Some interrelated properties of A and B form monoamine oxidase in monkey brain mitochondria

The multiplicity of monoamine oxidase (MAO) in monkey brain was studied by comparing the relationship between the selective substrates of MAO and the pH-activity curves obtained using these substrates. When mitochondrial and A-form MAO were used as the enzyme preparations with serotonin (5-HT) and norepinephrine (NE), preferential substrates for A-form MAO, the pH optima were 8.8 and 7.8 with 5-HT and 8.5 and 7.2 with NE. These substrates were also oxidized by B-form MAO after changing the pH of the incubation medium (shift to alkaline); these pH optima were 9.0 and 8.2, respectively. When common substrates of MAO were used (tyramine, octopamine, dopamine and tryptamine), the pH activity curves obtained were all broad and bell-shaped with pH optima for the 3 species of enzyme (mitochondria, A-form and B-form MAO) at 8.0, 7.8, and 8.0 with tyramine; 8.3, 7.5, and 8.5 with octopamine; 7.8, 7.5, and 8.5 with dopamine; and 8.0, 8.3, and 6.9 with tryptamine, respectively. The pH optima were 6.6 with beta-phenylethylamine (beta-PEA) and 9.0 with benzylamine, preferential substrates for B-form MAO, for either mitochondria or B-form MAO. The Km values obtained for tryptamine and beta-PEA were lower than those for the other substrates of MAO, regardless of the enzyme preparations. The Km and Vmax values of both forms MAO for 5-HT and NE were similar to those of the A-form MAO. The differences in the Km and Vmax values of the A-form MAO and B-form MAO for common substrates were comparable. Tyramine, octopamine and dopamine were substrates for both forms MAO, with only a slight preference for B-form MAO over A-form MAO. However, tryptamine may be deaminated predominantly by A-form MAO.     

Changes in MAO activities in several organs of rats after administration of l-thyroxine

Male rats were given daily injections of 200 micrograms/kg l-thyroxine, s.c., for a period of 10 days. Monoamine oxidase (MAO) activities in the heart, lung and liver mitochondria decreased rapidly to about 50% those of the control rats with 5-HT and beta-phenylethylamine (beta-PEA) as substrates on the first day. After that, heart MAO activity increased gradually and exceeded the control value after 10 days with 5-HT as the substrate. The level of liver MAO activity was maintained at about 50-70% during the same period of administration with 5-HT as the substrate. The thyroxine treated rats showed no marked change in brain MAO activity. In vitro, l-thyroxine and its metabolites had no discriminative actions on MAO activities in these organs of rats. The heart, lung and liver MAO have unaltered Km values for 5-HT and beta-PEA, but decreases in the Vmax for both substrates were observed between the control and l-thyroxine-treated rats. Addition of the brain, heart and liver cytosol fractions from l-thyroxine treated rats caused MAO activities of heart mitochondria to decrease with 5-HT as a substrate and caused them to increase with beta-PEA as a substrate. MAO activities in liver also were inhibited by adding all the cytosols when beta-PEA was the substrate, but on the contrary, lung MAO activities were increased when 5-HT was the substrate. These results indicate the possible presence of multiple modulators of MAO in the cytosol fractions of l-thyroxine treated rats.     

Effects of d-methamphetamine on monkey brain monoamine oxidase, in vivo and in vitro

A and B form MAO activities in mitochondria and synaptosome were measured in the brain of monkeys administered d-methamphetamine (d-MP) 2 mg/kg, i.m., daily for 7 days. When mitochondria were used as an enzyme preparation, the Km and Vmax values decreased with 5-HT (serotonin for A-form MAO substrate) and beta-phenylethylamine (beta-PEA for B-form MAO substrate), while in the synaptosome, a significant increase of the Km and Vmax values was observed with 5-HT and dopamine as substrates. The mitochondrial MAO treated with d-MP was inhibited strongly by clorgyline and deprenyl with beta-PEA as a substrate, while synaptosomal MAO was highly sensitive to these MAO inhibitors with 5-HT as a substrate. MP and amphetamine (AP) were found in brain mitochondrial and synaptosomal preparations of monkeys administered 2 mg/kg d-MP, i.m. daily for 7 days; MP and AP contents were 5.05 +/- 0.22 pg/mg protein and 37.3 +/- 3.8 ng/mg protein in mitochondria and 2.35 +/- 0.35 pg/mg protein and 46.4 +/- 1.5 ng/mg protein in synaptosomes, respectively. MAO was inhibited by MP and its metabolites, AP p-hydroxymethamphetamine (OH-MP) and p-hydroxyamphetamine (OH-AP), with 5-HT, beta-PEA and dopamine as substrates, in vitro. MP and its metabolites were more potent inhibitors of A-form MAO than B-form MAO.     

