Metabolism of octopamine in vitro by monoamine oxidase in some rat tissues.

The deamination $\text{in vitro}$ of DL-octopamine by MAO in rat brain, heart, kidney, liver and vas deferens has been studied by a radiochemical method. Kinetic constants for octopamine metabolism, as well as its sensitivity to inhibition by the irreversible MAO inhibitor clorgyline are described for each tissue. On the basis of the inhibition data, it was concluded that octopamine is metabolized preferentially by type A MAO in heart, kidney and vas deferens. However, in brain and liver, type B MAO is also responsible for a significant proportion of total octopamine metabolism. These studies are discussed in relation to current ideas about the regulation of octopamine concentrations in animal tissues, and the possible importance of this amine in mammalian physiology.     

Monoamine oxidase and catechol-O-methyltransferase activities in cultured human skin fibroblasts

Human fibroblasts obtained from normal male children and children with the Lesch-Nyhan syndrome were found to contain both the A and B forms of monoamine oxidase, with the A form predominating. Both forms of monoamine oxidase showed decreased activities in Lesch-Nyhan, as compared to normal cells; while catechol-O-methyltrans-ferase activities were similar. This study demonstrates the usefulness of fibroblasts cultured from human skin biopsies in analyses of alterations in catecholamine catabolism associated with inherited neurologic diseases.     

Evaluation of nitrocatechol chalcone and pyrazoline derivatives as inhibitors of catechol-O-methyltransferase and monoamine oxidase

Literature reports that chalcones inhibit the monoamine oxidase (MAO) enzymes, mostly with specificity for the MAO-B isoform, while nitrocatechol compounds are established inhibitors of catechol–O-methyltransferase (COMT). Based on this, nitrocatechol derivatives of chalcone have been proposed to represent dual-target-directed compounds that may inhibit both MAO-B and COMT. Both these enzymes play key roles in the metabolism of dopamine and levodopa, and inhibitors are thus relevant to the treatment of Parkinson’s disease. The present study expands on the discovery of dual MAO-B/COMT inhibitors by synthesising additional nitrocatechol derivatives of chalcones which include heterocyclic derivatives, and converting them to the corresponding pyrazoline derivatives. The newly synthesised chalcone and pyrazoline compounds were evaluated as inhibitors of human MAO and rat COMT, and the inhibition potencies were expressed as IC₅₀ values. A pyrazoline derivative, compound 8b, was the most potent COMT inhibitor with an IC₅₀ value of 0.048 μM. This is more potent than the reference COMT inhibitor, entacapone, which has an IC₅₀ value of 0.23 μM. The results indicated that the pyrazoline derivatives (IC₅₀ = 0.048–0.21 µM) are more potent COMT inhibitors than the chalcones (IC₅₀ = 0.14–0.29 µM). Unfortunately, the chalcone and pyrazoline derivatives were weak MAO inhibitors with IC₅₀ values > 41.4 µM. This study concludes that the nitrocatechol derivatives investigated here are promising COMT inhibitors, while not being suitable as MAO inhibitors. Using molecular docking, potential binding modes and interactions of selected inhibitors with COMT are proposed.     

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.     

Species differences in lung mitochondrial monoamine oxidase activities

Pulmonary mitochondrial monoamine oxidase (MAO) activity was examined in preparations from rat, rabbit and guinea-pig with 12 different amines as substrates: serotonin, norepinephrine, and octopamine (type A specific); tryptamine, benzylamine, 5-methoxytryptamine, 5-methyltryptamine, p-methoxyphenylethylamine, and 3,4-dimethoxyphenethylamine (type B specific); and tyramine, dopamine and 3-methoxytyramine (type A + B specific). The oxidation of type A and type A + B substrates was greater in guinea-pig lung mitochondria than in rat or rabbit preparations. Except for benzylamine, the oxidation of type B substrates was similar in all three species. Benzylamine was not oxidized by guinea-pig lung mitochondria but was actively metabolized by rat and rabbit preparations.     

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.     

