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.     

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.     

m-Octopamine as a substrate for monoamine oxidase.

$\text{m}$-Octopamine was characterized as substrate for monoamine oxidase (MAO) in rat brain and liver mitochondria. The $\text{K}$m and $\text{V}$max values of the brain enzyme were 735 μM and 32.5 nmoles/mg protein/30 min, and those of the liver enzyme 351 μM and 125 nmoles/mg protein/30 min, respectively. The inhibition experiments with clorgyline and deprenyl showed that $\text{m}$-octopamine was a common substrate for type A and type B MAO, though a major part of the activity was due to type A enzyme.     

Plasma 5-Hydroxyindoleacetic Acid as an Indicator of Monoamine Oxidase-A Inhibition in Rat Brain and Peripheral Tissues

We have examined the changes induced by the monoamine oxidase (MAO; EC 1.4.3.4) inhibitors tranylcypromine, clorgyline, and deprenyl on MAO activity and 5-hydroxytryptamine (serotonin, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content in rat brain and blood (plasma and whole blood). The decreases of MAO-A activity observed in the liver and lungs after different doses of clorgyline or tranylcypromine correlated significantly (r > 0.80 in all cases) with the decline of plasma 5-HIAA. This was unaffected by 0.25 and 5 mg kg^?1 of deprenyl, indicating that 5-HT was deaminated exclusively in the periphery by MAO-A. It is interesting that very potent and significant correlations (r > 0.75) were found between plasma 5-HIAA and MAO-A activity, 5-HIAA and 5-HT content in brain tissue. These results suggest that plasma 5-HIAA can be used confidently as a peripheral indicator of the inhibition of MAO-A in brain. This may represent a favorable alternative to the analysis of 5-HIAA in CSF in psychiatric patients undergoing antidepressant treatment with nonspecific MAO inhibitors or with the new selective MAO-A inhibitors.     

Monoamine oxidase activity in hepatopancreas of Octopus vulgaris

1. Monoamine oxidase activity has been studied in hepatopancreas of Octopus vulgaris using 5-HT and PEA as substrates.2. Time courses of MAO activity against 5-HT and PEA show that the enzyme has higher affinity for PEA than for 5-HT.3. MAO activity against 5-HT appears more sensitive than MAO activity against PEA, to variations of the temperature (range 17–67°C).4. The inhibition curves obtained with clorgyline and deprenyl indicate that MAO activity is due to a single form of the enzyme, not corresponding to type A and type B MAO.5. Semicarbazide 10⁻⁴ M does not affect the deamination of 5-HT and PEA, demonstrating that a semicarbazide-sensitive amine oxidase is not involved in this process.     

Oxidation of beta-phenylethylamine by both types of monoamine oxidase: effects of substrate concentration and pH.

β-Phenylethylamine (PEA) was characterized as substrate for both type A and type B monoamine oxidase (MAO) in rat brain mitochondria at different substrate concentrations and at different pHs of the reaction media. The experiments on sensitivity to clorygline and deprenyl showed that the inhibition patterns with PEA as substrate differed markedly at different substrate concentrations: at 10 μM, PEA acted as a specific substrate for type B MAO, but at 50–1000 μM it became a common substrate for both types of MAO. The inhibition patterns were also affected markedly by a small change in pH of the reaction medium, especially when PEA concentrations were 50 and 100 μM: the change in pH from 7.2 to 7.8 resulted in the incresse in the proportion of type A MAO by 20–30 per cent. To investigate the mechanisms of such changes in substrate specificity of PEA, kinetic analyses were carried out at pH 7.2 and 7.8 with the uninhibited, the clorgyline-treated (type B) and the deprenyl-treated (type A) enzyme. The Lineweaver-Burk plots for the uninhibited MAO showed strong substrate inhibition for both pHs, which is more marked at pH 7.8 than at pH 7.2. Pretreatment of the enzyme with 10^?7 M clorgyline resulted in generally similar K_m values for PEA to those of the uninhibited enzyme, and the substrate inhibition at pH 7.8 was also stronger than that at pH 7.2. After pretreatment with 10^?7 M deprenyl, the K_m values were higher and the V_max values were lower than those of the uninhibited or the clorgyline-treated enzyme; there was no or only slight substrate inhibition in these curves. These results suggest that the remarkable changes in substrate specificity observed at different PEA concentrations and at different pHs may be due to the strong substrate inhibition of type B MAO.     

