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.     

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.     

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.     

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.     

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.     

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.     

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.     

Studies on beta-phenylethylamine deamination by human placental monoamine oxidase

Kinetical properties of human placental monoamine oxidase (MAO) were investigated in studies on inhibitors and mixed substrates. MAO activity was determined by a radioisotopic assay. Lineweaver-Burk plots were linear at higher and lower concentrations of PEA, whereas at intermediate substrate concentrations, a downward curving plot was obtained. The Km values of the low- and high-affinity sites for PEA deamination were estimated. Studies with mixed substrates showed that 5-HT was a competitive inhibitor and tyramine a mixed-type inhibitor of deamination at high concentrations of PEA, whereas both were non-competitive inhibitors at lower concentrations of PEA. After pre-incubation of human placental mitochondrial preparations with deprenyl, Lineweaver-Burk plots were completely linear, and the Km value was the same as that obtained at low concentrations of PEA in the absence of deprenyl. Tyramine and 5-HT were competitive inhibitors of PEA deamination by deprenyl-treated MAO. From these results it is concluded that there are two kinds of MAO with high- and low-affinity sites for PEA in mitochondria of human placenta, corresponding to type B and A Mao, and that tyramine, 5-HT and PEA share a substrate-binding site on type A Mao, while tyramine and 5-HT bind to a site on type B MAO that is different from the PEA binding site.     

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.     

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.     

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.     

Monoamine oxidase activity of macrophages at rest and during phagocytosis

Mouse macrophages contain monoamine oxidase (MAO) A activity and traces of MAO B, as judged by a strong deamination of 5-hydroxytryptamine and tyramine and a marginal one of benzylamine. Significant inhibition of MAO activity occurred in the presence of the specific inhibitors clorgyline and deprenyl. MAO A activity was considerably depressed in phagocytizing cells.     

Studies on the subcellular localization of monoamine oxidase types A and B and its importance for the deamination of dopamine in the rat brain

The distribution of the MAO-forms A and B between intra- and extrasynaptosomal rat brain mitochondria was studied with the aid of their known substrate and inhibitor specificities. The activities with the selective substrates serotonin, PEA and benzylamine indicated that intrasynaptosomal mitochondria have about a 3.4-fold higher MAO A:MAO B ratio than extrasynaptosomal mitochondria. However, PEA was found to be a selective substrate for MAO B only at low concentrations (such as 5 × 10^?6M), whereas at higher concentrations (such as 10^?3M) it was a substrate for both forms of MAO. The different ratios of the two enzyme forms in the two mitochondrial populations were confirmed when the selective inhibitors clorgyline and deprenyl were used with dopamine or 10^?3M PEA. With these two amines, the ratios of MAO A: MAO B activities were 3–4.5 times higher in intrasynaptosomal than in extrasynaptosomal mitochondria. In particular, when the activity with dopamine was measured in intact synaptosomes, deamination being preceded by a specific uptake into these particles, the inhibitor sensitivities clearly showed that MAO activity was almost exclusively attributable to the A-form of the enzyme. Thus, mitochondria in the terminals of dopaminergic neurones have an even more pronounced enrichment in MAO A than the mitochondria obtained by osmotic lysis of a total brain synaptosomal preparation. It was also found that clorgyline and deprenyl have an inhibitory effect on the uptake of dopamine into nerve endings with IC₅₀ values in the range of 10^?5 to 10^?4M. These results are discussed in terms of possible physiological significancies of the properties and distribution of the two forms of MAO.     

Characterization of rat skeletal muscle monoamine oxidase.

Optimal conditions for deamination of 5-hydroxytryptamine in rat skeletal muscle were determined. The presence of monoamine oxidase (MAO) A and MAO B isozymes was demonstrated by the use of tyramine (a substrate of both forms), specific substrates (serotonin and benzylamine), and specific inhibitors (clorgyline and deprenyl) of MAO A and B respectively. A 6.5:3.5 ratio of MAO A to B was found using a whole muscle homogenate, while a 7.5:2.5 ratio was found with isolated mitochondria. Thermal inactivation studies demonstrated that skeletal muscle MAO A is more susceptible to heat inactivation than MAO B. The approximate proportion of muscle homogenate MAO which is present in sympathetic nerves was found to be 18 per cent, as determined by treating rats with 6-hydroxydopamine and quantifying the decrease in activity. Significant inhibition of MAO activity was observed after administration in vivo of the MAO inhibitors pargyline, tranylcypromine and harmaline.     

