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.     

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.     

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.     

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.     

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.     

Pharmacology and physiology of monoamine oxidase activity in vertebrates--a comparative study

Monoamine oxidase (MAO) activity is high in brain, where it regulates neurotransmitter levels, and in the 'detoxifying' organs. Two MAO isoenzymes (A and B) apparently exist in terrestrial tetrapods, but only one form (type A-like) can be detected in teleosts and in aquatic amphibia. MAO activity is regulated by both endogenous (hormones, substrates) and exogenous (daylength, temperature) factors.     

Some characteristics of monoamine oxidase activity in tissues of the adult female grassfrog (Rana pipiens)

1. Monoamine oxidase (MAO) activity was measured fluorometrically in tissues of the grass-frog, Rana pipiens. 2. Incubation with specific inhibitors revealed the presence of two forms of MAO, type A and type B, in all tissues examined, though the ratios were different in each tissue. 3. Liver contained the greatest, and oviduct and skeletal muscle the lowest amounts of MAO activity, and brain also contained relatively high MAO levels. 4. The apparent Michaelis constant (Km) was similar in most tissues but was higher in pancreas, muscle and oviduct. Values of Km were lower than those reported for mammalian tissues.     

LOWERED MONOAMINE OXIDASE ACTIVITY IN BRAINS FROM ALCOHOLIC SUICIDES

—Monoamine oxidase (MAO) activity in the brains of 15 suicides, of whom 8 were alcoholics, was compared to a control material of 20 individuals without known mental disorder. At autopsy 13 different parts of the brain were macroscopicaily dissected out and the MAO activity in the samples estimated with β-phenylethylamine and tryptamine as substrates. The MAO activity in all parts of the brain investigated was found to be significantly lower in the alcoholic suicides as compared to the controls, while there was no significant difference between the non-alcoholic suicides and controls. Different variables which might have influenced the MAO activity were investigated. There was no significant correlation between age and tryptamine-oxidizing activity, but a positive correlation between age and β-phenylethylamine-oxidizing activity was found. There was also a significant difference between the series in the time lapse between death and autopsy and in the time during which the dead body was kept at room temperature. However, neither of these variables could explain the differences between the series. The results thus demonstrate a connection between low MAO activity in the brain and suicidal behaviour among alcoholics.     

Monoamine oxidase: radiotracer development and human studies

Monoamine oxidase (MAO) is an integral protein of outer mitochondrial membranes and occurs in neuronal and nonneuronal cells in the brain and in peripheral organs. It oxidizes amines from both endogenous and exogenous sources, thereby influencing the concentration of neurotransmitter amines as well as many xenobiotics. It occurs in two subtypes, MAO A and MAO B, which are different gene products and have different substrate and inhibitor specificities. Both MAO A and B can be imaged and quantified in the living human brain using positron emission tomography (PET) and radiotracers labeled with carbon-11. PET studies have been carried out to measure the effects of age, MAO inhibitor drugs, tobacco smoke exposure, and other factors on MAO activity in the human brain.     

MONOAMINE OXIDASE (EC 1.4.3.4): ISOLATION AND CHARACTERIZATION OF MULTIPLE FORMS OF THE BRAIN ENZYME

Monoamine oxidase (MAO) in crude mitochondrial preparations from rat brain was solubilized, and different MAO-active fractions were separated by agarose columns and by Sephadex electrophoresis. Any combination of these techniques yielded at least three fractions possessing MAO activity as measured by assays using radioactive serotonin and benzylamine as substrates. The molecular weight of one of the MAO forms was found to be approximately 400,000 daltons while another was at least 1.5 × 10⁶ daltons. The crude mitochondria1 MAO was inhibited by [¹⁴C]-labelled pargyline and then solubilized and the radioactivity of the soluble and particulate MAO was compared to the enzyme activity found in the soluble and particulate fractions. Our studies suggest that appreciable MAO activity is lost upon solubilization and that the conformation of MAO may be altered.     

