Substrate specificity of the different forms of monoamine oxidase in rat liver mitochondria

Rat liver mitochondrial monoamine oxidase was inhibited by deprenil (selective inhibitor for the ‘B form’ of monoamine oxidase) to study the ‘A form’ of the enzyme separately. The activity towards serotonin (usually classified as a substrate for the ‘A form’) was estimated in the presence of additional monoamine oxidase substrates. All of the additional substrates investigated inhibited the activity towards serotonin competitively. In the deprenil inhibited preparation all of the residual activity towards β-phenylethylamine (usually classified as a substrate for the ‘B form’) was shown to be sensitive to the ‘A form’ inhibitor clorgyline, indicating that the ‘A form’ was also able to oxidize this substrate. The K_m values of the ‘A form’ for serotonin, tyramine and β-phenylethylamine did not differ significantly.When the ‘B form’ of monoamine oxidase was studied after inhibition of the ‘A form’ by clorgyline, all additional substrates investigated were able to inhibit the activity towards β-phenylethylamine in a competitive fashion. All of the remaining activity towards serotonin in the clorgyline inhibited preparation was sensitive to deprinil. Thus the ‘B form’ also appears to be able to oxidize this substrate. The K_m values for the ‘B form’ differed considerably: 4 μM for β-phenylethylamine, 102 μM for tyramine, and 2.5 mM for serotonin.     

The nature of the inhibition of rat liver monoamine oxidase types A and B by the acetylenic inhibitors clorgyline,l-deprenyl and pargyline

The kinetics of inhibition of rat liver mitochondrial monoamine oxidase by clorgyline, l-deprenyl and pargyline are consistent with a mechanism whereby a reversible interaction between the inhibitor and the enzyme active site under conditions of thermodynamic equilibrium is followed by a time-dependent formation of the covalently-bound enzyme-inhibitor adduct. The K_i value for the reversible interaction between clorgyline and monoamine oxidase A is about 1000 times lower than that towards the B-form of the enzyme, and this difference is sufficient to account for most, but not all, of the selectivity of the inhibition caused by this compound. The K_i value of the monoamine oxidase B selective inhibitor l-deprenyl towards that form of the enzyme is only about 40-fold lower than that towards the A-form. However, in this case, the rate of formation of the irreversible adduct is considerably faster for the B-form than for the A-form and this makes a major contribution to the selectivity of this compound. Pargyline shows a K_i value towards monoamine oxidase B that is only 8 times lower than that towards the A-form and in this case the rates of formation of the enzyme-inhibitor adducts are similar. The significance of these results are discussed in terms of the selective inhibition of monoamine oxidase.     

p-Methoxyamphetamine,a potent reversible inhibitor of type-A monoamine oxidase in vitro and in vivo

p-Methoxyamphetamine is over 20 times as potent as (+)-amphetamine as an inhibitor of 5-HT oxidation by monoamine oxidase in mouse brain in vitro, with a K_i value of 0·22 μM. It is highly selective towards A-type monoamine oxidase and possesses only weak activity against the B-type enzyme (Ki value about 500 μM with benzylamine as substrate and solubilized rat liver mitochondria as enzyme source). It is 10 times more active than (+)-amphetamine in protecting mouse brain monoamine oxidase from inhibition by phenelzine in vivo. o-Methoxy- and m-methoxyamphetamines inhibit monoamine oxidase both in vitro and in vivo with potencies comparable with, or less than that of (+)-amphetamine.     

Mixed substrate experiments with human brain monoamine oxidase

Michaelis constants for human brain monoamine oxidase have been determined with tyramine, benzylamine and dopamine as the substrates. In each case double reciprocal plots were linear over a 20-fold range of substrate concentrations. The method of mixed substrates failed to indicate heterogeneity in the enzyme preparation. The theory of the method of mixed substrates has been extended to cover systems in which two enzymes are each active toward two different substrates. It is shown that, regardless of the difference in K_m values of each of the individual enzymes for the two substrates, if the K_m values of the two enzymes are similar for each individual substrate the situation is indistinguishable from the case in which only a single enzyme is present. The observation that the mixed substrate experiments do not indicate the presence of more than one species of monoamine oxidase cannot therefore be regarded as providing firm evidence for homogeneity.     