EVIDENCE THAT 5-METHOXY-N, N-DIMETHYL TRYPTAMINE IS A SPECIFIC SUBSTRATE FOR MAO-A IN THE RAT: IMPLICATIONS FOR THE INDOLEAMINE DEPENDENT BEHAVIOURAL SYNDROME

A simple method for the separation of 5-hydroxyindoleacetic acid (5-HIAA) and 5-methoxyindoleacetic acid (5-MeOIAA) on columns of non-ionic polystyrene (Amberlite XAD-2) is described. Administration of 5-methoxy-N, N-dimethyl-tryptamine (5-MeODMT) 2 mg/kg i. p. to rats results in a sixfold increase in brain 5-MeOIAA within 15 min. This increase is blocked by the selective inhibitor of MAO-A, clorgyline, but not by the selective inhibitor of MAO-B, deprenyl, indicating that 5-MeODMT is deaminated almost entirely by MAO-A. The apparent 5-MeOIAA concentration in the brains of L-tryptophan loaded rats is not reduced by clorgyline and deprenyl, either singly or in combination, indicating that most of this fluorescence is due to other, unidentified substances. The apparent concentration of 5-HIAA in rat brain, minus 5-MeOIAA, is unaffected by deprenyl and reduced by clorgyline. However, clorgyline and deprenyl in combination reduced 5-HIAA values below those obtained with clorgyline alone. It is concluded that very little 5-MeODMT or other 5-methoxyindoleamines are formed endogenously in rat brain, and that the stereotyped syndrome of hyperactivity and tremors produced in rats by pretreatment with MAO inhibitors and L-tryptophan is dependent on the formation of an N-substituted derivative of 5-HT which is at least partly deaminated by MAO-B to 5-HIAA.     

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.     

Mechanism of the involvement of monoamine oxidase in the regulation of (Na+ + K+)-ATPase in rat brain

Increasing concentrations of dopamine fail to give a biphasic response to (Na⁺ + K⁺)-ATPase activity in various subcellular fractions of rat brain preincubated with monoamine oxidase inhibitors, viz. 1·10^?4 M clorgyline and 1·10^?4 M deprenyl. The product of the monoamine-oxidase-catalysed reaction with dopamine as substrate is 3-methoxy-4-hydroxyphenylacetaldehyde. An analogue of this product is 3-methoxy-4-hydroxybenzaldehyde. This analogue, when incubated with the subcellular fractions which had been preincubated with monoamine oxidase inhibitors and dopamine, gave a more pronounced biphasic response to (Na⁺ + K⁺)-ATPase activity than that observed in the fractions incubated with dopamine alone.     

Effects of inhibitors on monoamine oxidase activity in perch (Perca flavescens) brain in vitro

1. Perch brain homogenates were incubated in vitro and monoamine oxidase (MAO) activity was determined fluorometrically, using a kynuramine substrate. 2. Clorgyline, harmaline and deprenyl inhibited MAO activity in a concentration-related manner, with single sigmoid inhibition curves, and the type A inhibitors harmaline and clorgyline were more effective than the type B inhibitor deprenyl. 3. Two types of inhibition were recognized in vitro; a fast-onsetting inhibition, similar to that produced by a reversible inhibitor, and a slow-onsetting inhibition, which is time- and concentration-dependent and presumably represents inactivation of the enzyme.     

A new type of mitochondrial monamine oxidase distinct from type-A and type-B

The characteristics of mitochondrial monoamine oxidase (MAO) in carp liver were studied with MAO inhibitors and substrates. This enzyme was thermolabile, but was stabilized in the presence of bovine serum albumin. With clorgyline and deprenyl, single-sigmoidal curves for inhibition of the activity towards tyramine or 5-hydroxytryptamine were obtained; the sensitivities to the two inhibitors were identical. The activity towards β-phenylethylamine was not completely inhibited by clorgyline or deprenyl, but the remaining activity was inhibited by semicarbazide and the inhibition curves by either clorgyline or deprenyl and semicarbazide were also identical to the curves with the other two substrates. These results suggest that carp liver mitochondria contain “classical” MAO and a clorgyline- and deprenyl-resistant amine oxidase and that the classical MAO does not seem to be MAO-A or MAO-B, which are present in mitochondria of most mammalian tissues.     

1,4-Methylhistamine is a specific substrate for type B monoamine oxidase

1,4-Methylhistamine was characterized as substrate for monoamine oxidase (MAO) in rat liver mitochondria. The $\text{K}$_m and $\text{V}$_max values were 38.8 μM and 6.33 nmoles/mg protein/60 min, respectively. The inhibition experiments with clorgyline and deprenyl, the selective inhibitors for type A and type B MAO, showed that 1,4-methylhistamine was specific for type B MAO.     

