Characteristics of monoamine oxidase activity in brain and other organs of the adult bullfrog, Rana catesbeiana

1. Monoamine oxidase (MAO) activity was measured in brain, liver, kidney and intestine of the adult bullfrog by a fluorometric method. 2. All tissues contained both type A and type B MAO, on the basis of responses to specific inhibitors, but with different ratios in each tissue. 3. MAO activity was optimum at 30 degrees C. However, MAO type B showed greater activity changes related to incubation temperature than did type A. 4. The Michaelis constant (Km) of MAO also varied with temperature, with a nadir around 20 degrees C. The functional significance of this is not clear. 5. Arrhenius plots showed that the activation energy for MAO B was higher than for MAO A.     

Monoamine oxidase activity in several tissues of the goldfish. Carassius auratus

1. Monoamine oxidase (MAO) was determined fluorometrically in male goldfish tissues, and the effects of specific inhibitors determined. 2. MAO activity in kidney greater than intestine greater than pancreas greater than brain approximately liver greater than testis. Apparent Michaelis constant (Km) was higher in the first three and lower in the last three tissues. 3. Specific MAO type A inhibitors (harmaline, clorgyline) were much more effective than a type B inhibitor (deprenyl) in reducing MAO activity. 4. Apparently goldfish tissues contain only a single type A-like MAO.     

Biochemistry and physiology of monoamine oxidase (MAO) activity in the ring dove (Streptopelia risoria). II. Distribution of MAO in different tissues

Monoamine oxidase (MAO) activity was measured fluorometrically in liver, kidney, intestine and brain of adult male and female ring doves. Liver MAO was inhibited in a concentration-related fashion by clorgyline and harmaline (MAO type A inhibitors) where a plateau in the inhibition curve occurred with about 15% activity remaining, and also by the type B inhibitor deprenyl, which produced a plateau when about 85% activity remained. Kidney, intestine and brain MAO were inhibited in a biphasic manner by harmaline. Results with inhibitors suggest that 85% of liver MAO, 86% of kidney MAO, 88% of intestine and 75% of brain MAO is type A. Using 10(-6) M harmaline to differentiate between MAO-A and MAO-B type activities, the apparent maximal velocities (Vmax) and Michaelis constants (Km) were determined in different tissues. Most activity occurred in the intestine, with proportionally lesser amounts of kidney, liver and brain. The majority of MAO present was in the A form. Except for kidney, Km of MAO-B was higher than that of MAO-A. Both MAO-A and -B activities were higher in the intestines of male birds, although sex differences in content and type of MAO activity were not observed in other tissues of the ring dove.     

The characteristics and distribution of monoamine oxidase (MAO) activity in different tissues of the rainbow trout, Salmo gairdneri

Monoamine oxidase (MAO) activity was determined fluorometrically in brain, intestine, kidney and liver tissues of the rainbow trout, Salmo gairdneri. MAO activity was inhibited by various drugs in a concentration-related manner, with single sigmoid inhibition curves, the inhibitors of type A MAO, harmaline and clorgyline being more effective than deprenyl, an inhibitor of type B MAO. Intestine exhibited greatest MAO activity followed by liver and brain with kidney showing least activity. The Michaelis constants (Km) also showed variability between tissues. Inhibition of MAO by harmaline was non-competitive and dependent on the concentration of substrate present.     

Distribution of monoamine oxidase activity in tissues of the urodeles Ambystoma tigrinum (tiger salamander) and Necturus maculosus (mudpuppy)

1. Monoamine oxidase (MAO) activity was determined fluorometrically in tissues of adult mudpuppies, and pre- (young) and post- (adult) metamorphic tiger salamanders. 2. From responses to specific inhibitors it was determined that 95% activity was MAO type A in all tissues. 3. In young salamanders MAO activity was greater in brain and intestine of males than of females, and was considerably higher in kidney of both sexes and in intestine of males compared to adults. 4. MAO activity was distributed differently in the mudpuppy compared to the salamander. Intestine and liver contained high activity and brain had relatively little MAO activity compared to salamander. 5. The apparent Michaelis constant of MAO activity in the different groups and tissues was generally similar, suggesting a similarity of the MAO molecule.     

