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.     

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.     

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.     

Role of type A and type B monoamine oxidase in the metabolism of released [3H]dopamine from rat striatal slices

The effects of selective inhibition of multiple forms of monoamine oxidase (MAO) on the in vitro release and metabolism of newly-synthesized [³H]dopamine (DA) were examined using rat brain slices. Striatal slices were preincubated in the presence of [³H]l-tyrosine (20 μM) followed by a short incubation period in the presence of the selective irreversible MAO-inhibitor agents clorgyline (type A) and deprenyl (type B). Tissue pretreated in this manner was then subjected to a release incubation, and DA release and metabolism were determined under spontaneous and depolarizing conditions. Pretreatment with clorgyline (10^?7 M) significantly reduced the spontaneous, as well as K⁺-evoked, formation of both 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Deprenyl (10^?7 M) pretreatment did not significantly affect these variables, but clorgyline + deprenyl pretreatment resulted in a reduction of both DOPAC and HVA that was greater than that produced by clorgyline alone. By contrast, deprenyl pretreatment significantly decreased both DOPAC and HVA under depolarizing conditions, but only in the presence of the DA uptake inhibitor nomifensine (10^?5 M). In the absence of MAO inhibition, nomifensine increased K⁺-evoked formation of DOPAC and HVA, while spontaneous formation was not affected. The results suggest that released DA is deaminated primarily by the type A form of MAO; however, in the absence of the type A MAO, or under conditions that promote exclusive postsynaptic deamination, minor but significant metabolism occurs via the type B enzyme. Data obtained are further discussed in relation to the mechanism of MAO-inhibitor drug action and pre- versus postsynaptic formation of DOPAC and HVA.     

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.     

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.     

The effect of selective type A or type B monoamine oxidase inhibition on the intrasynaptosomal deamination of (3H)serotonin in rat spinal cord tissue

Summary The degree to which the type A and type B forms of monoamine oxidase participate in the intraneuronal deamination of (³H)serotonin (5-HT) was examined in synaptosomal-rich fractions of rat spinal cord tissue. Synaptosomes were labeled with (³H)5-HT and superfused with physiological buffers containing selective concentrations of a type A (clorgyline) or a type B (deprenyl) MAO inhibitor. The efflux of (³H)5-HT and newly-formed (³H)5-hydroxyindoleacetic acid (5-HIAA) was determined and compared to controls over time. In control samples, a slight decline in (³H)5-HT efflux occurred over the experimental superfusion period. However, a stable formation and efflux of (³H)5-HIAA was seen during this same period of time. When clorgyline was added to the superfusion buffer, a rapid decline in superfusate levels of (³H)5-HIAA was observed. Similar experiments in the presence of deprenyl were without effect. In order to elevate cytoplasmic concentrations of (³H)5-HT and therefore increase its chances for interaction with nerve terminal MAO, reserpine was added to the superfusion buffer. Reserpine caused a greater than 3-fold increase in (³H)5-HIAA formation with no change in (³H)5-HT efflux. Clorgyline inhibited this increase in (³H)5-HIAA formation but deprenyl was again without effect. In the presence of clorgyline, reserpine also caused an increase in (³H)5-HT efflux. These results strongly support the notion that 5-HT deamination within rat spinal cord nerve terminals occurs primarily, if not exclusively, through an interaction with type A MAO.Abbreviations 5-HT 5-hydroxytryptamine, serotonin - MAO monoamine oxidase - 5-HIAA 5-hydroxyindoleacetic acid Supported in part by N.I.H. grants GM 30002, 5-T32-GM 07039, and the West Virginia University Medical Corporation. L. M. B. was supported by a Swiger Fellowship     

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.     

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.     

K+-evoked [3H]-5-HT release from rat frontal cortex slices: the effect of 5-HT agonists and antagonists

The pharmacological characteristics of a pre-junctional 5-HT autoreceptor have been studied by following the Ca²⁺-dependent, K⁺-evoked release of [³H]-5-HT from preloaded rat frontal cortex slices. Added 5-HT, in the presence of the 5-HT uptake inhibitor chlorimipramine. caused a dose related inhibition of the K²⁺-evoked release of [³H]-5-HT in this system as did the 5-HT analogues 5-methoxytryptamine, N-methyltryptamine, 5-methoxy-NN'-dimethyltryptainine, N-methyl 5-hydroxytryptamine and tryptamine.The inhibitory effect of 1 μM 5-HT on the K⁺evoked release of [³H]-5-HT was reversed in a doserelated manner by the 5-HT antagonist drug, methiothepin (pA₁₀ value 6.7). At a concentration of 1 μM, the 5-HT antagonists drugs cinanserin and mianserin produced a small but significant reversal of the 5-HT induced inhibition of K⁺-evoked [³H]-5-HT release, but methysergide, metergoline and cyproheptadine were completely without effect at this concentration. The results are interpreted as evidence for a pre-junctional autoreceptor for 5-HT in the frontal cortex of the rat with a different pharmacological specificity for 5-HT antagonists from previously studied 5-HT receptors.     