Inhibition of the biosynthesis of rat testicular heme by 1,2-dibromo-3-chloropropane

N-Methylphenylethylamine (MPEA) and N-methylphenylethanolamine (MPEOA) were characterized as substrates for type A and type B monoamine oxidase (MAO) in rat brain mitochondria. The inhibition experiments with clorgyline and deprenyl showed that the inhibition patterns with MPEA as substrate were dependent on substrate concentrations but that this amine was a common substrate for both types of MAO at all substrate concentrations tested. When MPEOA was used as substrate, the inhibition patterns differed markedly at different substrate concentrations; at 10.0 μM, MPEOA acted as a specific substrate for type B MAO, but at 100 and 1000, μM it became a common substrate for both types. Kinetic analyses were carried out for MPEA and MPEOA with the uninhibited, the clorgyline-treated (type B MAO), and the deprenyl-treated enzyme (type A MAO). With the uninhibited enzyme, there were downward deflections in the curves of Lineweaver-Burk plots for both MPEA and MPEOA, suggesting the existence of different affinity components derived from type A and type B MAO. By means of the double-reciprocal plots, using the clorgyline- and the deprenyl-treated enzyme, it was confirmed that the high affinity corresponded to that for type B MAO and the low affinity to that for type A MAO for both MPEA and MPEOA. Therefore, the changes in the inhibition pattern at different substrate concentrations may be due to different affinities of the substrate for both types. By comparing the K_m and V_max values of both types observed for MPEA and MPEOA, it was pointed out that the β-hydroxylation of MPEA tended to increase the K_m value for type A MAO and to decrease the V_max values for both types.     

Inhibition of monoamine oxidase by amphetamine and related compounds

The α-methyl-substituted amines dl-α-methylbenzylamine, dl-α-methyltryptamine and the two stereoisomers of amphetamine were shown to be competitive inhibitors of the oxidation of benzylamine, tyramine and serotonin by rat liver monoamine oxidase. All these compounds were more potent inhibitors of serotonin oxidation than of benzylamine oxidation, with α-methyltryptamine showing the greatest selectivity and α-methylbenzylamine the least. The kinetics of the inhibition of tyramine oxidation were consistent with the presence of two enzyme species with different inhibitor sensitivities which were both active towards this substrate. The selectivity of these inhibitors was demonstrated with membrane-bound and solubilised preparations of the enzyme, but it could be abolished by treatment of the latter preparation with the chaotropic agent sodium perchlorate. The significance of monoamine oxidase inhibition in connexion with the pharmacological action of amphetamine is discussed.     