Monoamine Oxidase Activities in Catfish (Parasilurus Asotus) Tissues

Abstract

The substrate- and inhibitor-related characteristics of monoamine oxidase (MAO) were studied for catfish brain and liver. The kinetic constants for MAO in both tissues were determined using 5-hydroxytryptamine (5-HT), tyramine and β-phenylethylamine (PEA) as substrates. For both tissues, the V_max values were highest with 5-HT and lowest with PEA. The K_m value for the brain was highest with 5-HT, followed by tyramine and PEA; but for the liver its value was highest with PEA, followed by 5-HT and tyramine, although all values were in the same order of magnitude. The inhibition of MAO by clorgyline and deprenyl by use of 5-HT, tyramine and PEA as substrates showed that the MAO-A inhibitor clorgyline was more effective than the MAO-B inhibitor deprenyl for both catfish tissues; a single form was present since inhibition by clorgyline or deprenyl with 1000 μM PEA showed single phase sigmoid curves. It is concluded that catfish brain and liver contain a single form of MAO, relatively similar to mammalian MAO-A.     

Alteration of dopamine synthesis in rat striatum subsequent to selective type A monoamine oxidase inhibition

Abstract: Pretreatment of rat striatal slices with the selective type A monoamine oxidase (MAO) inhibitor clorgyline was found to produce significant inhibition of dopamine (DA) synthesis. DA synthesis was reduced by nearly 50%, but not until more than 90% of the type A enzyme was inhibited. In contrast, complete inhibition of the type B MAO following deprenyl treatment had no effect. It is suggested that interneuronal accumulation of DA following inhibition of type A MAO leads to feedback inhibition at the rate-limiting step in DA biosynthesis, tyrosine hydroxylation. These results are also consistent with the presence of a type A MAO within DA-containing neurons and provide evidence of a regulatory role for type A MAO in the synthesis of brain DA.     

Modification of substrate-inhibitor affinities of human platelet monoamine oxidase B in vitro

The rate of benzylamine utilization by monoamine oxidase (MAO)-B from human blood platelets was 2-4 times higher than that for octopamine. Both activities were inhibited 100% by 10(-7) M deprenyl (a specific MAO-B inhibitor) and were not affected by clorgyline (a specific MAO-A inhibitor) or by polyclonal antibodies to MAO-A. The preincubation of platelet MAO-B with purified MAO-A from mitochondrial membranes of human placenta resulted in appearance of excess octopamine activity. This additional activity was not precipitated by antibodies to MAO-A or inhibited by deprenyl but was inhibited by clorgyline. Incubation of the MAO-A preparation from placenta at 45 degrees C for 15 min before its preincubation with MAO-B caused 50% loss of both activities. Protease inhibitors had no effect on the modification of MAO. These data indicate that MAO-A or a factor tightly bound to it can modify MAO-B yielding a form of the enzyme with both MAO-A and MAO-B substrate and inhibitor affinities and MAO-B immunospecificity.     

The oxidation of adrenaline and noradrenaline by the two forms of monoamine oxidase from human and rat brain

The selective monoamine oxidase inhibitors clorgyline and (−)-deprenyl were used to study the distribution of monoamine oxidase-A and -B (MAO-A, MAO-B) activities towards (−)-noradrenaline and (+),(−)-adrenaline in homogenates from seven different regions of human brain. The activities towards 5-hydroxytryptamine and 2-phenethylamine, which are essentially specific substrates for the A- and B-forms, respectively, under the conditions used in this work, were also determined. Noradreanline and adrenaline were substrates for both forms of the enzyme in all regions studied. The total MAO activity was found to be highest in the hypothalamus and lowest in the cerebellar cortex. Use of the selective MAO inhibitors clorgyline and (−)-deprenyl also showed adrenaline and noradrenaline to be substrates for both forms of the enzyme in rat brain. In human cerebral cortex and rat brain the two forms were found to have similar K_m-values and maximum velocities towards adrenaline. These values for the two forms were also found to be similar in human cerebral cortex when noradrenaline was used as the substrate. In contrast MAO-A showed a significantly lower K_m and a higher maximum velocity towards noradrenaline in rat brain. These results suggest that the rat may not provide a close model of the human for studies on the effects of MAO inhibitors on brain noradrenaline metabolism.     

