Inhibition of brain monoamine oxidase activity by the generation of hydroxyl radicals: potential implications in relation to oxidative stress

Monoamine oxidase (MAO) is an enzyme involved in brain catabolism of monoamine neurotransmitters whose oxidative deamination results in the production of hydrogen peroxide. It has been documented that hydrogen peroxide derived from MAO activity represents a special source of oxidative stress in the brain. In this study we investigated the potential effects of the production of hydroxyl radicals (*OH) on MAO-A and MAO-B activities using mitochondrial preparations obtained from rat brain. Ascorbic acid (100 microM) and Fe2+ (0.2, 0.4, 0.8, and 1.6 microM) were used to induce the production of *OH. Results showed that the generation of *OH significantly reduced both MAO-A (85-53%) and MAO-B (77-39%) activities, exhibiting a linear correlation between both MAO-A and MAO-B activities and the amount of *OH produced. The reported inhibition was found to be irreversible for both MAO-A and MAO-B. Assuming the proven contribution of MAO activity to brain oxidative stress, this inhibition appears to reduce this contribution when an overproduction of *OH occurs.     

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.     

Brain region differences and some characteristics of monoamine oxidase type a and b activities in the vervet monkey

Monoamine oxidase in the vervet monkey showed greater variations in activity in six brain regions when tyramine or phenylethylamine was used as the substrate (3.8- to 4.1-fold differences) than when serotonin was the substrate (1.8-fold differences). With phenylethylamine and tyramine as substrates, the highest MAO specific activities were found in the hypothalamus and the lowest in the cerebellum and cortex. With serotonin as the substrate, the highest specific activities were in the mesencephalon and cortex. The inhibition of tyramine deamination by clorgyline and deprenyl yielded biphasic plots indicative of the presence of MAO-A and MAO-B enzyme forms in the vervet brain. On the basis of these inhibitor curves, the vervet brain could be estimated to contain approximately 85% MAO-B and 15% MAO-A, in contrast to rat brain which contains 45% MAO-B and 55% MAO-A. The inhibition of serotonin deamination by deprenyl in vervet brain yielded a biphasic plot, suggesting that some serotonin deamination in the vervet is accomplished by the MAO-B enzyme form. Estimations of the relative amounts of MAO-A and MAO-B based on inhibitor curves or based on substrate ratios yielded proportionate results which were in close agreement across the different brain regions, supporting the validity of these approaches to estimating MAO-A and MAO-B activities.     

Distribution of the Two Forms of Monoamine Oxidase Within Monoaminergic Neurons of the Guinea Pig Brain

The relative distribution of type A and type B monoamine oxidase (MAO) inside and outside the monoaminergic synaptosomes in preparations from hypothalamus and striatum of the guinea pig was determined by incubation of synaptosomal preparations of these regions with low concentrations of [14C]5-hydroxytryptamine (5-HT), noradrenaline, and dopamine. The deamination within the monoaminergic synaptosomes was hindered by selective amine uptake inhibitors. In the absence of these inhibitors, both intra- and extraneuronal deamination was measured. The two forms of the enzyme were differentiated with the irreversible and selective MAO-A and MAO-B inhibitors clorgyline and selegiline (l-deprenyl), respectively. [14C]5-HT was deaminated greater than 90% by MAO-A both inside and outside the 5-hydroxytryptaminergic synaptosomes prepared from the guinea pig hypothalamus. The deamination of [14C]noradrenaline within the noradrenergic synaptosomes of the hypothalamic preparation was in the ratio 75:25% for MAO-A:MAO-B; the corresponding ratio outside these synaptosomes was 45:55%. The deamination of [14C]dopamine within dopaminergic synaptosomes in the striatal preparation was 65% type A:35% type B, whereas outside these synaptosomes the ratio was 35:65%. Because the relative amounts and the distribution of the two forms of MAO in the guinea pig brain seem to be similar to those previously detected for the human brain, the MAO in the guinea pig brain may be a good model for the MAO in the human brain.     

Immobilized enzyme reactors based upon the flavoenzymes monoamine oxidase A and B

Monoamine oxidase (MAO) catalyzes the oxidative deamination of amines. The enzyme exists in two forms, MAO-A and MAO-B, which differ in substrate specificity and sensitivity to various inhibitors. Membrane fractions containing either expressed MAO-A or MAO-B have been non-covalently immobilized in the hydrophobic interface of an immobilized artificial membrane (IAM) liquid chromatographic stationary phase. The MAO-containing stationary phases were packed into glass columns to create on-line immobilized enzyme reactors (IMERs) that retained the enzymatic activity of the MAO. The resulting MAO-IMERs were coupled through a switching valve to analytical high performance liquid chromatographic columns. The multi-dimensional chromatographic system was used to characterize the MAO-A (MAO-A-IMER) and MAO-B (MAO-B-IMER) forms of the enzyme including the enzyme kinetic constants associated with enzyme/substrate and enzyme/inhibitor interactions as well as the determination of IC(50) values. The results of the study demonstrate that the MAO-A-IMER and the MAO-B-IMER can be used for the on-line screening of substances for MAO-A and MAO-B substrate/inhibitor properties.     