Possible heterogeneity of type B monoamine oxidase in pig heart mitochondria

The metabolism in vitro of 5-hydroxytryptamine (5-HT), tyramine and benzylamine by pig heart mitochondrial monoamine oxidase (MAO) has been studied. Linear Lineweaver-Burk plots yielded estimated K_m values (at pH 7.8) of 475 μM (5-HT) and 292 μM (tyramine). In contrast, linear regions of a downward-curving reciprocal plot revealed the presence of a high- and low-affinity metabolizing site (estimated K_m of 39 and 853 μm respectively) for benzylamine. Studies with the irreversible MAO inhibitor clorgyline indicated that metabolism of the three substrates in this tissue was brought about by type B MAO alone. However, the apparent sensitivity toward clorgyline of each substrate-metabolizing activity was not identical. This was due to different degrees of rapid or possibly instantaneous inhibition of enzyme activity toward each substrate. This rapid inhibition appeared to be both partially reversible and irreversible to a relative degree depending upon the substrate-metabolizing activity studied; additional time-dependent inhibition developing with prolonged preincubation was a first-order process, with a similar half-life, whichever substrate was used to assay MAO activity. Ackermann-Potter and Lineweaver-Burk plots also demonstrated differences in the inhibitory effects of clorgyline upon metabolism of each substrate. The ability of 5-HT, tyramine and benzylamine to inhibit each other's deamination in vitro was also investigated. Enzyme activity was measured by radiochemical assay with each labeled substrate in the presence and absence of the other non-labeled amines. Lineweaver-Burk analysis revealed a competitive interaction between tyramine and benzylamine, whereas mixed-type inhibition patterns were obtained for mixtures containing 5-HT/tyramine or 5-HT/benzylamine. In this latter case, the present inhibition data could only be assessed accurately with the low-affinity catalytic site for benzylamine. The kinetics of heat denaturation indicated both a thermolabile and thermostable component of each substrate-metabolizing activity. Some substrate-dependent differences in the relative proportions of these components were found. These experiments are discussed in relation to similar studies by other workers and suggest that pig heart MAO may, in fact, be heterogeneous.     

Binding and deamination of various substrates by types A and B monoamine oxidase in bovine brain mitochondria

The binding and deamination of four substrates by type A and type B monoamine oxidase (MAO) in bovine brain mitochondria were investigated in mixed substrate experiments. MAO activity in bovine brain mitochondria, with 5-hydroxytryptamine (5-HT) as substrate, was highly sensitive to clorgyline and less sensitive to deprenyl, while MAO activity with benzylamine or β-phenylethylamine (PEA) as substrate was highly sensitive to deprenyl and less sensitive to clorgyline. On the other hand, when tyramine plus PEA was used as substrate, the inhibition curves of clorgyline and deprenyl were both biphasic. These results indicate that 5-HT and benzylamine were preferentially deaminated by type A MAO and type B MAO, respectively, and that tyramine and PEA were deaminated by both types of MAO. Studies on the inhibition by clorgyline plus deprenyl of tyramine deamination (in the absence and presence of another substrate) showed that the deamination of tyramine by both type A and type B MAO was inhibited by PEA or benzylamine, while only type A MAO was inhibited significantly by 5-HT. The KA_i value, the dissociation constant of the type A MAO and 5-HT complex, and the KB_i values, the dissociation constants of the type B MAO and PEA or benzylamine complex, were almost equal to the K_m values of type A MAO and type B MAO respectively. The KA_i values for PEA and benzylamine were 78 and 58 μM respectively. For the type B MAO-5-HT complex, the dissociation constant KB_i was 1447 μM. These results show that type A MAO deaminates tyramine and 5-HT whereas benzylamine is not deaminated, but only binds to the substrate binding site of type A MAO with almost the same rate as that for deamination by type B MAO; with type B MAO, tyramine, PEA and benzylamine are deaminated, whereas 5-HT is not deaminated and binds to the substrate binding site of type B MAO with low affinity.     

Concentration dependence of the oxidation of tyramine by the two forms of rat liver mitochondrial monoamine oxidase

The two forms of monoamine oxidase in rat liver mitochondria were shown to have different K_m and maximum velocity values with tyramine as the substrate. K_m values of 107±15 μM and 579±45μM were determined for the A- and B-form respectively at pH 7.2 in air-saturated buffer. The maximum velocity of the A-form was found to be approximately half of that of the B-form under these conditions. A consequence of these differences is that the ratio of activities of MAO-A:MAO-B determined from clorgyline inhibition curves will be dependent upon the concentration of tyramine used to assay for enzyme activity. As a fixed concentration of tyramine, the height of the plateau in a clorgyline inhibition curve will also be affected by the presence of a selective competitive inhibitor. Procaine, an MAO-A selective competitive inhibitor, was found to increase the K_m value of the MAO-A towards tyramine to 505 ± 172 μM and under these conditions the plateau-height of the clorgyline inhibition curve was not significantly affected by variations of the tyramine concentration over a 20-fold range.     