Comparative enzymological study of catalytic properties of liver monoamine oxidases in frogs

Comparative enzymological study of catalytical properties of monoamine oxidase (MAO) of liver of the marsh frog Rana ridibunda and common frog Rana temporaria has revealed certain features of similarity and differences between these enzymes. The MAOs from both studied biological sources show catalytic properties resembling those of the classical MAO of terrestrial vertebrates: they deaminate tyramine, tryptamine, serotonin, and benzylamine and do not deaminate histamine, have sensitivity to clorgyline, the specific inhibitor of the MAO A form, and deprenyl, the specific inhibitor of the MAO B form, and are not inhibited by 10⁻² M semicarbazide. Based on data of substrate-inhibitor analysis, a suggestion is put forward about the existence of two molecular forms of the enzyme in liver of the studied frog species. Quantitative interspecies differences have been revealed between liver MAO of Rana ridibunda and Rana temporaria in values of kinetic parameters of reactions of deamination of several substrates and in sensitivity to the inhibitors, deprenyl and clorgyline. In the species Rana temporaria the MAO activity in reaction of deamination of serotonin and benzylamine were virtually identical, whereas in the species Rana ridibunda these parameters for serotonin were almost one order higher than for benzylamine. In the species Rana ridibunda, selectivity of action of deprenyl was expressed many times weaker, while selectivity of the clorgyline—one order of magnitude stronger than in the species Rana temporaria. The catalytic activities towards all studied substrates of liver MAO of both studied amphibian species were several times lower as compared with the enzyme of rat liver.     

5-HYDROXYTRYPTAMINE CATABOLISM IN THE RAT BRAIN DURING ONTOGENESIS

Although the serotoninergic innervation is immature in the brains of young rats, the 5-HIAA content is similar to that found in adults. As indicated by the ratio of 5-HIAA to 5-HT levels in the brain stem and the forebrain, the catabolism of the indolamine was more rapid during the first 3 postnatal weeks than in adults. This was contirmed by measuring the total formation of [³H]5-HIAA from [³H]5-HT newly synthesized from L-[³H]tryptophan in brain stem slices of young and adult rats. Electrolytic lesions of midbrain raphe nuclei (B7 and B8) performed on the 5th postnatal day resulted in parallel decreases in brain 5-HT and 5-HIAA levels; this ruled out the possibility that 5-HIAA might be formed from 5-HT synthesized outside serotoninergic neurons, using peripheral 5-hydroxytryptophan. Inhibition of 5-HT storage by reserpine pretreatment did not alter the higher capacity of newborn tissues to catabolize exogenous [³H]5-HT. Therefore, possible differences in 5-HT binding in serotoninergic neurons between newborn and adult rats were not likely to account for the differences in 5-HT catabolism. Estimation of the rate of 5-HIAA efflux from the brain after MAO inhibition did not reveal marked changes with age. The activity of MAO type A, the enzyme involved in 5-HT catabolism, was higher during early life than later on. This could be shown by using 5-HT as substrate and clorgyline as a selective inhibitor. An opposite pattern of development was seen for MAO B, measured with benzylamine as substrate and deprenyl as selective inhibitor. These results suggest that the high 5-HIAA levels found in the brains of young rats can be attributed mainly to the presence of high MAO A activity during early life.     

Comparative study of substrate and inhibitory specificity of monoamine oxidase of squid optic ganglia

Comparative study of substrate specificity of monoamine oxidase (MAO) of optic ganglia of the Pacific squid Todarodes pacificus and the Commander squid Berryteuthis magister has been carried out. The enzyme of the Pacific squid, unlike that of the Commander squid, has been established to be able to deaminate not only tyramine, tryptamine, serotonin, benzylamine, and β-phenylethylamine, but also histamine-substrate of diamine oxidase (DAO). In relation to all studied substrates, the MAO activity of optic ganglia of T. pacificus is several times higher as compared with that of B. magister. In the case of deamination of serotonin this difference was the highest and amounted to 5 times. Semicarbazide, the classic DAO inhibitor, at a concentration of 10 mM did not inhibit catalytic activity of both studied enzymes. The substrate-inhibitory analysis with use of deprenyl and clorgyline, specific inhibitors of different MAO forms, indicates homogeneity of the enzyme of the Pacific squid and heterogeneity of the Commander squid enzyme whose composition seems to contain at least two MAO forms. There are obtained quantitative differences in substrate specificity and reaction capability with respect to the inhibitors clorgylin and deprenyl for MAO of optic ganglia of the studied squid species. These differences probably can be explained by significant differences in the evolutionary level of these biological species.     

Brain monoamine oxidase in suckling rabbits treated with organophosphorus compound. I. Acute intoxication

In 16-day-old rabbits treated with dichlorvos 8 mg/kg per os the activity of monoamine oxidase A form (MAO A) in brain was studied. A short-lasting decrease of brain MAO A was followed by a recovery of enzyme activity within 120 min. In parallel to changes in MAO A activity breathing disturbances were observed. It is concluded that the decrease of MAO A was due rather to hypoxia than to a direct effect of dichlorvos. In 16-day-old rabbits clorgyline (2 mg/kg) and tranylcypromine (10 mg/kg) were potent inhibitors of MAO A but deprenyl and pargyline were ineffective.