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.     

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.     

Selective influences of age and thyroid hormones on type A monoamine oxidase of the rat heart.

The specific actiivty of rat heart MAO, towards both tyramine and benzylamine as substrates, was found to increase with the age of the animal, and also after administration of (-)-thyroxine to young male rats. Conversely, enzyme activity was decreased in animals made hypothyroid by including 2-thiouracil in their diet. However, with both age and altered thyroid status, relatively greater changes in the deamination of tyramine rather than in that of benzylamine, were obtained. Clorgyline and deprenyl, used as inhibitors of rat heart MAO, indicated that tyramine is metabolized solely by MAO-A, whereas benzylamine is a substrate for both MAO-A and -B, and also a clorgyline- and deprenyl-resistant enzymic activity. The proportional contribution of MAO-A, -B and the clorgyline-resistant enzyme towards the total benzylamine deamination in the rat heart was found to vary with the age and with altered thyroid status of the animal in such a way that selective changes in the activity of MAO-A appear to be largely responsible for the overall changes in the specific activity of rat heart MAO which occur in response to these developmental factors.     

Selective influences of age and thyroid hormones on Type A monoamine oxidase of the rat heart

The specific activity of rat heart MAO, towards both tyramine and benzylamine as substrates, was found to increase with the age of the animal, and also after administration of (?)-thyroxine to young male rats. Conversely, enzyme activity was decreased in animals made hypothyroid by including 2-thiouracil in their diet. However, with both age and altered thyroid status, relatively greater changes in the deamination of tyramine rather than in that of benzylamine, were obtained. Clorgyline and deprenyl, used as inhibitors of rat heart MAO, indicated that tyramine is metabolized solely by MAO-A, whereas benzylamine is a substrate for both MAO-A and -B, and also a clorgyline- and deprenyl-resistant enzymic activity. The proportional contribution of MAO-A, -B and the clorgyline-resistant enzyme towards the total benzylamine deamination in the rat heart was found to vary with the age and with altered thyroid status of the animal in such a way that selective changes in the activity of MAO-A appear to be largely responsible for the overall changes in the specific activity of rat heart MAO which occur in response to these developmental factors.     

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.     

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.     

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)     

Characteristics of mitochondrial and synaptosomal monoamine oxidase in monkey brain

Enzymic properties of monoamine oxidase (MAO) from monkey brain were studied. High MAO activity was observed in the mesencephalon and dienecephalon of the brain. Highest activity in every region of the brain was found with tyramine as a substrate. Monkey brain mitochondrial MAO showed a different substrate specificity and different Km and Vmax values than the enzyme from mice, rats, guinea pigs and rabbits. The pH activity curves were all bell-shaped, but the pH optima were remarkably different with the various substrates used. The activities of various substrates at pH 7.2 were compared with those at the pH optimum. At the pH optima, the activity was about 1.2-fold higher with tyramine and dopamine, 2-fold higher with beta-phenylethylamine (beta-PEA) and 3-fold higher with serotonin (5-HT) and benzylamine. These results were almost similar when synaptosomes from monkey brain were used. MAO activities with 5-HT and beta-PEA were strongly inhibited by much lower concentrations of clorgyline and deprenyl, respectively. Plateau-shaped inhibition curves by these inhibitors were obtained with tyramine as the substrate. These results indicate that both the A- and B-form of MAO appear to be uniformly distributed in monkey brain, and the A-form of MAO represents approximately 35% and 50% of the total MAO activity in mitochondria and synaptosomes, respectively.     

The effect of age and thyroid hormones upon the ability of the chick heart to deaminate monoamines

The effect of age and thyroid hormones upon the ability of chick heart homogenates to metabolize monoamines has been investigated. 5-Hydroxytryptamine is entirely metabolized by a monoamine oxidase (MAO) with the characteristics of MAO-A, whereas some of the tyramine and all of the benzylamine are oxidatively deaminated by a clorgyline-resistant, but semicarbazide-sensitive enzyme, with a similar subcellular distribution to that of MAO. The remainder of the tyramine deamination is brought about by MAO-A and MAO-B. The specific activities of both clorgyline-sensitive and resistant enzymes are increased by the same proportion by increase in age or by treatment with (-)-thyroxine, and decreased by 2-thiouracil. The significance of these results is discussed.     

The effect of age and thyroid hormones upon the ability of the chick heart to deaminate monoamines.

The effect of age and thyroid hormones upon the ability of chick heart homogenates to metabolize monoamines has been investigated. 5-Hydroxytryptamine is entirely metabolized by a monoamine oxidase (MAO) with the characteristics of MAO-A, whereas some of the tyramine and all of the benzylamine are oxidatively deaminated by a clorgyline-resistant, but semicarbazide-sensitive enzyme, with a similar subcellular distribution to that of MAO. The remainder of the tyramine deamination is brought about by MAO-A and MAO-B. The specific activities of both clorgyline-sensitive and resistant enzymes are increased by the same proportion by increase in age or by treatment with (--)-thyroxine, and decreased by 2-thiouracil. The significance of these results is discussed.     

