Substrate-selective activation of rat liver mitochondrial monoamine oxidase by oxygen

Curcumin supplementation in the maturation medium improves the maturation, fertilisation and developmental competence of porcine oocytes

This study was conducted to determine the effects of supplementing the maturation medium with the antioxidant curcumin on the in vitro maturation (IVM), fertilisation and development of porcine oocytes. Curcumin supplementation was performed at concentrations of 0, 5, 10, 20, and 40 µM. At concentrations of 5-20 µM, curcumin had significant positive effects (P < 0.05) on maturation and fertilisation rates compared to the non-treated group. Of the groups cultured with 5-20 µM curcumin, the number of oocytes with DNA-fragmented nuclei after IVM was significantly lower than in groups matured without curcumin. Moreover, curcumin supplementation at 10 µM also gave a significantly higher rate of blastocyst formation compared with oocytes matured without curcumin. Increasing the curcumin concentration to 40 µM yielded negative effects on fertilisation and embryonic development compared with the groups treated with lower concentrations of curcumin. Supplementation with 10 µM curcumin had beneficial effects on the oocyte maturation rate and DNA fragmentation index compared to the non-treated group both in the presence and absence of hydrogen peroxide. These results indicate that curcumin supplementation at a suitable concentration (10 µM) is potentially useful for porcine oocyte culture systems, in terms of protecting oocytes from various forms of oxidative stress.

In-vitro maturation of oocytes: biological aspects

Natural cycle and in-vitro maturation (IVM) of oocytes are becoming interesting alternatives to classical assisted reproduction technology approaches for patients, especially in those at high risk for ovarian hyperstimulation syndrome or with poor ovarian reserve. More than for their clinical and biological indications, natural cycle and IVM of oocytes can also be considered as good social and economic alternatives to the classical IVF treatment, based on their financial cost-effectiveness with exclusion of expensive medications. To be successful, IVM must entail both nuclear and cytoplasmic maturation, and its maturation and success rates are affected by the number of collected cumulus layers, the degree of atresia and the maturation rate between 24 and 48 h. Endogenous regulation of oocyte maturation is a complex sequence of events regulated by endocrine parameters, oocyte/follicular cross-talk, and intra-oocyte kinase/phosphatase interactions. This complex process requires a better definition of each contributing factor affecting oocyte development and the resulting embryo quality. The clinical aspects of IVM have been documented earlier; the present paper will mainly focus on the biological aspect of oocyte maturation in vitro and the quality of derived embryos.

Effects of delipidation and oxygen concentration on in vitro development of porcine embryos

The effects of delipidation and the oxygen (O(2)) concentration in the atmosphere during culture on in vitro development and H(2)O(2) content were investigated in porcine in vivo fertilized embryos and embryos after in vitro maturation and in vitro fertilization (IVM/IVF embryos). There was no significant difference in the developmental rates to the blastocyst stage between the intact and delipidated IVM/IVF embryos. However, the mean number of cells in blastocysts derived from delipidated IVM/IVF embryos (19.8 +/- 0.8 cells) was significantly smaller than that from intact embryos (24.2 +/- 1.2 cells). Although there were no significant differences in the developmental rates to the blastocyst stage of intact and delipidated IVM/IVF embryos between the cultures under 5% O(2) and 20% O(2), the developmental rate of intact IVM/IVF embryos cultured under 5% O(2) (27.1%) was significantly higher than that of the delipidated embryos cultured under 20% O(2) (19.3%). On the other hand, there was no difference in the developmental rate to the blastocyst stage between in vivo fertilized embryos cultured under 5% O(2) and 20% O(2). Hydrogen peroxide (H(2)O(2)), one of the reactive oxygen species (ROS), is thought to cause damage to embryos. The H(2)O(2) content per embryo derived from oocytes cultured under 5% O(2) (in vivo fertilized, 58.0 +/- 2.5 pixels; IVM/IVF, 79.6 +/- 3.2 pixels) was significantly lower than that (in vivo fertilized, 100.2 +/- 3.8 pixels; IVM/IVF, 103.9 +/- 3.2 pixels) under 20% O(2). Furthermore, the level of H(2)O(2) in delipidated IVM/IVF embryos (94.7 +/- 3.9 pixels) was significantly lower than that in intact embryos (103.9 +/- 3.2 pixels) cultured under 20% O(2). The present results indicate that the delipidation of porcine IVM/IVF embryos and reduction of the O(2) concentration decreased the H(2)O(2) level rather than the in vitro developmental rate to the blastocyst stage.