Central monoamine oxidase and phenolsulfotransferase activities in spontaneously hypertensive rats

The activities of monoamine oxidase and phenolsulfotransferase in the hypothalamus and anterior pituitary gland of spontaneously hypertensive rats and the normotensive control (Wistar Kyoto rat) rats were investigated. The monoamine oxidase activity (determined using dopamine as substrate) in both these tissues was not significantly different between the normo- and hypertensive animals. Hypothalamic phenolsulfotransferase does not sulfate-conjugate dopamine at pH of 6.5 and pituitary phenolsulfotransferase does not sulfate-conjugate dopamine or 3,4-dihydroxyphenylacetic acid at the same pH. Hypothalamic phenolsulfotransferase activity determined using 3,4-dihydroxyphenylacetic acid as substrate was significantly higher in the spontaneously hypertensive than the Wistar Kyoto rats, while pituitary enzyme (determined using phenol as substrate) was the same in both strains of animals. We proposed that in the spontaneously hypertensive rats the higher level of hypothalamic phenolsulfotransferase could (by removing 3,4-dihydroxyphenylacetic acid as sulfated acid) increase the deamination of dopamine by monoamine oxidase. This could in turn result in the presence of high amount of sulfated 3,4-dihydroxyphenylacetic acid in the anterior pituitary gland reported in our earlier study, and be partly responsible for the reduced central dopaminergic activity found in the hypertensive rats.     

Localization of biogenic amines and monoamine oxidase in the rabbit hippocampus

Comparison of the localization of monoamines and monoamine oxidase in the rabbit hippocampus shows that most pyramidal and all granular neurons contain no monoamines. Up to 1% of pyramidal and about 3% of polymorphic neurons are noradrenergic, and some of the latter are basket cells. Their terminals, containing both noradrenalin and monoamine oxidase, are in contact with the bodies and processes of the pyramidal and granular neurons. Single serotoninergic polymorphic neurons are found in sectors H₁ and H₂ of the cornu ammonis and in sector H₅ of the fascia dentata of the hippocampus; they have few terminals. Noradrenergic afferents enter the cornu ammonis in the external bundle of the alveus and in the septal tract, which runs along the inner surface of the fimbria. Noradrenergic terminal plexuses surround the bodies of the pyramidal cells and are concentrated at the level of the apical dendrites of the pyramids and at the base of the granular neurons of the fascia dentata. Convergence of noradrenergic and serotoninergic terminals is found on some pyramidal, granular, and polymorphic neurons. The high concentration of serotonin in the hippocampus despite the minimal number of serotoninergic neurons can be explained by the large number of serotonin-containing stellate cells of nonneural nature. They are localized on groups of pyramidal neurons in sectors H₁ and H₂ and also on blood vessels. Individual variations are found in the number of serotonin-containing neurons in the hippocampus and in the number and distribution of serotonin-containing stellate cells.A. A. Bogomolets' Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 5, No. 1, pp. 40–46, January–February, 1973.     

Quantitative histochemical study of monoamine oxidase in the rabbit hippocampus

The distribution of monoamine oxidase activity in the rabbit hippocampus was studied by quantitative histochemical analysis. The presence of monoamine oxidase activity was found in str. lacunosum-moleculare of areas CA₁ and CA₂ of the hippocampus and in str. moleculare of the dentate fascia. A strong positive reaction was found in layers containing many myelinated fibers (the alveus and Shaffer's collaterals). However, when the reaction was carried out without substrate, considerable deposition of diformazan was observed in these layers. These observations and the ability of myelin to reduce nitro-BT spontaneously, described in the literature, suggest that the positive Glenner's reaction in the alveus and Shaffer's collaterals is not specific. The deposition of diformazan in the layer of pyramidal and granular neurons likewise is nonspecific, as is confirmed by the results of experiments with preincubation in iproniazid and with incubation without substrate.Institute of Biological Physics, Academy of Sciences of the USSR, Pushchino-on-Oka. Translated from Neirofiziologiya, Vol. 9, No. 2, pp. 135–141, March–April, 1977.     

The effects of disulfiram on rat liver mitochondrial monoamine oxidase.

Monoamine oxidase (MAO) of rat liver mitochondria was found to be inhibited by disulfiram. The inhibition is pH and time dependent: 50% inhibition was observed by 16.5 μM of disulfiram at pH 9.1 after 30 min of preincubation. At pH 7.4 only slight inhibition was produced despite the high concentration of disulfiram (330 μM) and the preincubation period. The inhibition is irreversible and appears to be of mixed type: noncompetitive at low concentration range of the substrate and uncompetitive at high concentration range. Glutathione at twice the concentration of disulfiram abolished the inhibitory effect of the drug. Ethanol, while by itself has only slight effect on MAO activity, enhanced the inhibitory effect of disulfiram at pH 7.4. At pH 9.1, ethanol alone has no effect on MAO; however, it was found to weaken the inhibitory effect of disulfiram.     