Effect of chlorpromazine, imipramine and lithium on MAO-A and MAO-B activity in rat brain mitochondria

Drugs with efficacy in psychiatric disorders affect the function of central neurotransmitter amines, which are inactivated primarily by monoamine oxidase (MAO). Effect of these drugs on the two types of MAO (MAO-A and MAO-B) has been studied in rat brain. The result showed that chlorpromazine (CPZ) and imipramine (IMI) at concentrations of 1x10(-2), 5x10(-3) and 2.5x10(-3) M inhibited rat brain mitochondrial MAO-A activity in vitro by 82, 50, 39 and 86, 74, 38 %, respectively. CPZ at concentrations of 5x10(-3), 2.5x10(-3), 1x10(-3) M inhibited rat brain mitochondrial MAO-B activity in vitro by 83, 55, 39 %, respectively, while IMI at concentrations of 5x10(-4), 2.5x10(-4), 1x10(-4) M inhibited the in vitro enzyme activity by 43, 35, 21 %, respectively. Lithium at concentration of 5x10(-3) M could not either inhibit MAO-A or MAO-B in the mitochondrial fraction of rat brain.     

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.     

Inhibition of rat brainstem monoamine oxidase activity by CGP 6085 A

CGP 6085 A [4-(5,6-dimethyl-2-benzofuranyl)piperidine] HCl, a known serotonin inhibitor, also inhibits rat brainstem monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B) in both in vivo and in vitro experiments. Serotonin (5-HT) deamination by MAO-A is inhibited 35% at a dose of 100 mg/kg i.p. in vivo. Similar experiments show a maximal 20% decrease in phenylethylamine (PEA) deamination by MAO-B at a dosage of 30 mg/kg i.p. Over the range of 0.1 to 10 mg/kg i.p., CGP 6085 A decreases 5-HIAA levels in the brainstem. This in vivo inhibition of MAO activity is confirmed by in vitro experiments. In vitro studies in rat brainstem mitochondrial preparations show a dose-dependent, reversible, inhibition of MAO using tyramine as the substrate for the enzyme reaction. With an in vitro IC50 of 2-3 microM, the potency of CGP 6085 A is comparable to pargyline.     

Monoamine oxidase and semicarbazide-sensitive amine oxidase in spontaneously hypertensive and in normotensive control rats

The aim of the present work was to compare monoamine oxidase (MAO) and semicarbazide sensitive amine oxidase (SSAO) activity in several tissues from spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY). Contribution of MAO-A, -B and SSAO to the metabolism of each substrate in each tissue was defined from experiments where the decrease of oxidative deamination of each substrate at a given concentration was measured as a function of increasing concentrations of a selective MAO-A, -B or SSAO inhibitor. In the heart, aorta and, to a lesser extent, the femoral arteries MAO-A activity was higher in SHR than in WKY. Similarly in the liver the enzyme activity was higher in SHR than in WKY but was due to the -B form of MAO. In all the other tissues studied (duodenum, brain, lungs, adrenals and kidneys) no difference in MAO-A, MAO-B or SSAO activity was found between SHR and WKY, except for the kidneys and brain, if the differences in the weights of these organs in SHR are taken into account.     

Effect of benzamide derivatives on rat brain monoamine oxidase activity in vitro

The ability of moclobamide and other benzamide derivatives to inhibit the activity of monoamine oxidase in the rat brain was studied. Distinct effects of these compounds on the deamination of serotonin and norepinephrine (MAO-A substrates); 2-phenylethylamine (selective MAO-B substrate); tyramine and dopamine (MAO-A and MAO-B substrates) are shown. It was demonstrated that among all the compounds studied moclobamide appeared to be the most active and selective inhibitor of MAO-A: at a concentration of 100 microM it caused a 100% inhibition of serotonin and norepinephrine deamination, which might be explained by the presence of C1 atom in the para-position of benzene ring in moclobamide molecule. Other benzamide derivatives were less active in inhibiting MAO-A and had but a negligible effect on dopamine- and 2-phenylethylamine deamination.     