Effect of lignocaine on monoamine oxidase activity of brain and liver

In vivo administration of a single dose (100-150 mg/kg, i.p.) of lignocaine produces no change in MAO activity, while long-term treatment (50 mg/kg/day for 15 and 30 consecutive days, i.p.) produces a slight but appreciable inhibition of MAO activity with tyramine or serotonin but not with benzylamine as substrate in both rat brain and liver mitochondria. Lignocaine (2-20 mM) inhibits (in vitro) both brain and liver mitochondrial MAO activity, using tyramine, serotonin and benzylamine as substrates, in a concentration-dependent manner. Furthermore, lignocaine produces a marked in vitro inhibition of serotonin and tyramine oxidation in MAO-A and not in MAO-B preparation of rat brain. Ackermann-Potter plots of MAO indicate that lignocaine-induced inhibition of MAO activity is reversible in nature. Lineweaver-Burk plots show that lignocaine (2-10 mM) produces a significant increase in Km and decrease in Vmax of MAO for tyramine and serotonin in both brain and liver. Similarly Km and Vmax values are changed using benzylamine as substrate in the presence of relatively higher concentrations of lignocaine (5-20 mM). These results suggest that lignocaine-induced inhibition of mitochondrial membrane-bound MAO activity of both neuronal and non-neuronal tissues is associated with its conformational change.     

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.     

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.     

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.     

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.     

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.     

The effect of lipophilic compounds upon the activity of rat liver mitochondrial monoamine oxidase-A and -B

The effect of various lipophilic compounds on the activity of monoamine oxidase (MAO) was determined. The local anaesthetics procaine, procainamide, tetracaine and lignocaine were all MAO-A selective inhibitors, whereas benzyl alcohol, butan-l-ol, hexan-l-ol and octan-l-ol inhibited MAO-B selectively. Procaine was found to be a competitive inhibitor of the deamination of 5-hydroxy-tryptamine (5-HT), tyramine, β-phenethylamine and benzylamine. Benzyl alcohol was competitive towards β-phenethylamine and benzylamine, but a mixed-type inhibitor towards 5-HT and tyramine. The same patterns of inhibition for both drugs were found when the activity was assayed under atmospheres of either oxygen or air. The inhibition produced by both compounds was fully reversible. Triton X-100 appeared to inhibit the activity of MAO-A selectively when preincubated with the enzyme for 30 min at 30°. This selectivity was lost when the preincubation temperature was raised to 37°. The inhibition of MAO activity by Triton X-100 after preincubation at 37° was found to be irreversible. Sodium deoxycholate and SDS were also found to inhibit the activity of MAO after preincubation with the enzyme at 37°. The significance of these results is discussed.     

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.     

An in vitro interethnic comparison of monoamine oxidase activities between Japanese and Caucasian livers using rizatriptan, a serotonin receptor 1B/1D agonist, as a model drug

AIMS: Monoamine oxidase (MAO) is located in human liver, and catalyses the oxidative deamination step of many xenobiotics. However, whether there exists an interethnic difference in MAO activities has, to our knowledge, not been clarified. We aimed to assess the MAO type A (MAO-A) involvement in the metabolic pathway of rizatriptan (RIZ), an antimigraine 5-hydroxytryptamine (5-HT)1B/1D agonist, and the interethnic difference in MAO activities between Caucasians and Japanese using RIZ as a model drug in in vitro experiments. METHODS: Oxidative deaminase activities were determined with the subcellular fractions of Japanese livers and the microsomal fraction of Caucasian livers using RIZ, 5-HT (MAO-A substrate) and 2-phenylethylamine (PEA) (MAO-B substrate) as substrates. RESULTS: The oxidative deaminase activities of RIZ vs. 5-HT were highly (r = 0.87 and 0.96, P < 0.001) correlated with each other in both the microsomal and mitochondrial fractions of Japanese livers. Subsequent results were obtained from in vitro experiments using liver microsomes based upon these findings. The oxidative deaminase activities of RIZ were inhibited completely by the nanomolar-order concentration of clorgyline and Ro 41-1049 (MAO-A selective inhibitors), but not by that of Ro 16-6491 (MAO-B selective inhibitor). The majority of the mean Michaelis-Menten values for three substrates toward MAO obtained from six Japanese and six Caucasian liver microsomes reached no significant differences between the two ethnic groups. The mean microsomal oxidative deaminase activities assessed in 18 Japanese and 20 Caucasian livers using the three substrates also showed no significant differences between the two ethnic groups. CONCLUSIONS: RIZ is mainly metabolized by MAO-A and the in vitro oxidative deaminase activities mediated via MAO-A and -B do not appear to differ between Japanese and Caucasians.