Evidence for existence of A and B form monoamine oxidase in mitochondria from dog platelets

It is known that platelet MAO appears to behave more like the B-form enzyme than the A-form enzyme based on inhibitor sensitivity and substrate specificity. However, dog platelets showed a different substrate specificity such as high activity with 5-HT and beta-PEA as substrates. Moreover, dog platelet MAO was sensitive to the drugs clorgyline and harmaline with 5-HT as the substrate, while it was sensitive to the drug deprenyl with beta-PEA as the substrate. These results also indicate the existence of two forms of MAO in dog platelets unlike in other platelets such as those from humans. A-form MAO from dog platelets was more stable against heat treatment at 55 degrees C than A-form MAO from dog liver and brain. On the other hand, there was no difference in the heat resistance of the three enzymes with beta-PEA as the substrate. After digestion with trypsin at 37 degrees C for 30 min, it was found that MAO from dog platelets, brain and liver were mostly inhibited with 5-HT as the substrate. In contrast, MAO in brain and liver were inhibited about 10%, but platelet MAO was inhibited about 50% with beta-PEA as the substrate. From these results, it is considered that dog platelet MAO exists as the two forms of the enzyme and has different enzymic properties in comparison with those of MAO from dog liver and brain mitochondria.     

Styrene inhibits monoamine oxidase A, but not monoamine oxidase B in monkey brain mitochondria

The effects of styrene on mitochondrial monoamine oxidase (MAO) activity in rat and monkey brains were compared in vitro. After preincubation at 25 degrees C for 20 min with 1 mM styrene monomer MAO-A activity in monkey brain was inhibited potently using 5-HT (for MAO-A substrate), but MAO-B activity in monkey brain and platelets were slightly inhibited using beta-PEA (for MAO-B substrate). Styrene monomer also competitively inhibited MAO-A activity in a dose-dependent manner. MAO-A in monkey brain was inhibited by styrene in ascending order of potency: styrene trimer>styrene dimer>styrene monomer. In contrast styrene monomer slightly inhibited both MAO-A and MAO-B activities in rat brain mitochondria. In the present study styrene monomer potently inhibits MAO-A activity, but not MAO-B activity, in monkey brain mitochondria in vitro. These results indicate the inhibiting action of styrene differs depending on animal species and MAO isoforms.     

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.     

Endogenous monoamine oxidase inhibitor-like substances in monkey brain

The extraction and partial purification of endogenous "monoamine oxidase (MAO) inhibitor-like" material from the monkey brain are described. The endogenous material (F-1 and F-2) obtained after Bio-Gel P-2 gel filtration and silica column chromatography inhibited MAO in the monkey brain mitochondria toward 5-hydroxytryptamine (5-HT), beta-phenylethylamine (beta-PEA), tyramine and dopamine as substrates. The inhibitory effects of F-1 and F-2 were non-linear concentration dependent, and F-1 non-competitively inhibited A-form MAO, while F-2 inhibited A-form MAO competitively and inhibited B-form MAO non-competitively. These substances were more potent inhibitors of A-form than of B-form MAO. F-2 was heat stable but liable to the treatment with pepsin and trypsin. F-1 was not inactivated by heat treatment and digestion with pepsin and trypsin. F-1 may be a low molecular weight (less than 1350) compound, including certain monoamines or their metabolites or other unidentified compounds, while F-2 was a low molecular weight (about 2500) peptide.     

Studies on monoamine oxidase. XVIII. Enzymic properties of placental monoamine oxidase.

Enzymic properties of partially purified monoamine oxidase (MAO) from human placenta were studied with tyramine, serotonin and benzylamine as substrates. The highest activity was obtained with serotonin and almost no activity was observed with benzylamine. These results are similar to those obtained with rat placental MAO, but different from those with rabbit placental MAO. The Km values for serotonin and tyramine were found to be 0.21 mM and 0.23 mM, respectively and the pH optimum was 8.1 with either substrate. The thermal inactivation curves of this enzyme with the two substrates were identical. The pI curves for inhibition of MAO activity by harmine, pargyline and iproniazid were similar and almost the same pI 50 values for the respective inhibitors were obtained with the two substrates. MAO in human placenta differs from that in other organs, such as liver, brain and plasma from the standpoint of the substrate specificity and the inhibitor sensitivity. The possibility that human placenta contains a single form of MAO is discussed on the basis of the present results.     

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.     

Substrate and inhibitor selectivity of human heart monoamine oxidase.