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.     

Effect of nonselective and selective inhibitors of monoamine oxidases A and B on pethidine toxicity in mice.

The LD50 of pethidine was determined in mice pretreated (4 h) either with the nonselective monoamine oxidase (MAO) inhibitor, phenelzine or with clorgyline, a selective inhibitor of MAO A or deprenyl, a selective inhibitor of MAO B. Phenelzine or combined clorgyline plus deprenyl pretreatments decreased pethidine LD50. Clorgyline or deprenyl alone did not affect pethidine toxicity. Whole brain 5-hydroxytryptamine (5-HT) concentrations were measured in the pretreated mice. 5-HT levels were approximately doubled (P less than 0.001) after phenelzine or clorgyline plus deprenyl treatment, but not after clorgyline or deprenyl given alone. These results indicate that both MAO A and MAO B need to be inhibited to increase pethidine toxicity and brain 5-HT levels. They support the involvement of 5-HT in the toxic interaction between pethidine and MAO inhibitors.     

β-Phenylethylamine and benzylamine as substrates for human monoamine oxidase A: A source of some anomalies?

Monoamine oxidase (MAO) A predominates both in human placenta and lung. With 5-hydroxytryptamine (5-HT), β-phenylethylamine (PEA) and benzylamine (Bz) as substrates and clorgyline and deprenyl, respectively, as selective MAO A and B inhibitors, their activity pattern has been defined and compared with that of human liver. PEA had a much higher V_max with placental MAO A than did Bz; it behaved largely as an A substrate in placenta, and partly as an A substrate in lung. At commonly used substrate concentrations, deamination of Bz (sensitive to 10^?7 M deprenyl) was a better indicator of MAO B activity than deamination of PEA. The divergence between PEA and Bz as MAO A and B substrates may be one reason for some of the apparent discrepancies in the behaviour of MAO A and B noted in a variety of tissues in the literature.However, Bz reacts with benzylamine oxidase (BzAO) as well as MAO B. Depending on the tissue, deprenyl-resistant Bz activity may indicate the presence of BzAO rather than MAO A. As there is a widespread distribution of BzAO in man and rat, BzAO should be considered among the alternatives of enzyme activity when Bz is used as substrate.     

Serotonin oxidation by type B MAO of rat brain

The two MAO types in rat brain can be selectively inhibited by administering intraperitoneal injections of clorgyline or pargyline in suitable doses. Brain mitochondria prepared from such animals exhibit type B or type A MAO activity, respectively. In vitro clorgyline and deprenyl dose-response curves confirmed the purity of the enzyme preparations. Specific activities and K_m values of such preparations were determined for tyramine, serotonin and benzylamine. Type B and type A MAO were found to oxidize serotonin and benzylamine. respectively, although they had low affinities. Serotonin oxidation by mitochondria prepared from clorgyline treated animals showed type B characteristics also in its heat inactivation time course.     

Cumulative effects of irreversible MAO inhibitors in vivo

The effects of repeated treatment with clorgyline, pargyline, deprenyl and tranylcypromine on MAO activity in rat brain and liver were investigated. MAO was measured with the substrates serotonin (5HT), phenethylamine (PEA) and, in some cases, t brain tissue after single and repeated administrations of 10 mg/kg s.c. clorgyline or deprenyl were also compared. Single doses of clorgyline (1 and 10 mg/kg s.c.) completely blocked the deamination of 5-HT. PEA deamination gradually decreased during the 14-day treatment. Pargyline in a dose of 0.3 mg/kg s.c. reduced both 5-HT and PEA deamination progressively over the same period. In the course of repeated treatment the effects of clorgyline and deprenyl on 5-HT and PEA deamination increased in intensity, by a factor of about 10 in the brain and about 3 in the liver. The potentiation of the effect of tranylcypromine was less marked (brain: × 4; liver: × 2). The rates of recovery of MAO activity were not greater after repeated than after single administrations of high doses of clorgyline and deprenyl, suggesting that the withdrawal of the drugs is not followed by a rebound phenomenon. Our results indicate that repeated treatment with suitable doses of clorgyline or deprenyl leads to specific reduction of either MAO A or B activity in brain, without producing any appreciable effect in the liver.     

Substrate specificity and inhibitor sensitivity of monoamine oxidase in rat kidney mitochondria.