Histochemical detection of monoamine oxidase and alcohol dehydrogenase activities in the Syrian hamster Harderian glands: existence of a sexual dimorphism

Summary Monoamine oxidase (MAO) and alcohol dehydrogenase (AD) activities were studied histochemically in the Syrian hamster Harderian gland using tryptamine as substrate and Nitroblue Tetrazolium as the final electron acceptor. No dark: light-related changes were observed. Male type I secretory cells showed an intense MAO reaction. Female type I cells exhibited a moderate MAO activity. Both male and female glands showed a moderate/intense AD-positive reaction. Male type II cells were lacking MAO and AD activities. MAO activity found in the hamster Harderian glands corresponded mainly to MAO type A since treatment with chlorgyline (0.01, 0.1 and 0.5mm) totally inhibited it. The possible role of these two enzymes in Harderian gland indolalkylamine metabolism is discussed.     

Expression of A and B types of monoamine oxidase in neuroblastoma hybrid cells

Monoamine oxidase (MAO) activity towards kynuramine as substrate was measured in 6 hybrid cells derived by fusion of neuroblastoma and glioma, liver or brain cells, and was compared with that of parental or non-parental clones. Activities varied from the lowest level of less than 0.15 pmol/min/mg protein in a neuroblastoma clone NB2A to the highest level of 127 pmol/min/mg protein in NCB20 mouse neuroblastoma × Chinese hamster embryo brain hybrid cells. The relative proportions of A and B types of MAO activities were determined in homogenates of each cell line by inhibition curves with clorgyline and deprenyl. Although the A type activity was found in all cell lines measured, MAO A was predominant in 9 clones, except for NCB20 hybrid cells, N4G-B-a neuroblastoma × glioma hybrid cells, and G8-1 myoblast. The ratio of type A/type B activity in NCB20, N4G-B-a and G8-1 cells was 20/80, 75/25 and 95/5, respectively. The results suggest that NCB20 cells are highly enriched in MAO type B, and that the NCB20 cell is an excellent model for studying the type B activity found in the brain in vivo.     

Increase of monoamine oxidase-B activity in the brain of scrapie-infected hamsters

In the present study, the purpose is to determine activities of monoamine oxidases (MAO) in the brain of 263K scrapie-infected hamsters during the development of this experimental prion disease. Indeed, MAO activity modifications which have already been related in aging and neurodegenerations is suspected to be involved in the neuron loss process by elevated hydrogen peroxide formation. Monoamine oxidase type A (MAO-A) and B (MAO-B) activities were followed in the brain at different stages of the disease. MAO-A activity did not change significantly during the evolution of the disease. However, concerning the MAO-B activity, a significant increase was observed from 50 days post-infection and through the course of the disease and reached 42.9+/-5.3% at its ultimate stage. Regarding these results, MAO-B could be a potential therapeutic target then we have performed a pre-clinical treatment with irreversible (Selegiline or L-deprenyl) or and reversible (MS-9510) MAO-B inhibitors used alone or in association with an anti-scrapie drug such as MS-8209, an amphotericin B derivative. Our results show that none of the MAO-B inhibitors used was able to delay the onset of the disease. Neither these MAO-B inhibitors nor R-NMDA inhibitors (MK-801) can enhance the effects of MS-8209. The present findings clearly indicate a significant increase of cerebral MAO-B activity in scrapie-infected hamsters. Furthermore, inhibitors of MAO-B do not have any curative or palliative effect on this experimental model indicating that the raise of this activity is probably more a consequence rather than a causal event of the neurodegenerative process.     

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.     

Monoamine oxidase in the brain of European corn borer larvae,Ostrinia nubilalis (Hübner)

Monoamine oxidase (MAO) activity was demonstrated in intact brains of larvae from the European corn borer, Ostrinia nubilalis using 2-(2′-benzothiazolyl)-5-stryl-3-(4′-phthalhydrazidyl) tetrazolium chloride (BSPT). With tryptamine as the substrate, MAO specific activity was restricted to mitochondria within perineurial cells. The basic BSPT methodology was modified by the substitution of 2% dimethylsulfoxide for dimethylformamide in the incubation medium. This yielded increased permeability of the brain to the incubation medium, presumably by disrupting the integrity of the blood-brain barrier. A discussion of possible reasons for the previous inability to demonstrate insect neuronal MAO activity is presented.     