Monoamine Oxidase in Rat and Bovine Endocrine

Monoamine oxidase (MAO) was characterized in tissue homogenates from rat pancreatic islets, rat neurohypophysis and adenohypophysis, and rat and bovine adrenal medulla and adrenal cortex. Phenylethylamine was preferentially deaminated by rat pancreatic islet and bovine adrenal medulla MAO and with slight preference by rat neurohypophysis MAO, whereas 5-hydroxytryptamine was preferentially deaminated by MAO from all other endocrine tissues. Tyramine was a good substrate for all tissues. Clorgyline, a selective inhibitor of MAO-A, preferentially inhibited deamination of 5-hydroxytryptamine by all tissue homogenates, whereas deprenyl, a selective inhibitor of MAO-B, preferentially inhibited deamination of phenylethylamine. Km values for 5-hydroxytryptamine and tyramine were higher by one to two decimal powers than for phenylethylamine in homogenates from all endocrine tissues. Km values were significantly lower for 5-hydroxytryptamine and significantly higher for phenylethylamine in rat and bovine adrenal cortex than in adrenal medulla. According to these results, the contributions of MAO-B to total enzyme activity were 70% for rat pancreatic islets, 45% for rat neurohypophysis, 15% for rat adenohypophysis, 20% for rat adrenal medulla, 10% for rat adrenal cortex, 60% for bovine adrenal medulla, and 20% for bovine adrenal cortex. PC 12 cells also contained predominantly MAO-A (90%); however, an increased Km for phenylethylamine and a sensitivity of deamination of this MAO-B substrate to inhibition by clorgyline are indicators of abnormal behavior of MAO in this clonal rat pheochromocytoma cell line.     

Monoamine oxidase and catechol-O-methyl transferase activity in Tetrahymena.

Tetrahymena pyriformis strain HSM was found to have monomine oxidase (MAO) and a catechol-3-methyl transferase-like (COMT) activity. As in mammalian tissues, the MAO activity is predominantly localized in the mitochondrial pellet and COMT in the cytosol. The COMT-like activity was present in amounts comparable to several mouse tissues and was inhibited by tropolone. MAO activity was much lower than in any of the mouse tissues tested, and its activity varied greatly from preparation to preparation. The substrate preference of Tetrahymena MAO was tryptamine greater than serotonin greater than dopamine, and activity increased with increasing pH from pH 6.5 to pH 7.8, as does that of mouse liver MAO. Teh Km of Tetrahymena MAO for tryptamine was approximately 4 micrometer, an order of magnitude lower than that of mouse liver MAO. Sensitivity of inhibition by MAO inhibitors was variable. In some preparations, no inhibition was observed. In others clear inhibition was obtained, harmine and clorgyline being among the most potent inhibitors.     

Some properties of monoamine oxidase in carp heart

Studies using clorgyline, deprenyl and semicarbazide as inhibitors showed that carp heart homogenate contained a new type of monoamine oxidase (MAO) and a clorgyline- and deprenyl-resistant amine oxidase (CRAO). The deamination of monoamines by carp heart MAO proceeded in two steps by a double-displacement (ping-pong) mechanism. The Km values of the MAO for oxygen (K0 values) with tyramine, 5-hydroxytryptamine (5-HT) and beta-phenylethylamine (PEA) as substrates were identical (59 microM).     

Ganglioside GM1 Causes Expression of Type B Monoamine Oxidase in a Rat Clonal Pheochromocytoma Cell Line, PC12h

The effects of ganglioside supplementation of culture medium on monoamine oxidase (MAO) type A and B activities in a rat clonal pheochromocytoma cell line, PC12h, were examined. The MAO activity in PC12h cells proved to be mainly due to type A MAO, and type B MAO activity was negligible. After supplementation of the culture medium with ganglioside GM1, the PC12 cells were found to express type B MAO activity after 4 days of culture, and the amount of type B activity increased with the number of days of culture. After 3 weeks of culture in the presence of GM1, type B activity was about 10% of the total, whereas in control cells type B MAO activity was only about 0.6% of the total. By kinetic analyses of type A and B MAO in PC12h cells after 3 weeks of culture, the increase of type B MAO activity was found to be due to the increase in amount of type B MAO; the Km values were almost the same and only the Vmax values were increased in the cells supplemented with GM1. Among gangliosides tested GM1 was the most effective in causing expression of type B MAO activity, whereas nerve growth factor was not effective. These results suggest that GM1 and other gangliosides may be involved in the expression of type B MAO in nerve cells and in the regulation of levels of the biogenic amines in the brain.     