The deamination of the substrates 5-hydroxytryptamine (5-HT), tyramine, dopamine, β-phenylethylamine and benzylamine by rat kidney mitochondrial monoamine oxidase (MAO) was studied, and kinetic constants are reported for each substrate. By the use of the selective MAO inhibitors, clorgyline and deprenyl, 5-HT and benzylamine were found to be substrates for types A and B MAO, respectively, in this tissue, whereas the other substrates were metabolized by both forms of MAO. No evidence for any significant metabolism of 5-HT or benzylamine by other amine oxidases was obtained. However, some conditions under which the carbonyl reagents semicarbazide, isoniazid and aminoguanidine may interfere with assays for MAO, without actually affecting enzyme activity directly, are described. Preincubation of kidney mitochondria with histamine resulted in a time- and oxygen-dependent irreversible inhibition of both type A and type B MAO activity; the exact nature of the inhibitory agent and its mode of action remain to be determined.     

Properties of a semicarbazide-sensitive amine oxidase in human umbilical artery

The metabolism of some aromatic amines by amine oxidase activities in human umbilical artery homogenates has been studied. The inhibitory effects of clorgyline showed that 5-hydroxytryptamine (5-HT) and tryptamine, 1 mM, were predominantly substrates for monoamine oxidase (MAO) type A, whereas MAO-A and B were both involved in the metabolism of beta-phenylethylamine (PEA), 100 microM, and tyramine, 1 mM. About 20-30% of tyramine and PEA metabolism was resistant to 1 mM clorgyline, but sensitive to inhibition by semicarbazide, 1 mM, indicating the presence of a semicarbazide-sensitive amine oxidase (SSAO). Benzylamine, 1 mM, appeared to be metabolized exclusively by SSAO with a Km (161 microM) at pH 7.8 similar to that found for SSAO in other human tissues. Tyramine and PEA were relatively poor substrates for SSAO, with very high apparent Km values of 17.6 and 13.3 mM, respectively, when determined in the presence of clorgyline, 10(-3) M, added to inhibit any metabolism of those amines by MAO activities. However, kinetic studies with benzylamine indicated that clorgyline, 10(-3) M, also appears to inhibit SSAO competitively such that the true Km values for tyramine and PEA may be about 60% of those apparent values given above. No evidence for the metabolism of 5-HT or tryptamine by SSAO was obtained. The aliphatic amine methylamine was recently shown to be a specific substrate for SSAO in umbilical artery homogenates. We have used benzylamine and methylamine as SSAO substrates in histochemical studies to localize SSAO in tissue sections.(ABSTRACT TRUNCATED AT 250 WORDS)     

MAO types in guinea pig liver mitochondria

MAO of guinea pig liver mitochondria actively deaminated dopamine, tyramine, serotonin and 5-methoxy-tryptamine, while tryptamine, 5-methyl-tryptamine and 7-methyl-tryptamine were moderately deaminated. Very little deamination occurred when benzylamine. noradrenaline and β-phenylethylamine were used as substrates. The in vitro inhibition patterns of MAO of guinea pig liver mitochondria by some selective inhibitors were investigated in the presence of tyramine, tryptamine and serotonin. Tryptamine oxidation showed biphasic inhibition pattern with harmaline, clorgyline and Lilly 51641, while the inhibition curves in the presence of pargyline and deprenyl were sigmoidal. The inhibition curves for tyramine oxidation were biphasic with all the inhibitors except pargyline. Serotonin-MAO inhibition curves, on the other hand, were sigmoidal with all the inhibitors except Lilly 51641. Thermal treatment of guinea pig liver mitochondria produced rapid inactivation of serotonin and tryptamine oxidizing activity, while benzylamine oxidizing activity was found to be most thermostable.     

Properties of monoamine oxidase in mouse neuroblastoma N1E-115 cells

Monoamine oxidase (MAO) from adrenergic mouse neuroblastoma N1E-115 cells was compared to MAO found in rat and mouse brain, rat superior cervical ganglion, and human platelet. In comparison to MAO from brain and ganglion, mouse neuroblastoma MAO deaminated 5-hydroxytryptamine (5-HT) to a proportionately greater extent than all other substrates studied, with benzylamine deamination representing only 1 per cent that of 5-HT. Neuroblastoma MAO was over 1000 times more sensitive to inhibition by clorgyline than by deprenyl. With increasing concentrations of clorgyline, inhibition of tyramine deamination was represented by a simple sigmoid curve, suggesting the presence of primarily one form of MAO. Our results are consistent with evidence for a specific form of MAO associated with sympathetic neurons and suggest that neutoblastoma N1E-115 cells are highly enriched in MAO type A.     

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.