4-(O-Benzylphenoxy)-N-Methylbutylamine (Bifemelane) and Other 4-(O-Benzylphenoxy)-N-Methylalkylamines as New Inhibitors of Type A and B Monoamine Oxidase

4-(O-Benzylphenoxy)-N-methylbutylamine (Bifemelane, BP-N-methylbutylamine), a new psychotropic drug, was found to inhibit monoamine oxidase (MAO) in human brain synaptosomes. It inhibited type A MAO (MAO-A) competitively and type B (MAO-B) noncompetitively. BP-N-methylbutylamine had a much higher affinity to MAO-A than an amine substrate, kynuramine, and it was a more potent inhibitor of MAO-A than of MAO-B. The Ki values of MAO-A and -B were determined to be 4.20 and 46.0 microM, respectively, while the Km values of MAO-A and -B with kynuramine were 44.1 and 90.0 microM, respectively. The inhibition of MAO-A and -B by BP-N-methylbutylamine was found to be reversible by dialysis of the incubation mixture. MAO-A in human placental and liver mitochondria and in a rat clonal pheochromocytoma cell line, PC12h, was inhibited competitively by BP-N-methylbutylamine, while MAO-B in human liver mitochondria was inhibited noncompetitively, as in human brain synaptosomes. BP-N-methylbutylamine was not oxidized by MAO-A and -B. The effects of other BP-N-methylalkylamines, such as BP-N-methylethylamine, -propylamine, and -pentanylamine, on MAO activity were examined. BP-N-methylbutylamine was the most potent inhibitor of MAO-A, and BP-N-methylethylamine and -propylamine inhibited MAO-B competitively, whereas BP-N-methylbutylamine and -pentanylamine inhibited it noncompetitively. Inhibition of these BP-N-methylalkylamines on MAO-A and -B is discussed in relation to their chemical structure.     

Differential Postnatal Development of Monoamine Oxidases A and B in the Blood-Brain Barrier of the Rat

Abstract: We studied the monoamine metabolizing mitochondrial enzyme, monoamine oxidase (MAO), in cerebral microvessels obtained from postnatally developing rats by measuring the specific binding of [³H]pargyline, an irreversible inhibitor of MAO, and the rate of oxidation of three known MAO substrates: benzylamine, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, and tryptamine. MAO activity increased postnatally, with the greatest increase occurring in the second week and reaching a peak at 3 weeks of age. A concomitant increase in MAO of the cerebral cortex also occurred, but was several-fold less than that of cerebral microvessels. Using clorgyline and deprenyl, relatively specific inhibitors of MAO-A and MAO-B, we showed that cerebral microvessels contain both forms of MAO at all ages, but there was a major preponderance in the postnatal development of MAO-B. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analyses of rat microvessels after [³H]pargyline binding also showed two distinct bands of radioactivity at all ages. These two bands corresponded to molecular weights of ∼6.5,000 for MAO-A and -60,000 for MAO-B. SDS-PAGE resuits of brain microvessels obtained from 1-, 14-, and 42-day-old rats confirm the differential postnatal development of MAO-B in rat brain microvessels.     

Selective Effects of Proteases and Phospholipase A2 on Monoamine Oxidases A and B of Human Brain and Liver

Human brain and liver mitochondria contain membrane-bound monoamine oxidase of both A and B types. Monamine oxidase-A (MAO-A), either membrane-bound or in detergent-solubilized extracts from these tissues, was selectively inhibited during incubations with trypsin, chymotrypsin, thermolysin, or papain. MAO-A in solubilized, but not in membrane-bound, preparations was also very sensitive to the action of phospholipase A2, while MAO-B was unaffected. Membrane-bound MAO-A of rat brain mitochondria was more sensitive to phospholipases and less sensitive to proteases than was human brain enzyme, indicating that these agents may reveal species differences in MAO properties. Human brain and liver MAO-A, either solubilized or bound in mitochondrial membranes, apparently contains basic and aromatic peptide moieties that are available to proteases. Hydrolysis of these peptide bonds leads to rapid denaturation unless substrate molecules stabilize the active site. Phospholipase A2 may disrupt the phospholipid microenvironment of MAO-A, the integrity of which is essential for MAO-A activity, but not for MAO-B. No interconversion of the two activities was observed. After phospholipase A2 treatment, remaining MAO-A activity was recovered in low-molecular-weight regions of a gel filtration gradient, suggesting that MAO-A subunits were released. Although these experiments argue against the proposal that phospholipids may regulate the ratio of A/B activities of a single enzyme molecule, it is conceivable that endogenous phospholipases or proteases in mitochondrial membranes may influence MAO-A activity independently of MAO-B activity.     