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.

Antioxidant requirements for bovine oocytes varies during in vitro maturation, fertilization and development

Antioxidants may be beneficial additives to synthetic culture media because these well defined media lack serum or other macromolecules that serve as reactive oxygen species scavengers. In this study, three separate experiments were performed to determine the effects of antioxidants on the development of oocytes to the morula and blastocyst stage when added during in vitro maturation (IVM) of bovine oocytes, during in vitro fertilization (IVF), and during embryo culture for the first 72 h of the development period. Bovine oocytes were matured, fertilized (under 20% O(2)), and embryos were cultured (under 7% O(2)) in defined conditioned medium in vitro with or without supplementation with the antioxidant cysteine, N-acetyl-L-cysteine (NAC), catalase and superoxide dismutase (SOD). Significant improvements in the proportion of oocytes undergoing morula and blastocyst development (33.3% versus 20.3%, P<0.05) were achieved when cysteine (0.6 mM) was added to the maturation medium as compared to control medium without antioxidant supplementation. However, the addition of NAC (0.6mM), catalase (5 or 127 U/ml) or SOD (10 or 1000 U/ml) to the maturation medium did not improve the proportion of oocytes undergoing morula and blastocyst development. During the IVF period, addition of antioxidants (cysteine or NAC 0.6mM, catalase 127U/ml, SOD 100U/ml) significantly reduced the subsequent rate of bovine embryo development to the morula and blastocyst stage (P<0.05). In a defined medium for embryo culture (7% O(2)), the addition of cysteine improved the development of bovine embryos while NAC, catalase and SOD had no positive effect on embryonic development. Our study showed that medium supplementation with cysteine during IVM and in vitro culture (IVC) improved the rate of bovine embryo development, in contrast to extracellular antioxidants like catalase and SOD that caused no improvement.

Oxygen dependency of monoamine oxidase activity in the intact lung

Hydrogen peroxide generated by monoamine oxidase (MAO)-mediated deamination of biogenic amines has been implicated in cell signaling and oxidative injury. Because the pulmonary endothelium is a site of metabolism of monoamines present in the venous return, this brings into question a role for MAO in hyperoxic lung injury. The objective of this study was to evaluate the O(2) dependency of the MAO reaction in the lung. To this end, we measured the pulmonary venous effluent concentrations of the MAO substrate [(14)C]phenylethylamine and its metabolite [(14)C]phenylacetic acid after the bolus injection of either phenylethylamine or phenylacetic acid into the pulmonary artery of perfused rabbit lungs over a range of PO(2) values from 16 to 518 Torr. The apparent Michaelis constant for O(2) was approximately 18 microM, which is more than an order of magnitude less that measured for purified MAO. The results suggest a minimal influence of high O(2) on MAO activity in the normal lung and demonstrate the importance of measuring reaction kinetics in the intact organ.

Oxygen-induced embryopathy and the significance of glutathione-dependent antioxidant system in the rat embryo during early organogenesis

We investigated the effect of glutathione (GSH)-dependent antioxidant system against hydrogen peroxide (H2O2) formation in oxygen-induced embryopathy. Exposure of rat embryos to a high concentration of oxygen (20%) during early neurulation (day 9 to 10) significantly increased the incidence of neural tube defects compared with control embryos (10% vs 0%, p < 0.01) exposed to a low O2 concentration (5%). The concentration of GSH in 20% O2-exposed embryos was significantly reduced compared with that in control embryos (10.68 +/- 0.72 vs 12.34 +/- 0.65 nmol/mg protein, p < 0.001). The activity of gamma-glutamylcysteine synthetase (gamma-GCS), the rate-limiting GSH synthesizing enzyme increased in 20% O2-exposed embryos (24.83 +/- 0.71 vs 21.00 +/- 0.94 microunits/mg protein). Increased activity of gamma-GCS was associated with increased expression of gamma-GCS mRNA. Substantial increases were also observed in the activities of glutathione peroxidase (GPX) and glutathione S-transferase (GST) in 20% O2-exposed embryos. The formation of intracellular H2O2, measured by flow cytometer using 2',7'-dichlorofluorescein diacetate (DCFH-DA), increased in isolated embryonic cells of 20% O2-exposed embryos. The addition of buthionine sulfoxamine (BSO), a specific inhibitor of gamma-GCS, to culture media exposed to 20% O2 produced a marked decrease in the concentration of GSH in association with a further increase in the incidence of embryonic malformations (24.4% vs. 10%, P < 0.01). The addition of 2.0 mM GSH ester to culture media exposed to 20% O2 prevented the development of embryonic malformations through the restoration of normal GSH contents and reduction of H2O2. Our results demonstrated that oxygen-induced embryonic malformations were induced by increased production of H2O2 in the presence of an immature free radical scavenger system. We suggest that impaired responsiveness of the GSH dependent antioxidant system against oxidative stress plays a crucial role in oxygen-induced embryopathy.