Inhibition of rat brain monoamine oxidase by formamidines and related compounds

The efficacy of the pesticide chlordimeform or N′-(4-chloro-o-tolyl)-N,N′-dimethylform-amidine, six chlordimeform metabolites, and eleven related compounds as inhibitors of the oxidative deamination of radiocarbon-labelled biogenic amines by rat brain monoamine oxidase was examined. The I₅₀ value for chlordimeform with tyramine as substrate was 6.0 × 10^?5M. Inhibition following prolonged pre-incubation of chlordimeform with monoamine oxidase increased with time. and this was attributed, at least in part, to the formation of the more potent monoamine oxidase inhibitor 4′-chloro-o-formotoluidide, a known chlordimeform metabolite and degradation product. 4′-Chloro-o-formotoluidide was the most potent monoamine oxidase inhibitor examined yielding I₅₀ values of 2.6 × 10^?6m. 1.5 × 10^?6M and 3.2 × 10^?6M, with tyramine, dopamine, and serotonin, respectively, as substrates. The N-demethyl (demethylchlordimeform) and N-didemethyl chlordimeform metabolites gave I₅₀ values with tyramine of 3.3 × 10^?5M and 7.5 × 10^?5M, respectively. Three additional metabolites, 4-chloro-o-toluidine, 5-chloroanthranilic acid, and N-formyl-5-chloroanthranilic acid, were weak inhibitors with I₅₀ values of 1 × 10^?4m or higher. The other formamidine compounds also inhibited the oxidative deamination of tyramine; I₅₀ values ranged from 9.3 × 10^?5m to 7.5 × 10^?6m. Lineweaver-Burk plots revealed that chlordimeform, demethylchlordimeform, and 4′-chloro-o-formotoluidide were competitive inhibitors of the oxidative deamination of β-phenylethylamine, tyramine, dopamine, tryptamine, and serotonin. Inhibition was reversible since activity was restored by washing.     

The oxidation of tryptamine by the two forms of monoamine oxidase in human tissues

The selective monoamine oxidase inhibitors clorgyline and (-)-deprenyl have been used to determine the activities of monoamine oxidase-A and -B towards tryptamine in several human tissues. The results were compared with those obtained with the A-form-selective substrate 5-hydroxytryptamine, the B-form-selective substrate 2-phenethylamine and the common substrate tyramine. Tryptamine was found to be a substrate for both forms of the enzyme in human liver, kidney cortex and medulla and in seven different brain regions. The Km values of the two forms towards this substrate were similar in all the human tissues examined but the maximum velocities differed. Thus the A-form would contribute approximately 50% of the total monoamine oxidase activity towards this substrate in human cerebral cortex, whereas it would contribute about 60% in kidney cortex and medulla and 75% in liver. These results suggest that both forms of monoamine oxidase would contribute to the metabolism of tryptamine in human tissues and are difficult to reconcile with suggestions that tryptamine excretion may provide a simple index of monoamine oxidase-A inhibition.     

Characterization of N-methylphenylethylamine and N-methylphenylethanolamine as substrates for type A and type B monoamine oxidase