Formation of the Neurotransmitter Glycine from the Anticonvulsant Milacemide Is Mediated by Brain Monoamine Oxidase B

Milacemide (2-n-pentylaminoacetamide) is a secondary monoamine that in the brain is converted to glycinamide and glycine. This oxidative reaction was suspected to involve the reaction of monoamine oxidase (MAO). Using mitochondrial preparations from tissues that contain MAO-A and -B (rat brain and liver), MAO-A (human placenta), and MAO-B (human platelet and bovine adrenal chromaffin cell), it has been established that mitochondria containing MAO-B rather than MAO-A oxidize (H2O2 production and glycinamide formation) milacemide. The apparent Km (30-90 microM) for milacemide oxidation by mitochondrial MAO-B preparations is significantly lower than that for milacemide oxidation by mitochondrial MAO-A (approximately 1,300 microM). In vitro MAO-B (l-deprenyl and AGN 1135) rather than MAO-A (clorgyline) selectively inhibited the oxidation of milacemide. These in vitro data are matched by ex vivo experiments where milacemide oxidation was compared to oxidation of serotonin (MAO-A) and beta-phenylethylamine (MAO-B) by brain mitochondria prepared from rats pretreated with clorgyline (0.5-10 mg/kg) and l-deprenyl (0.5-10 mg/kg). Furthermore, in vivo experiment demonstrated that l-deprenyl selectively increased the urinary excretion of [14C]milacemide and the total radioactivity with a concomitant decrease of [14C]glycinamide. Such changes were not observed after clorgyline treatment, but were evident only at doses beyond clorgyline selectivity. The present data therefore demonstrate that milacemide is a substrate for brain MAO-B, and its conversion to glycinamide, further transformed to the inhibitory neurotransmitter, glycine, mediated by this enzyme may contribute to its pharmacological activities.     

Serotonin and benzylamine oxidation by type A and type B MAO of rat brain in presence of organophosphate pesticides

Organophosphate (OP) pesticides, monocrotophos (MCP), dichlorvos (DDVP) and phosphamidon significantly inhibit both MAO-A and MAO-B activities in rat brain mitochondria. The inhibition of MAO-A by MCP is reversible whereas the inhibition by DDVP and phosphamidon is irreversible. MAO-B is inhibited irreversibly by all these organophosphates suggesting that the mechanism of action of OP pesticides is through phosphorylation of serine residue present in active centre of MAO.     

Differential substrate specificity of monoamine oxidase in the rat heart and renal cortex

Although it is known that substrate specificities differ with species and within each species with the tissues, in the rat heart no natural substrate was found for MAO-B. beta-phenylethylamine (beta-PEA) has always been considered the "endogenous" substrate of MAO B. We thought worthwide to evaluate the effect of Ro 41-1049 and lazabemide, both members of a class of highly selective, mechanism-based and reversible inhibitors for MAO-A and MAO B, respectively on the metabolization of beta-PEA by the rat heart. Also the lack of molecular data on rat heart MAOs, prompted us to better characterize rat heart MAOs, both kinetically and using molecular biology techniques. K(m) values for deamination of beta-PEA in the rat heart were 13-fold those in the kidney, by contrast, K(m) values for deamination of 5-HT were quite similar in both tissues. Unexpectedly, the selective MAO-A inhibitor Ro 41-1049 was by far the most potent inhibitor of beta-PEA (20 microM) deamination in the rat heart, while clorgyline, another MAO A inhibitor, and lazabemide, a MAO B inhibitor, had intermediate efficacy; selegiline was found unable to inhibit deamination of beta-PEA. In the rat renal cortex lazabemide and selegiline both inhibited beta-PEA deamination. The reduction of beta-PEA concentration to just 200 nM, the use of heart membranes instead of tissue homogenates or the use of heart membranes pre-treated with 1% digitonine failed to change this pattern of inhibition. Semicarbazide was found not to alter deamination of beta-PEA. Western blot showed the presence of both isoforms (55 kd and 61 kd) in the renal cortex. In the heart there was a predominance of the A form, the B form being undetected. The RT-PCR products for both MAO-A and MAO-B, were found to have the expected sizes. In conclusion, we found mRNA for MAO-B but were unable to detect the protein itself or its activity when using beta-PEA as the substrate.     