Pseudo-phaeochromocytoma after multiple drug interactions involving the selective monoamine oxidase inhibitor selegiline

A patient presented with paroxysmal hypertension and typical clinical features of phaeochromocytoma, but with a normal adrenal computed tomographic scan and much higher plasma noradrenaline than adrenaline concentrations. Urinary vanillylmandelic acid concentrations were only moderately elevated. This syndrome probably arose as a consequence of an interaction between the monoamine oxidase inhibitor selegiline, the sympathomimetic agent ephedrine, and a tricyclic antidepressant. The mechanism of the interaction is thought to be related to increased sympathetic release of noradrenaline by ephedrine, inhibition of catabolism by selegiline, and inhibition of reuptake of noradrenaline by the tricyclic. Although newer selective monoamine oxidase inhibitors are considered to be safer than earlier non-selective inhibitors, they can also contribute to drug interactions mimicking phaeochromocytoma.

Increased generation of reactive oxygen species in embryos cultured in vitro

Previous studies have suggested that oxygen toxicity is closely related to the developmental blockage of embryos cultured in vitro. In this study, to obtain an actual proof of the increase in production of reactive oxygen species within embryos, we have measured the level of H2O2 in individual embryos using a fluorimetric method. Mouse (ICR) pronuclear stage embryos from the oviducts were cultured for a specified time under various conditions in a medium to which 2',7'-dichlorodihydrofluorescin diacetate was added. After washing the embryos, the fluorescence emissions of the H2O2-dependent oxidative product in embryos were measured. The fluorescent emissions were lowest in embryos cultured under 5% O2 and highest under 40% O2 (5% < 20% < 40%), just the inverse of the culture efficacy. The fluorescence emmissions of embryos cultured in Ham's F-10, which contains hypoxanthine and transition metals such as Cu and Fe, were higher than those cultured in BWW and alpha MEM, which do not contain these components (alpha MEM < BWW < Ham's F-10; again this is the inverse of the culture efficacy). The fluorescence emissions of embryos increased with the time of the exposure to visible light. L-cysteine and thioredoxin, both of which have been shown to promote embryo development, decreased the fluorescence emissions of embryos. All of these results would provide direct evidence for the hypothesis that oxygen radicals are involved in the developmental blockage.

Development of hamster one-cell embryos recovered under different conditions to the blastocyst stage

One-cell stage embryos, recovered from superovulated golden hamsters (8 to 12 weeks of age) 12 hours after egg activation, were cultured in HECM-1 medium at 37 degrees C and 5% CO(2) in air. The culture conditions investigated were the time and temperature required for embro recovery, the pH shift of the washing medium, and the oxygen concentration of the gas phase during and after embryo recovery. Each condition was assessed by the developmental efficiency of the embryo as determined by morphological criteria. As the time required for embryo recovery was reduced, the developmental rates of the embryos were improved: 2.3% (3 128 ) 26.9% (35 130 ) at 5 and 3 minutes, respectively, as determined by the number of embryos developed to the blastocyst stage. No blastocysts were obtained when more than 10 minutes were required for embryo recovery. As the oxygen concentration was reduced from 40 to 20% or to 5%, rather high developmental rates were obtained even when the time required for embryo recovery was prolonged: 6.9% (9 130 ) and 21.7% (28 129 ) of the embryos developed to the blastocyst stage when they were recovered under 5% oxygen within 10 and 5 minutes, respectively. Neither the temperature during embryo recovery (37 degrees C and 25 degrees C) nor the pH shift (pH 7.22 to 7.52) of the washing medium used in embryo recovery procedures influenced the development of the embryos. These findings suggest that the developmental block in hamster embryos may involve oxidative stress, which may result from exposure to high oxygen concentration and light during the manipulation of oocytes and embryos.