Chromenylchalcones with inhibitory effects on monoamine oxidase B

Structure–activity relationship (SAR) calculations were used to find monoamine oxidase-B (MAO-B) inhibitors by identifying pharmacophores exhibiting high inhibitory activities. Several such chromenylchalcones were designed and synthesized accordingly. Their inhibitory effects on MAO-B were determined using an HPLC-based method and an MAO-B enzyme assay kit. (E)-3-(6-Methoxy-2H-chromen-3-yl)-1-(2-methoxyphenyl)prop-2-en-1-one exhibited a half-maximal inhibitory concentration of 320 nM. Its molecular-level binding mode with the three-dimensional structure of MAO-B was elucidated using an in silico docking study. The chromenylchalcone scaffold, which is derived from natural products including isoflavonoids and chalcones, had not been previously reported as an MAO-B inhibitor.     

Influence of prolonged fasting on monoamine oxidase and semicarbazide-sensitive amine oxidase activities in rat white adipose tissue

Monoamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO) activities are very high in white adipose tissue (WAT). SSAO, also known as Vascular Adhesion Protein-1 in vessels, is present at the surface of fat cells and independent approaches have evidenced its impressive increase during adipogenesis. However, the factors that might regulate the expression SSAO and MAO in adipose tissue are still poorly defined. Here, we report the influence of fasting on MAO and SSAO activities in adipose depots. A decrease of MAO activity occurred after three days of starvation in the intra-abdominal adipose tissue (INWAT) of male Wistar rats, regardless of their initial adiposity or fat loss. The reduced fat stores of seven-week old rats, loosing 59% of INWAT mass during fasting, contained only one half of the MAO activity found in fed control. The same reduction of MAO was observed after prolonged fasting in older rats which lose only 26% of their INWAT during the same starvation duration, leading to a fat mass comparable to that of younger fed control rats. It was therefore the endocrine and metabolic changes occurring during fasting that were responsible for the reduced MAO activity and not the amount of INWAT. Surprisingly, SSAO activity remained unchanged during starvation. In subcutaneous WAT, the changes in MAO and SSAO activities exhibited the same tendencies than those found in INWAT. Taken together, these data show that both MAO and SSAO activities increase in INWAT with age-dependent fattening, and indicate that only MAO diminishes during fasting.     

Development of monoamine oxidase activity and monoamine effects on glutamate release in cerebellar neurons and astrocytes

Activities of monoamine oxidase (MAO) A and B were measured during the first month of postnatal development in mouse cerebellum and in primary cultures of either cerebellar granule cells or cerebellar astrocytes, derived from 7-day-old cerebella. In addition, effects of the two monoamines, serotonin (a MAO A substrate) and phenylethylamine (a MAO B substrate) on the release of glutamate under resting conditions and in a transmitter related fashion (i.e., potassium-induced, calcium-dependent glutamate release) were studied during the same period. Both MAO A and MAO B activities increased during in vivo development (beginning around postnatal day 14) and in cultured astrocytes (during a comparable time period and to a similar extent), but remained constant at a low level in granule cells. In 4-day-old cerebellar granule cell cultures there was no potassium-induced glutamate release but serotonin as well as phenylethylamine reduced the release in both the presence and absence of excess potassium. In 8- and 12-day-old granule cell cultures and in 8- and 18-day old astrocyte cultures there was a pronounced glutamate release during superfusion with 50 mM K⁺. In both neurons and astrocytes this response was inhibited by 1 nM of either serotonin or phenylethylamine. In the astrocytes the inhibition was followed by an increased release of glutamate in both the presence and absence of the high potassium concentration, whereas the 8-day-old neurons showed only a slight increase in glutamate release after the with-drawal of the monoamine and only in the absence of excess potassium. The response was almost identical in 8-and 18-day-old astrocytes in spite of the marked difference in MAO activities.Special issue dedicated to Dr. Paola S. Timiras.