Human brain monoamine oxidase: one molecular entity-multiple binding sites?

Monoamine oxidase (MAO) of human brain cortex was partially characterized by using different substrates and inhibitors. Two K_m values were calculated for each of the three substrates tested, i.e., phenethylamine (PEA), benzylamine (BA) and 5-hydroxtryptamine (5-HT). Clorgyline and 5-HT, both known as MAO-A occupants, were able to abolish the second (high) K_m deamination of PEA. 5-HT, while non-competitively inhibiting the deamination of low BA concentrations, competitively inhibited the deamination of high concentrations of this type B substrate. The kinetics of 5-HT deamination showed positive cooperation which indicates the involvement of subunits in the enzyme structure. The ability of some phospholipids to change the enzyme behaviour was considered as indication that these molecules might play a role in determining the ratio between the so-called A and B types of MAO, and in the regulation of the enzyme's activity.     

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)     

Studies of monoamine oxidases. Inhibition of bovine brain MAO in intact mitochondria by tricyclic antidepressant drugs.

Tricyclic antidepressant drugs (TCA) were found to reversibly inhibit monoamine oxidase (MAO) in intact mitochondria of beef brain cortex, I₅₀, values were in the range of 10^?4 to 10^?3 M, using chlorimipramine, amitriptyline, desimipramine, imipramine and doxepin. Unlike TCA inhibition reported for MAO in rabbit tissues, the inhibition observed with beef brain MAO was greater for the A-type enzyme, indicated by serotonin (5-HT) deamination, than for the B-type enzyme, indicated by phenylethylamine (PEA) deamination. Chlorimipramine was the most effective of the five tricyclic antidepressant drugs tested for the inhibition of 5-HT deamination, while amitriptyline was the most effective for inhibiting PEA deamination. Kinetic analyses also revealed marked differences in the interaction of the tricyclics with the A form and the B form of MAO. Inhibition was found to be of a mixed type by reciprocal plots, but Dixon plots indicated that the inhibition was parabolic with 5-HT and either linear or hyperbolic with PEA, depending on the TCA used. Mixed inhibitor studies were also carried out, combining a TCA with a selective (clorgyline or deprenyl) or a non-selective (tranylcypromine) MAO inhibitor. Such combinations did not result in a potentiation of inhibition of either the MAO-A or MAO-B type enzyme activity. The present results indicate that the inhibition of MAO may be of only minor significance in the therapeutic efficacy of TCA in the treatment of depression, especially in combined therapy. However, this conclusion must be tempered by the knowledge that there are marked variations in MAO properties from different enzyme sources, as evidenced by these results.     

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.     

Characterization of eight biogenic indoleamines as substrates for type A and type B monoamine oxidase

Tryptamine, N-methyltryptamine, N,N-dimethyltryptamine, 5-hydroxytryptamine (5-HT), 5-hydroxy-N-methyltryptamine, bufotenine, 5-methoxytryptamine, and 5-methoxy-N,N-dimethyltryptamine were characterized as substrates for type A and type B monoamine oxidase (MAO) in rat liver mitochondria. Experiments on sensitivity to clorgyline and to deprenyl, using two substrate concentrations, showed that tryptamine and its N-methylated and N,N-dimethylated derivatives were common substrates for both types of MAO at a substrate concentration of 20.0 μM; at 1000 μM, tryptamine and N-methyltryptamine were common, but N,N-dimethyltryptamine became specific for type B MAO. All the 5-hydroxy- or 5-methoxy-indole derivatives were almost completely specific for type A MAO at a substrate concentration of 20.0 μM; when the concentration was 1000 μM, some of the MAO activity was due to type B MAO for 5-HT, bufotenine and 5-methoxytryptamine. The rat liver mitochondrial enzyme was pretreated with 10^?7M clorgyline and 10^?7M deprenyl to obtain, respectively, the type B-rich and the type A-rich enzyme. These enzyme preparations were subjected to kinetic analyses for the eight amines. From the kinetic analyses, together with data on inhibitor sensitivity, the following phenomena can be described. N-Methylation of tryptamine or of 5-HT did not cause appreciable changes in the specificity of the substrates toward each type of MAO, but it elevated the K_mvalue of type B MAO when the values for tryptamine and N-methyltryptamine were compared. N,N-Dimethylation of tryptamine and 5-HT tended to increase the specificity for type B MAO. All the dimethylated compounds had very low activities with either type A or type B MAO. Either the 5-hydroxy- or the 5-methoxy-group contributed to the specificity of the substrates for type A MAO.