Effects of oxygen toxicity on early development of mouse embryos

To examine the effects of oxygen toxicity on embryonic development, mouse pronuclear embryos were cultured under low oxygen conditions with or without superoxide dismutase (SOD), and the blastulation rate was compared with that of embryos cultured under standard conditions. The blastulation rate of mouse pronuclear embryos cultured under standard conditions was only 1.5% (2/131). This rate was increased significantly, to 28.5% (43/151), when the embryos were cultured under low oxygen conditions; and to 31.0% (35/113) when SOD (500 micrograms/ml) was added to the medium under standard conditions; the rate was increased to 75.2% (115/153) when the embryos were cultured under low oxygen conditions in the presence of SOD. The minimum effective concentration of SOD in the culture medium was 50 micrograms/ml under conditions of 5% O2. The blastulation rate was significantly decreased after 1-hr exposure of pronuclear embryos to room atmospheric oxygen concentration (20% O2), and subsequent culture under 5% O2 with SOD did not result in an improved blastulation rate. Culture with SOD under 5% O2 promoted the development of two-cell stage embryos to the blastocyst stage. When two-cell stage embryos were collected 48 hr after hCG and cultured for 66 hr, their blastulation rate was similar to that of embryos collected from mice 114 hr after hCG. These results suggested that embryonic development in vitro is greatly affected by atmospheric oxygen throughout the early embryonic stages and that this harmful effect can be prevented by culturing embryos under low oxygen conditions and in the presence of SOD.

Oxidative stress on mouse embryo development in vitro

Oxygen radicals are involved in the in vitro block phenomenon of embryo development, because a low oxygen tension and superoxide dismutase (SOD) have been shown to promote the in vitro development of mouse embryos. One of the target molecules damaged by oxygen radicals may be the thiol (SH) group of proteins because it is readily oxidized. In this study, we evaluated the effects of thioredoxin, which is a powerful protein disulfide reductase, on mouse (Institute of Cancer Research, ICR) preimplantation embryo development. Culture of mouse pronuclear embryos recovered 17 h after human chorionic gonadotrophin (hCG) administration in the presence of thioredoxin (200 micrograms/mL) significantly increased the blastulation rate (75.3%) when compared to the control culture system (8.9%). The effects of thioredoxin were observed only from the pronuclear stage to the two-cell stage (17-48 h after hCG administration). An additive effect of thioredoxin and SOD, or thioredoxin and a low oxygen tension, was observed. These results suggest that the oxidation of the SH group of proteins is one of the causes of developmental blockage of embryos in vitro. The target protein for reduction by thioredoxin has not been identified yet, but thioredoxin will be a new clue for clarifying the mechanism of blocking development in vitro.

Comparison of monoamine oxidase-A inhibition by moclobemide in vitro and ex vivo in rats

Inhibition of MAO activity was measured in rat brain homogenates using 5-HT as MAO-A substrate and phenylethylamine as MAO-B substrate. Moclobemide rather selectively inhibited MAO-A. Its inhibitory potency is rather low, like that of toloxatone, whereas clorgyline, harmaline, cimoxatone and brofaromine were all found to be at least 100 times more potent. Phenelzine, isocarboxazid and tranylcypromine were nonspecific, inhibiting MAO-A and MAO-B to about the same extent. The same drugs were also tested ex vivo. Here again moclobemide preferentially inhibited MAO-A; it was equipotent to clorgyline and brofaromine in these tests, and 2-4 times as potent as cimoxatone and harmaline. Moclobemide is a relatively weak MAO-A inhibitor in vitro and yet more potent in vivo than other reversible inhibitors, suggesting that the compound may be converted in vivo to an active form. Nevertheless, it has not been possible so far to identify activated derivatives, and recent findings that moclobemide markedly inhibits liver MAO-A within 5 min of an intravenous injection strongly suggests that the compound itself is responsible for the inhibition.