N-Methylphenylethylamine (MPEA) and N-methylphenylethanolamine (MPEOA) were characterized as substrates for type A and type B monoamine oxidase (MAO) in rat brain mitochondria. The inhibition experiments with clorgyline and deprenyl showed that the inhibition patterns with MPEA as substrate were dependent on substrate concentrations but that this amine was a common substrate for both types of MAO at all substrate concentrations tested. When MPEOA was used as substrate, the inhibition patterns differed markedly at different substrate concentrations; at 10.0 /smM, MPEOA acted as a specific substrate for type B MAO, but at 100 and 1000, μM it became a common substrate for both types. Kinetic analyses were carried out for MPEA and MPEOA with the uninhibited, the clorgyline-treated (type B MAO), and the deprenyl-treated enzyme (type A MAO). With the uninhibited enzyme, there were downward deflections in the curves of Lineweaver-Burk plots for both MPEA and MPEOA, suggesting the existence of different affinity components derived from type A and type B MAO. By means of the double-reciprocal plots, using the clorgyline- and the deprenyl-treated enzyme, it was confirmed that the high affinity corresponded to that for type B MAO and the low affinity to that for type A MAO for both MPEA and MPEOA. Therefore, the changes in the inhibition pattern at different substrate concentrations may be due to different affinities of the substrate for both types. By comparing the K_m and V_max values of both types observed for MPEA and MPEOA, it was pointed out that the β-hydroxylation of MPEA tended to increase the K_m value for type A MAO and to decrease the V_max values for both types.     

A comparison of cardiac and vascular clorgyline-resistant amine oxidase and monoamine oxidase: Inhibition by amphetamine,mexiletine and other drugs

Clorgyline-resistant amine oxidase (CRAO) and monoamine oxidase (MAO) were studied in homogenates of rat heart and aorta, using benzylamine and tyramine as substrates. In heart, benzylamine at 0.001 mM was deaminated solely by CRAO. With higher concentrations of benzylamine (0.01, 0.1 and 1.OmM), an increasing involvement of MAO-A and MAO-B became apparent in the deamination of benzylamine such that, at 1.0 mM benzylamine, deaminated products resulted equally from MAO-A, MAO-B and CRAO. In aorta, benzylamine was deaminated solely by CRAO irrespective of the concentration used. Tyramine (0.01, 0.1, 1.0 and 5.0 mM) was deaminated entirely by MAO-A in heart, whereas in the aorta both MAO-A and CRAO participated. In aorta the ratio of product formation from MAO-A and CRAO did not vary with changes in the concentration of tyramine, indicating similar K_m values for both enzymatic activities. Further studies with tyramine (0.1 mM) and clorgyline showed biphasic inhibition curves suggestive of two distinct MAO-A components in both heart and aorta. The two components showed different properties in the heart when compared with aorta. When homogenates of hearts were heated at 50° for 1 hr, their sensitivity to inhibition by clorgyline increased, while in homogenates of aorta sensitivity to clorgyline decreased. CRAO was investigated further with benzylamine as substrate. Kinetic studies gave similar K_m values for both heart and aorta (4–6 μM at pH 7.8), and these values were not altered by flushing the assay tubes with oxygen. However, flushing with nitrogen caused uncompetitive inhibition in the heart and noncompetitive inhibition in aorta. These results suggest a difference in the catalytic mechanism between CRAO of heart and aorta. In both heart and aorta, CRAO was inhibited by semicarbazide, (+)-amphetamine, phenelzine and (+)- and (?)-mexiletine, with the (+)-form being more potent. Straight-chain diamine and polyamine compounds failed to inhibit in concentrations up to 10^?4 M. Thus, CRAO is not a typical diamine or polyamine oxidase. The results show differences between heart and aortic CRAO and MAO-A, and the possibility exists for heterogeneity within each of these two distinct forms of amine oxidase. Additionally, drugs known to inhibit MAO-(+)-amphetamine, phenelzine and mexiletine also inhibit CRAO. However, the biological significance of since the physiological role of CRAO is unknown.     

Inhibition of rat liver monoamine oxidase by α-methyl- and N-propargyl-amine derivatives

The inhibition of rat liver monoamine oxidase by a number of N-propargyl and α-methyl amine derivatives has been examined. The results indicate that α-methyl-substituted primary and secondary amine derivatives tend to show selectivity as reversible inhibitors towards the A-form of the enzyme. The structural features that result in selectivity in irreversible inhibitors are less easy to define and substitution of an N-propargyl group into a compound that is a selective reversible inhibitor of monoamine oxidase will not necessarily result in retention of that selectivity. Replacement of the acetylenic group in a B-selective irreversible inhibitor by an ethylenic group resulted in a compound that was a reversible inhibitor showing slight selectivity for the A-form of the enzyme.     