Metabolism of biogenic amines in neuroblastoma and glioma cells in culture

The kinetic parameters of monoamine oxidase (MAO; E.C 1.4.3.4) and catechol-O-methyltransferase (COMT; EC 2.1.1.6) were evaluated in extracts of adrenergic and non-adrenergic mouse neuroblastoma cells and in rat glioma cells. Using the naturally-occurring substrates tyramine, tryptamine, serotonin and norepinephrine, the affinity of MAO for a given substrate was independent of the presence of the catecholaminergic pathway or cell type used, with apparent Km values ranging from 8-14 microM for tryptamine to 510-580 microM for norepinephrine. The MAO activity in glioma cells was substantially greater than in either neuroblastoma clone, but Vmax values varied little with substrate among cell lines. Both the neuronal and glial COMT had a similar Km for 1-norepinephrine (200 microM); the corresponding Vmax values were also similar among the different cell lines, but represented only 2-10% of the maximal MAO activity. Neuroblastoma and glioma cells, when grown from early logarithmic to stationary phase, showed no significant changes in specific activity of either MAO or COMT. Growth of cells for 3 days with 1 mM-N6,O2'-dibutyryl adenosine-3',5'-cyclic monophosphate resulted in no marked change in either MAO or COMT activity. These results suggest that in neurons neither MAO nor COMT plays a major role in the type of transmitter inactivation that is analogous to that of acetylcholinesterase in cholinergic synapses. The occurrence of considerable MAO and acetylcholinesterase activities in glioma cells may indicate a role for these cells in neurotransmitter inactivation.     

Type B monoamine oxidase activity in human brain malignant tumors

An increase of monoamine oxidase (MAO) activity was observed in Central Nervous System (CNS) malignant tumors, but the isoform responsible was not identify (Marcozzi et al., 1985). In the present work we report additional data in order to ascertain whether the type A or B MAO isoform is increased in some malignant human tumors of CNS. In the homogenated tissues the amine oxidase activity was determined by the chemiluminescent method, using different and specific substrates or inhibitors of MAO A and B and copper-dependent enzymes. 19 samples from 4 different types of tumors and relative peritumoral tissues were analysed. The highest activity of was imputable to type B MAO.     

Steroid regulation of monoamine oxidase activity in the adrenal medulla

Administration of different steroid hormones in vivo has distinct and specific effects on the MAO activity of the adrenal medulla. In an effort to reconstitute these effects in defined cells, we have isolated endothelial cells and chromaffin cells from the bovine adrenal medulla and tested each cell type for sensitivity to these steroids. As in the intact animal, we found that endothelial cell MAO activity was stimulated 1.5- 2.5-fold by 10 microM progesterone, hydrocortisone, and dexamethasone, inhibited by ca. 50% by 17-alpha-estradiol, but unaffected by testosterone. The type of MAO in the endothelial cells was found to be exclusively of the A type. The chromaffin cells had MAO B exclusively and were inert to treatment with dexamethasone. The mode of action of the various steroids on MAO A activity in endothelial cells seemed to be that of affecting the number of MAO molecules, as binding of [3H]pargyline, an MAO inhibitor, changed in proportion to changes in enzyme activity. Consistently, the kinetic parameters for MAO A showed changes in Vmax but not Km under all conditions. The specificity of steroid action on MAO A activity was also supported by the fact that steroid-induced changes in total cell division ([14C]thymidine incorporation) and total protein synthesis ([14C]leucine incorporation) were seen after changes in MAO A. We conclude that the differential effects of steroids on MAO activity in the intact adrenal medulla can be reproduced in cultured adrenal medullary endothelial cells but not in chromaffin cells. Therefore we suggest that the action of these steroid hormones on the intact adrenal medulla may be restricted to the endothelial cell component of this tissue.     