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

Inhibition by Zn(2+) of A-form monoamine oxidase in monkey brain mitochondria

The effects of ZnSO(4) on mitochondrial monoamine oxidase (MAO) activity in monkey brain were compared with those in rat and rabbit, in vitro. After preincubation at 25 degrees C for 20 min with 1 microM ZnSO(4), MAO-A activity in monkey brain was about 50% using serotonin (5-HT) as a substrate, and the inhibition was proportional to the concentration of ZnSO(4). However, ZnSO(4) had no effect on MAO-B activity in monkey brain using beta-phenylethylamine (beta-PEA) as a substrate. The inhibition by ZnSO(4) of MAO-A activity was competitive and reversible. CdSO(4) also inhibits MAO-A, but not MAO-B in monkey brain mitochondria. ZnSO(4) did not inhibit either MAO-A or MAO-B activity in rat and rabbit brain mitochondria. These results indicate that the inhibiting action of Zn(2+) differs depending on animal species. In monkey brain mitochondria, MAO-A was highly sensitive to Zn(2+) and MAO-B was less sensitive. These results also suggest that Zn(2+) may regulate the level of catecholamine content in monkey brain.     

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.     

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.     

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.     

Changes in amine oxidase in plasma of rats treated with hepatotoxins

When allyl formate (AF) was administered to rats, the marked elevation of B-form MAO activity in plasma was found with beta-PEA as a substrate. In contrast, in the case of carbon tetrachloride (CCl4), A-form MAO activity elevated predominantly. The deaminations of 5-HT and beta-PEA in these plasma treated with AF or CCl4 were not inhibited completely by a high concentration of MAO inhibitor, deprenyl or clorgyline. These results indicate that there may be two or more distinct amine oxidases released from the liver and other organs in response to CCl4 or AF.     

Distribution and inhibition characteristics of human brain monoamine oxidase.

Monoamine oxidase (MAO) activity in 14 regions of 10 normal post-mortem human brains using 5-hydroxytryptamine (5HT), benzylamine, tyramine and dopamine as substrates is presented. Regional distribution with 5HT, benzylamine and tyramine was generally similar with the highest activities observed in the hypothalamus. However, with dopamine as substrate, highest MAO activity occurred in the nucleus accumbens. Although there was relatively greater MAO activity towards 5HT than towards benzylamine in all four cerebral cortical areas studied compared with the caudate, putamen, accumbens and hypothalamus this apparently greater proportion of type A MAO in cortex could not be confirmed with the use of the specific inhibitor clorgyline. In some cases inhibition curves with clorgyline (and correspondingly with deprenyl) were not the expected double sigmoid shape. It is suggested that characterisation of MAO by techniques dependent on the use of specific inhibitors in samples of human brain collected and stored in the usual manner may prove difficult to interpret.     

Studies on the nature of the increased monoamine oxidase activity in the rat heart after adrenalectomy

The increased activity, induced by adrenalectomy, of the enzyme monoamine oxidase (MAO) in the rat heart was found to resemble closely that present in the hearts of control animals. No significant differences were observed in the nature of the response to heat denaturation, changes in pH or to inhibition by pargyline or clorgyline. The relative activities using the substrates tyramine, 5-hydroxytryptamine, dopamine or benzylamine were the same. No evidence was found to suggest the presence of a heat-stable or dialysable inhibitor of enzyme activity. In young rats there was an increase in the relative enzyme activity using benzylamine as substrate, compared with the activity using tyramine, in the first few days after adrenalectomy. No effect of adrenalectomy could be detected upon the MAO activity in the rat brain or liver. It is concluded that the increase in rat heart MAO following adrenalectomy cannot be due to the synthesis of an enzyme with different catalytic properties, nor to the transformation of the existing enzyme into one of increased catalytic ability but with different properties.     

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 monoamine oxidase. (Report 37) Effects of oxygen concentration on rat liver and brain monoamine oxidase (author's transl)]

MAO activity in rat brain mitochondria with tyramine as substrate at 100% oxygen concentration was three times as much as that at 20%. When serotonin served as substrate, difference in activities between the two oxygen concentrations was not significant. Similar results were obtained when rat liver MAO was used as the enzyme source. At 100% oxygen concentration, pargyline showed the most potent inhibition of MAO activity in liver mitochondria with tyramine as substrate, but inhibitions caused by pheniprazine and harmaline were not remarkable. At 100% oxygen concentration, harmaline showed the most potent inhibition of MAO activity in the liver when serotonin served as substrate, while inhibitions of the MAO activity by pargyline and pheniprazine were weak. At 20% oxygen concentration, harmaline showed the most potent inhibition of MAO activity in the brain when serotonin was used as substrate. These inhibitions were studied using Lineweaver-Burk plots. Pargyline revealed a noncompetitive inhibition to MAO activity in liver and brain with tyramine and serotonin as substrate, harmaline a competitive inhibition to MAO activity in liver and brain with tyramine as substrate, while noncompetitive inhibition to MAO activity in liver and brain was evident when serotonin was used as the substrate.     

Comparative lipophilicities of substrates of monoamine oxidase

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