Oxidation of phenylethanolamine and octopamine by type A and type B monoamine oxidase. Effect of substrate concentration.

Phenylethanolamine (PEOA) and octopamine (OA) were characterized as substrates for type A and type B monoamine oxidase (MAO) at various substrate concentrations, using rat brain mitochondria. The experiments on sensitivity to clorgyline and deprenyl showed that the inhibition patterns with PEOA as substrate differed markedly at different substrate concentrations: at 12.5 μM, PEOA acted as a specific substrate for type B MAO, but at 125 and 1250 μM it became a common substrate for both types of MAO. However, when OA was used as substrate, there were only slight or no differences in the inhibition patterns among the various concentrations tested; OA was found to be a common substrate for both types of MAO. Benzylamine was also examined for comparison and confirmed to be highly specific for type B MAO over a wide concentration range of the substrate. Kinetic analyses were carried out for PEOA and OA. High and low affinities for MAO were identified for PEOA: K_m values were 22.7 and 465 μM, and V_max values were 6.90 and 19.2 nmoles/mg of protein/30 min respectively. Pretreatment of the enzyme with 10^?6 M clorgyline resulted in the disappearance of the low affinity component, and pretreatment with 10^?6 M deprenyl resulted in the disappearance of the high affinity component. Therefore, the high affinity corresponded to that for type B MAO and the low one to that for type A MAO. For OA, however, the double reciprocal plots were linear with a single affinity component showing K_m and V_max values of 455 μM and 90.9 nmoles/mg of protein/ 30 min respectively. From the present study, it can be concluded that, when sensitivity of MAO to clorgyline or deprenyl is studied, it is necessary to check the effect of substrate concentration for each substrate and enzyme preparation, suspecting the different affinities of the substrate for type A and type B MAO.     

Monoamine oxidase. I. Specificity of some substrates and inhibitors

A number of synthetic substrates and inhibitors of monoamine oxidase have been studied, using the enzyme from porcine brain. K_m and V_max values have been calculated for substrates using Lineweaver-Burk plots. In many cases large variations in K_m and V_max were observed for relatively small changes in the structure of substrates. Similar observations were made concerning K_i values for some competitive inhibitors. The effects of hydrogen-ion concentration on enzyme activity are consistent with the view that unprotonated amines are the species which bind to the enzyme. This finding, together with the observation that tertiary amines can act as substrates has led to formulation of a proposed mechanism of dehydrogenation which does not depend upon intermediate formation of Schiff base from the substrate amine.     

Effects of aldehyde dehydrogenase inhibitors on enzymes involved in the metabolism of biogenic aldehydes in rat liver and brain

The effects of the aldehyde dehydrogenase inhibitors disulfiram, coprine and cyanamide on enzymes involved in the metabolism of biogenic aldehydes in rat liver and brain were studied. Both liver and brain aldehyde dehydrogenase activities were significantly decreased in rats pretreated with these drugs. In the liver, the low-K_m aldehyde dehydrogenase activity was markedly decreased by all three drugs after 2 and 24 hr whereas only cyanamide inhibited the high-K_m enzymes. The brain ALDH-activity with a low acetaldehyde concentration was significantly decreased by coprine and cyanamide at both times tested, whereas disulfiram caused no change after 2 hr but an inhibition of 38% after 24 hr. The brain ALDH-activity with a high acetaldehyde concentration was significantly decreased by coprine and cyanamide but not by disulfiram. The activity of the substrate specific enzyme succinate semialdehyde dehydrogenase in brain was slightly but significantly decreased in rats pretreated with cyanamide but not in rats pretreated with disulfiram or coprine. None of the drugs caused any changes in the activities of aldehyde reductase and monoamine oxidase in brains in vivo. The activity of monoamine oxidase in liver was significantly decreased by coprine after 24 hr. In contrast to the effects obtained in vivo, disulfiram was found to be an inhibitor in vitro of brain succinate semialdehyde dehydrogenase and liver monoamine oxidase. Aldehyde reductase was slightly inhibited by both disulfiram and 1-aminocyclopropanol in vitro.     