Alterations in monoamine oxidase activity of the mouse brain and liver after mixed neutron-gamma irradiation

Monoamine oxidase (MAO) plays an important role in the metabolism of neuro-transmitter biogenic amines. Its activity was determined in mouse brain and liver after exposure to different kinds of ionizing radiation and after pretreatment with a radioprotective agent. After a lethal dose of mixed neutron-gamma irradiation the MAO activity decreased in the brain and increased in the liver. In contrast, after a lethal dose of 60Co-gamma irradiation enzyme activity was considerably increased in the brain while in the liver it increased like after mixed neutron-gamma irradiation. AET (S2-aminoethyl-isothiuronium-Br X HBr), when administered in a radio-protective dose, inhibited MAO activity in the brain, while it increased in the liver. Even more marked changes of enzyme activity were observed in both brain and liver after AET pretreatment and mixed neutron-gamma irradiation. On the basis of the results it is suggested that different kinds of ionizing radiation lead to different types of lipid peroxidation in the lipid environment surrounding MAO, an event leading to altered enzyme activity. AET itself inhibited MAO in the brain and increased the activity in the liver but did not prevent the alterations caused by ionizing radiation in enzyme activity.     

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.     

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.     

Influence of age on brain vascular and cardiovascular monoamine oxidase activity in the rat

Monoamine oxidase (MAO) activity in brain microvessels and cardiovascular tissues was examined in rats of different age. MAO activity continued to increase with age in the heart, but in contrast, reached maximum activity in three weeks in the aorta, mesenteric artery and mesenteric vein. Between 7 and 60 weeks, there was a small decline in the MAO activity in the testicular artery. The highest MAO activity was found in the cerebral microvessels and increased with age. The half-life of MAO was estimated in the heart and peripheral blood vessels in young and old animals. The half-life of cardiac MAO was increased with age whereas that of the mesenteric vein, mesenteric artery and aorta remained constant between 7 and 112 weeks. Thus an explanation for this increased cardiac MAO activity in old rats was a reduced rate of degredation of this enzyme. The high activity of the enzyme in the brain microvessels suggests that it may participate in regulating the influx and efflux of monoamines in the central nervous system.     

Determination of 3-Methoxytyramine in Rat Brain by HPLC with Electrochemical Detection and Its Correlation with Dopamine Function After Administration of a Monoamine Oxidase Inhibitor and L-3,4-Dihydroxyphenylalanine

3-Methoxytyramine (3-MT), normally a minor metabolite of 3,4-dihydroxyphenylethylamine (dopamine) in brain, becomes the sole product of metabolism following the administration of a monoamine oxidase (MAO) inhibitor. A simplified reverse-phase HPLC method for 3-MT employing electrochemical detection is fully described. This method has a detection limit of 0.1 microgram/g brain wet weight and is sensitive enough to detect 3-MT in individual brain regions after rats have been pretreated with an MAO inhibitor. Administration of tranylcypromine (TCP, 10 mg/kg) and L-3,4-dihydroxyphenylalanine (L-DOPA) (10-50 mg/kg) produced a dose-dependent linear increase in 3-MT concentrations in the dopaminergic brain regions n. caudatus (r = 0.95; p less than 0.01) and n. accumbens (r = 0.96; p less than 0.01). This treatment also produced a dose-dependent increase in behavioural activity in rats (r = 0.88; p less than 0.01). Furthermore, a good correlation was found between the activity responses of individual rats and the accumulation of 3-MT after TCP/L-DOPA in both n. caudatus (r = 0.76; p less than 0.01) and n. accumbens (r = 0.84; p less than 0.01). These data describe a simple and sensitive HPLC analysis technique for 3-MT and demonstrate that following administration of an MAO inhibitor this metabolite may provide a useful monitor of central dopamine function.