Oxidation of N-Methyl-1,2,3,4-Tetrahydroisoquinoline into the N-Methyl-Isoquinolinium Ion by Monoamine Oxidase

N-Methyl-1,2,3,4-tetrahydroisoquinoline (NMTIQ) was found to be oxidized by monoamine oxidase (MAO) into N-methylisoquinolinium ion, which was proved to inhibit enzymes related to the metabolism of catecholamines, such as tyrosine hydroxylase, aromatic-L-amino acid decarboxylase, and MAO. NMTIQ was oxidized by both types A and B MAO in human brain synaptosomal mitochondria. Oxidation was dependent on the amount of MAO sample and the reaction time. Enzyme activity with respect to NMTIQ reached optimum at a pH of approximately 7.25, as was the case with other substrates. Type A MAO had higher activity for this substrate than type B. The Km and Vmax values of the oxidation by types A and B MAO were 571 +/- 25 microM and 0.29 +/- 0.06 pmol/min/mg protein, and 463 +/- 43 microM and 0.16 +/- 0.03 pmol/min/mg protein, respectively. The Vmax values of types A and B MAO for NMTIQ were much smaller than those for other substrates such as kynuramine. NMTIQ was the first tetrahydroisoquinoline shown to be oxidized into the isoquinolinium ion by MAO in the brain.     

Deamination of Aliphatic Amines by Monoamine Oxidase A and B Studied Using a Bioluminescence Technique

Deamination of n-octylamine and n-decylamine has been studied in various tissues using a new bioluminescence technique. Selectivity of n-octylamine and n-decylamine as substrates for monoamine oxidase (MAO) A or B has been determined using both clorgyline and (-)-deprenyl inhibition curves and kinetic parameters. Homogenates of rat brain, liver and heart containing predominantly MAO-A or -B were prepared by preincubation for 60 min with (-)-deprenyl or clorgyline (30 nM), respectively. Human placenta (MAO-A) and platelet (MAO-B) were used as reference tissues containing only one MAO form. In tissues (rat liver, brain) containing both MAO forms in equal proportion, inhibition curve studies showed a preference of both substrates for the B form of the enzyme; however, where MAO-A was the major form (rat heart, human placenta), clorgyline was the more effective inhibitor. In the beef brain cortex n-octylamine showed marked preference for MAO-B, whereas n-decylamine was selective toward-MAO-A. Kinetic studies in general supported the picture of greater selectivity of the aliphatic amine substrates for deamination by MAO-B, as reflected by lower Km values for this enzyme type. However, n-octylamine was more selective for MAO-B than n-decylamine in both kinetic and inhibition curve studies. The deamination of these aliphatic amine substrates cannot be explained only by reference to the binary classification of MAO into types A and B.     

Biochemical characteristics of liver and brain monoamine oxidase from pacu

Monoamine oxidase (MAO) activity in the liver and brain of the pacu, Piaractus mesopotamicus was determined using a fluorescence assay with kynuramine as substrate. Apparent Michaelis constant values (20·33 μM for liver and 25·85 μM for brain) were similar in these tissues, but in terms of tissue protein MAO activity from liver was 4·5 times higher than from brain. The greater inhibitory effects of clorgyline than of deprenyl on MAO activity from liver and brain of this species suggest that pacu's MAO is a type A-like enzyme.     

In vivo and in vitro effects of temperature on monoamine oxidase activity in brain and other tissues of the goldfish. Carassius auratus L

1. The maximum velocity (Vmax) and apparent Michaelis constant (Km) of brain and liver monoamine oxidase (MAO) in goldfish were different in fish acclimated to 22 degrees C and to 7 degrees C ambient temperature. 2. In brain, Vmax and Km were dependent upon incubation temperature, but both parameters were lower in 7 degrees C, adapted fish over most of the incubation temperature range. 3. The values obtained for Km showed a plateau at incubation temperatures at and below 25 degrees C for warm water fish, and at and below 20 degrees C for cold water fish. The activation energy of brain MAO was lower in fish adapted to the colder water. 4. These results show that goldfish MAO displays changes in functional activity in response to a change in environmental temperature. Apparently the purpose of this adaptation is to compensate for a reduction in enzyme concentration.     