Review article: hypoxia and hepatic drug metabolism--clinical implications

Most major pathways of hepatic drug metabolism are dependent on oxygen. Hepatic mixed-function oxidases use oxygen directly as a substrate, while many other enzyme systems are indirectly dependent on oxygen for the generation of essential co-factors, such as NAD+ and ATP. Studies in vitro show that many of these oxygen-dependent reactions are impaired by relatively minor reductions in oxygen supply, of a magnitude likely to be encountered in vivo. Phase I metabolism by mixed-function oxidases appears to be more sensitive to hypoxia than phase II drug conjugation, although the oxygen requirements of conjugation reactions, such as glucuronidation, may be greatly enhanced by poor nutrition or fasting. Studies in humans are few, but in general they affirm the potential importance of the effects of hypoxaemic states on hepatic drug elimination. On present evidence, special care should be taken in hypoxic patients with drugs extensively metabolized by the liver, particularly those which have a low therapeutic ratio.

Contribution of monoamine oxidase (MAO) to the binding of tertiary basic drugs in lung mitochondria

The effects of tertiary amine-containing basic drugs on the enzymes located in the mitochondria and the effect of monoamine oxidase inhibitors (MAOIs) on drug accumulation in lung mitochondria have been studied. Various basic drugs inhibited MAO activity but not other mitochondrial marker enzymes. The potency of MAO inhibition correlated well with their lipid solubility, and the basic drugs inhibited MAO activity dose dependently and competitively. Further, MAO inhibition correlated well with binding affinity to lung mitochondria, and the binding of tertiary amine drugs to lung mitochondria was decreased by treatment with MAOIs. A good correlation was observed between the potency of MAOIs to inhibit the binding of the basic drug to the high-affinity site in mitochondria and the MAO inhibitory activity in mitochondria. These results indicate that mitochondrial MAO is one of the binding sites for tertiary basic drugs in the lung. We think that the action and/or adverse reaction of some drugs may result from inhibition of mitochondrial MAO to metabolize various biogenic amines and that mitochondrial MAO may function as a reservoir for basic drugs.

Monoamine oxidase in rat and bovine endocrine tissues

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.

Benzylamine metabolism at low O2 concentrations. Relative sensitivities of monoamine oxidase, aldehyde dehydrogenase and hippurate synthesis to hypoxia

The O2 dependence of the metabolism of benzylamine to benzaldehyde, benzoate and hippurate was studied in isolated rat hepatocytes. The initial oxidation to benzaldehyde, catalyzed by monoamine oxidase, had an apparent Kmo2 value of 34 microM in cells and 40 microM in isolated rat liver mitochondria. The conversion of benzaldehyde to benzoate was essentially independent of O2 concentration in spite of the dependence of the reaction upon cytosolic NAD+. The conversion of benzoate to hippurate was half-maximal at 2.4 microM O2 in hepatocytes and at about 0.5 microM O2 in rat liver mitochondria. These values are consistent with the O2 dependence of bioenergetic changes in these preparations and indicate that the O2 dependence of hippurate formation is due to ATP availability for synthesis of benzoyl-CoA. These studies show that the three metabolic processes involved in benzylamine metabolism have markedly different dependences upon O2 and that metabolism of benzylamine by monoamine oxidase is O2 dependent over a physiologically important range.

Monoamine oxidase in pancreatic islets, exocrine pancreas, and liver from rats. Characterization with clorgyline, deprenyl, pargyline, tranylcypromine, and amezinium

Monoamine oxidase (MAO) was characterized in tissue homogenates from pancreatic islets, exocrine pancreas, and liver from rats. Phenylethylamine was preferentially deaminated by pancreatic islet MAO while 5-hydroxytryptamine was preferentially deaminated by MAO from exocrine pancreas, and tyramine was a good substrate for both tissues. All three substrates were well deaminated by liver tissue. Clorgyline, a selective inhibitor of MAO-A, preferentially inhibited deamination of 5-hydroxytryptamine by all three tissue homogenates, while deprenyl, a selective inhibitor of MAO-B, preferentially inhibited deamination of phenylethylamine. In the case of pargyline, a less selective MAO-B inhibitor, the preference in favour of phenylethylamine was less pronounced. According to these results, MAO in pancreatic islets can be classified as predominantly type B enzyme species and MAO in exocrine pancreas as predominantly type A enzyme species while both types of the enzyme are present in the liver. Using the same three MAO substrates and compared with the effects of the selective enzyme inhibitors, clorgyline and deprenyl, tranylcypromine can be classified as a potent nonselective inhibitor of MAO in homogenates of all three tissues investigated with a slight preference in favour of the inhibition of the B-form of the enzyme, while in contrast amezinium can be classified as a weak nonselective inhibitor of MAO with a slight preference in favour of the inhibition of the A-form of the enzyme. All MAO inhibitors tested also inhibited insulin secretion by isolated incubated rat pancreatic islets, however only at IC50 which were two to three decimal powers higher than those necessary for the inhibition of the MAO activity, thus indicating that inhibition of MAO activity and inhibition of insulin secretion are apparently not closely related.