Evaluation of the oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to toxic pyridinium cations by monoamine oxidase (MAO) enzymes and its use to search for new MAO inhibitors and protective agents

Monoamine oxidase (MAO) enzymes catalyze the oxidative deamination of amines and neurotransmitters and inhibitors of MAO are useful as neuroprotectants. This work evaluates the human MAO-catalyzed oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a dopaminergic neurotoxin, to the directly-acting neurotoxic metabolites, 1-methyl-4-phenyl-2,3-dihydropyridinium (MPDP⁺) and 1-methyl-4-phenylpyridinium (MPP⁺) measured by High-Performance Liquid Chromatography (HPLC), and this approach is subsequently used as a new method for screening of MAO inhibitors and protective agents. Oxidation of MPTP by human MAO-B was more efficient than by MAO-A. R-Deprenyl, a known neuroprotectant, norharman (β-carboline), 5-nitroindazole and menadione (vitamin K3) inhibited MAO-B and reduced the formation of toxic pyridinium cations. Clorgyline and the β-carbolines, harman and norharman, inhibited the oxidation of MPTP by MAO-A. Cigarette smoke, as well as the naturally occurring β-carbolines (norharman and harman) isolated from smoke and coffee inhibited the oxidation of MPTP by MAO-B and/or MAO-A, suggesting protective effects against MPTP. The results show the suitability of the approach used to search for new MAO inhibitors with eventual neuroprotective activity.     

Multiple forms of monoamine oxidase in rabbit platelets.

Rabbit platelets were found to contain both types A and B MAO activities. The specific enzymatic activity of rabbit platelet MAO was higher for the substrate serotonin than for phenylethylamine. The K_m's for rabbit platelet MAO indicated that the MAO-B enzyme was similar to human platelet MAO and that both MAO-A and MAO-B enzymes in the rabbit platelet are similar to the corresponding forms in the rabbit brain. The drugs clorgyline and deprenyl confirmed the existence of types A and B MAO in the platelet and furthermore indicated that the type A form accounted for approximately 90% of the total enzymatic activity. Amitriptyline at low (micromolar) concentrations selectively inhibited MAO-B activity in both rabbit platelets and brain.     

Serotonin Metabolism by Monoamine Oxidase in Rat Primary Astrocyte Cultures

The oxidative deamination of serotonin (5-HT) to 5-hydroxyindoleacetic acid (5-HIAA) by rat primary astrocyte cultures was investigated in intact cells using HPLC. All detectable 5-HIAA accumulated in the extracellular medium, and its rate of production was proportional to the 5-HT concentration over the tested range of 5 x 10(-7) to 10(-4) M. At 5 x 10(-7) M 5-HT, intracellular 5-HT was detectable only in astrocytes treated with monoamine oxidase (MAO) inhibitors. These findings are consistent with the idea that 5-HT taken up into astrocytes is not stored for re-release, but is rapidly metabolized to 5-HIAA, which is then extruded from the cell. At 5 x 10(-7) M 5-HT, 5-HIAA formation in intact cells was blocked 63% by the selective high-affinity 5-HT uptake inhibitor fluoxetine. 5-HT oxidation to 5-HIAA is carried out principally by MAO-A, because clorgyline was more effective at inhibiting the production of 5-HIAA than was pargyline. Radioenzymatic determinations of MAO activity in cell homogenates supported these findings, because under these conditions clorgyline was 1,000-fold more effective than pargyline at inhibiting MAO activity toward 14C-labelled 5-HT. However, the relatively selective MAO-B substrate beta-phenylethylamine (PEA) was also oxidized, showing that these cultures also contained MAO-B activity; the Km values for MAO-A oxidation of 5-HT and MAO-B oxidation of PEA were 135 and 45 microM, and Vmax values were 88 and 91 nmol/mg of total cell protein/h, respectively. Higher concentrations of PEA (greater than 20 microM) were oxidized by both MAO-A and MAO-B isozymes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cytochemical localization of type-A and -B monoamine oxidase in the rat pineal gland

Summary In the mammalian pineal gland, serotonin (5-HT) is located both in the pinealocytes and in the noradrenergic nerve terminals. Pineal 5-HT can be metabolized by three different routes, one of these being its deamination, catalized by monoamine oxidase (MAO). MAO is known to exist as two isozymes, MAO-A and MAO-B. Using two different cytochemical methods at the ultrastructural level, we have localized the presence of MAO in the pineal gland of the rat. The use of selective inhibitors of A-type (clorgyline) and B-type (deprenyl) has shown that MAO-A is localized in the noradrenergic nerve terminals, while pinealocytes contain MAO-B. Taking into account that 5-HT is only deaminated by MAO-A, the specific association of each MAO isozyme with a defined cell type implicates that two cellular compartments are needed in the pineal gland for the biosynthesis of 5-methoxytryptophol and 5-methoxyindole acetic acid, while for the synthesis of melatonin and 5-methoxytryptamine just one cellular compartment, the pinealocyte, is appropriate.