MAO-B inhibiting) dose of pargyline (15mg/kg) resulted in a time-dependent increase in punished responding. In contrast, chronic administration of the MAO-A selective inhibitor (clorgyline; 1.0mg/kg, 2mg/kg), the MAO-B selective inhibitor deprenyl (5mg/kg) or MAO-B inhibiting doses of pargyline (1.0mg/kg, 5mg/kg) were without effect. Finally, chronic treatment with the combination of a low dose of clorgyline (1.0mg/kg) and a low dose of pargyline (1.0mg/kg) did result in a time-dependent increase in punished responding. These results suggest that inhibition of both MAO-A and MAO-B is required for the eventuation of the anxiolytic effect resulting from chronic MAOI treatment.     

Effects of monoamine oxidase inhibitors on levels of catechols and homovanillic acid in striatum and plasma

Levels of homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC) and dihydroxyphenylglycol (DHPG) in plasma and the striatium were measured after inhibition of monoamine oxidase type A (MAO-A) by clorgyline (4 mg/kg i.p.), MAO-B by (-)deprenyl (1 mg/kg i.p.), both MAO-A and MAO-B by nialamide (75 mg/kg i.p.) or peripheral neuronal MAO by debrisoquin (40 mg/kg i.p.). Levels of HVA in plasma decreased by about 60% after single doses of nialamide or clorgyline, by about 80% after repeated doses of nialamide, by about 40% after a single dose of debrisoquin and by about 50% after repeated doses of debrisoquin. The administration of clorgyline, nialamide or debrisoquin significantly decreased concentrations of DOPAC and DHPG in plasma, whereas (-)deprenyl did not affect levels of DHPG or HVA. None of the MAO inhibitors produced more than about 80% depression of levels of any of the deaminated metabolites. The results suggest that most of the HVA in plasma is derived from deamination of DA by MAO-A in peripheral neurons; that DOPAC in plasma is derived from cells outside the central nervous system; that DHPG in plasma is derived virtually exclusively from the metabolism of norepinephrine in sympathetic nerve endings and that residual levels of HVA after treatment with debrisoquin provide an improved but limited indication of central dopaminergic activity.     

Studies of monoamine oxidases: Inhibition of bovine brain MAO in intact mitochondria by selective inhibitors

The inhibition of mitochondrial monoamine oxidase (MAO) from beef brain cortex by the selective inhibitors, clorgyline, harmaline, Deprenyl and pargyline, was compared using five substrates: serotonin (5-HT), β-phenylethylamine (PEA), tyramine, tryptamine and dopamine. Dose-response studies, consistent with the classification of MAO, types A and B, indicated that serotonin deamination was more sensitive to clorgyline and harmaline inhibition than was phenylethylamine. However, the curves for all substrates were double-sigmoidal, rather than being a single sigmoid curve for 5-HT and PEA. Deprenyl and pargyline did not exhibit any marked selectivity for inhibiting PEA deamination without prior preincubation of enzyme and inhibitor. The rate of inhibition was variable and was dependent upon the substrate, the nature of the inhibitor and the inhibitor concentration. Dual inhibitor studies, using the “type A” inhibitor, clorgyline, and the “type B” inhibitor, Deprenyl, together, resulted in almost complete MAO inhibition, regardless of substrate. Combining the two type A inhibitors, clorgyline and harmaline, or the two type B inhibitors, deprenyl and pargyline, resulted in inhibitions that were equal to or only slightly greater than the inhibition produced by a single inhibitor. These results suggested that there are at least two distinct sites in beef brain MAO from cortical mitochondria which may be interacting. The deamination of all substrates occurs at both sites.