Monoamine oxidase in human platelets: Kinetics and methodological aspects

The kinetic properties of human platelet monoamine oxidase (MAO) were examined in 20 apparently healthy controls. The mean value (±S.D.) of the maximum velocity (V) was found to be 5.36 ± 1.97 pmoles of product formed/10° platelets/min and the Michaelis-Menten constants were for phenethylamine (K^PEA_m) 14.6 ± 8.20 μM and for oxygen (K_m^O₂)254 ± 125 μM, when assayed in 0.1 M phosphate buffer, pH 7.4. The relation between the value of the corresponding apparent constants was studied. Inhibition of the enzyme activity was seen at 20 μM of PEA and 180 μM of oxygen. The enzyme kinetics were also studied at different pH. Two pK values were found, pK₁ = 6.65 and pK₂ = 6.95. The influence of homogenization on the MAO activity was compared with the activity in the undisrupted platelet. At PEA concentrations below 10 μM higher MAO activities were found in the intact cell. A 15 per cent loss of activity was detected in platelet samples after storing at ?20° for three and a half years.     

A benzylamine oxidase distinct from monoamine oxidase B—Widespread distribution in man and rat

Benzylamine oxidase (BzAO) and monoamine oxidase type B(MAO-B) both selectively catalyse the oxidative deamination of benzylamine (Bz). We define the former as that benzylamine-metabolizing activity insensitive to 4 × 10^?4 M deprenyl, a concentration which totally inhibits all forms of MAO. Although both enzymes are widespread in human and rat tissues, their organ distribution differs. Liver and brain show highest MAO-B activity, whilst BzAO activity predominates in aorta and lung. Relatively low BzAO and no MAO-B activity is present in plasma. In the rat, phenylethylamine (PEA) and dopamine (DA) are both substrates for a deprenyl-resistant enzyme with a distribution similar to BzAO, but in man these amines are solely oxidized by MAO. At pH 7.2 the K_m of BzAO for benzylamine is 2.2 × ^?4 M in the rat; μn man, it is 1.1 × 10^?4M. The K_m of MAO-B for benzylamine is 1.0 × 10^?4M in the rat and 5 × 10^?5 in man. Semicarbazide, procarbazine and carbidopa are potent inhibitors of BzAO and inhibit it selectively, leaving MAO substantially unaffected.     

Characteristics of monoamine oxidase in mitochondria isolated from chick brain,liver, kidney and heart

The substrate- and inhibitor-related characteristics of monoamine oxidase (MAO) were studied with mitochondria of chick brain, liver, kidney and heart. The kinetic constants for MAO in these organs were determined, using 5-hydroxytryptamine (5-HT), tyramine and β-phenylethylamine (PEA) as substrates. For all the substrates, the V_max values were highest in kidney, followed in decreasing order by brain, liver and heart. For tyramine and PEA, the K_m values were lowest in liver, but for 5-HT it was lowest in heart. Inhibition experiments with clorgyline and deprenyl were carried out on mitochondria of the four organs with the three substrates at their K_m concentrations. From the plateaus observed of inhibition by clorgyline, it was concluded that 5-HT was oxidized by both types of MAO in mitochondria of all the organs; PEA was fairly specific for type B MAO in brain, liver and kidney, but non-specific in heart. In heart mitochondria, appreciable amounts of the activities toward tyramine and PEA were due to an amine oxidase distinct from mitochondrial MAO; 5-HT, however, was oxidized exclusively by mitochondrial MAO in this organ. The above atypical characteristics in substrate specificity found in chick tissues support the idea that the type A and type B concept cannot be applied uncritically to all tissues from all species.