Substrate Selectivity of Type A and Type B Monoamine Oxidase in Rat Brain

Abstract: Use of the irreversible inhibitors clorgyline and deprenyl showed that rat brain mitochondria contain type A and type B monoamine oxidase (MAO). Tyramine is a substrate for both types of MAO, whereas serotonin is a preferential substrate for type A MAO. In contrast to MAO in other tissues, type A MAO in brain tissue oxidizes β-phenylethylamine (PEA) at high concentrations (0.5 and 1.0 mM). The proportions of type A and type B MAO activities in the mitochondria estimated from the double-sigmoidal inhibition curves of tyramine oxidation were about 70:30 irrespective of the concentration of tyramine. With PEA as substrate, the ratios of type A to type B activities were found to increase from low values at low concentrations to about 1 at 0.5-1.0 mM-PEA, and even higher at further increased concentrations of PEA. At very low (0.01 mM) and high (10.0 mM) concentrations of PEA, single-sigmoidal curves were obtained; with the high PEA concentration the activity was highly sensitive to clorgyline, whereas with the low concentration it was highly sensitive to deprenyl. In deprenyl-pretreated mitochondrial preparations, all the remaining activity towards 0.5-1.0 mM-PEA was shown to be highly sensitive to clorgyline, demonstrating that this activity was indeed due to oxidation by type A MAO. The opposite result was obtained with deprenyl as inhibitor of clorgyline-pretreated preparations, demonstrating that PEA at this concentration was also oxidized by type B MAO in rat brain mitochondria. The K₃ values of type A and type B MAO for PEA were significantly different. On Lineweaver-Burk analysis, plots with PEA as substrate for type A MAO in a deprenyl-treated preparation were linear over a wide concentration range, whereas those for type B MAO in a clorgyline-treated preparation were not linear, but showed substrate inhibition at higher concentrations of the substrate. It is concluded from the present findings that the effect of the substrate concentration must be considered in studies on the characteristics of multiple forms of MAO in various organs and species.     

Kinetic analysis of platelet monoamine oxidase in chronic schizophrenia

Km and Vmax values for platelet monoamine oxidase (MAO) were determined in 16 chronic schizophrenics and 18 controls utilizing three substrates, tyramine (TYR), benzylamine (BZ), and phenylethylamine (PEA). In the chronic schizophrenics decreased Km and Vmax values were found for TYR and BZ but not PEA. When prior neuroleptic drug exposure was considered, a trend toward lower kinetic parameters was found in schizophrenics with a history of prior neuroleptic usage. We conclude that platelet MAO activity is, in chronic schizophrenics, both quantitatively reduced and qualitatively different from control enzyme. We suggest that the measurement of Km in addition to the measurement of Vmax may be a useful biological marker for chronic schizophrenia providing that the appropriate substrates are employed.     

Differences in the Structure of A and B Forms of Human Monoamine Oxidase

Abstract: [³H]Pargyline-labeled polypeptides associated with the A and B types of monoamine oxidase (MAO) activity in human tissues were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). [³H]Pargyline was bound to MAO A in a crude mitochondrial fraction from the placental trophoblast of a male newborn and to MAO B in blood platelets from the umbilical vein of the same newborn. [³H]Pargyline was also bound to MAO A and B in a crude mitochondrial fraction from cultured skin fibroblasts of a male adult and to MAO B in blood platelets from the same individual. Specific labeling of proteins associated with type A or type B activity in fibroblast cells was achieved by preincubation with selective B or A inhibitors, respectively. For all tissues, SDS-PAGE of [³H]pargyline-bound samples revealed a labeled protein band of apparent molecular weight 63,000 for MAO A and 60,000 for MAO B. When SDS-solubilized, [³H]pargyline-labeled MAO A and B proteins from the same male newborn were subjected to limited proteolysis and one-dimensional peptide mapping in SDS gels, different patterns of [³H]pargyline-labeled peptides were obtained. These findings indicate that distinct enzyme molecules are associated with the A and B types of human MAO activity.     

Effect of prolonged cold exposure on monoamine oxidase activity and kinetics and on serotonin metabolism in the rat brain

Effects of long-term cold exposure on the content of serotonin and its metabolite 5-hydroxyindolacetic acid (5-HIAA) and monoamine oxidase (MAO) activity and kinetic parameters (Km and Vmax) of oxidative deamination of serotonin in rat brain stem. The increase of 5-HIAA level in the initial period of chronic cold exposure was determined by the blockade of active metabolite transport from the brain. The level of serotonin and the rate of its catalytic deamination by MAO were found to be decreased in cold-adapted rats. The magnitude of the Km of serotonin deamination was unchanged.