Time-dependent inhibition of monoamine oxidase by beta-phenethylamine

Several reports have suggested that monoamine oxidase activity towards beta-phenethylamine is inhibited by high concentrations of that substrate. This inhibition is not found if initial velocities are measured, but there is a slower time-dependent inhibition at higher beta-phenethylamine concentrations. Such time-dependent inhibition is not found with tyramine as substrate or upon incubation of the enzyme with the reversible inhibitor amphetamine. The inhibition is not due to the accumulation of phenacetaldehyde, phenylethanol or phenacetic acid, or to a reaction of any of these three products either with each other or with beta-phenethylamine. Although the inhibition is time-dependent, the inactivated enzyme slowly regains activity upon removal of the beta-phenethylamine. A model is proposed to explain the observed inhibition.

Characteristics and specificity of phenelzine and benserazide as inhibitors of benzylamine oxidase and monoamine oxidase

The selectivity of benserazide and phenelzine toward inhibition of benzylamine oxidase (BzAO) and monoamine oxidases (MAO-A and MAO-B) was studied in homogenates of rat skull and lung. In addition, the kinetic interaction and reversibility of BzAO inhibition were assessed. Both drugs inhibited BzAO but only phenelzine inhibited MAO, whether tested in vitro or in vivo. Neither compound acted as an irreversible inhibitor of BzAO. Benserazide was found to be a noncompetitive inhibitor. Phenelzine acted as a substrate for BzAO followed by product-induced noncompetitive inhibition which was labile at 37 degrees but not at 4 degrees. A reversible component in phenelzine-induced inhibition of MAO-A and -B is also suggested from in vivo studies.

Monoamine oxidase-catalyzed metabolism of 3,4-dihydroxyphenylethylamine in the dopaminergic synaptosomes from rat corpus striatum

This communication describes conditions under which the monoamine oxidase-catalyzed metabolism of 3,4-dihydroxyphenylethylamine can be assayed in dopaminergic synaptosomes from the rat striatum. In contrast to the activity of the isolated enzyme or that of free mitochondria, the synaptosomal reaction exhibits sigmoidal kinetics with respect to substrate concentration. This is consistent with a kinetic mechanism in which intrasynaptosomal substate partitions between reaction and saturable storage in synaptic vesicles. The reaction is inhibited at moderately decreased oxygen tension, where catecholamine uptake is unaffected. The specificity of this effect suggests that it reflects limited availability of oxygen as an enzyme substrate.

Activity of platelet monoamine oxidase in apparently health subjects

Monoamine oxidase activity was assayed in platelets from 32 apparently healthy subjects using phenethylamine as substrate and two concentrations of oxygen (0.06 and 0.12 mM). Apparent Km (microM) and Vmax (nmol/mg protein/5 min) values were estimated from double reciprocal plots. The means of the Km and Vmax values were both increased by a factor of about 1.6 when the oxygen concentration was elevated from 0.06 mM (Km = 2.96 and Vmax = 4.47) to 0.12 mM (Km = 4.82 and Vmax = 7.18). The frequency distribution of the Vmax values was not clearly unimodal, especially at the higher oxygen concentration. The Vmax values of the women were significantly higher than those of the men but no sex difference was obtained for the Km values. The overall velocity (v) at 10 microM phenethylamine was highly correlated to the Vmax values both at low and high oxygen concentrations (r = 0.99 and 0.98, respectively).

The effect of lipid-depletion on the kinetic properties of rat liver monoamine oxidase-B

The extraction of lipids from rat liver mitochondrial membranes by 2-butanone treatment inhibited the activity of membrane-bound monoamine oxidase -A but not -B. For the -B form, the apparent Michaelis constants of the enzyme towards oxygen and the maximum molecular turnover numbers obtained when beta-phenethylamine and benzylamine were used as substrates were not significantly changed by the lipid-depletion procedure, but the values of the Michaelis constant towards benzylamine was significantly increased after lipid-depletion. The differential sensitivity of beta-phenethylamine and benzylamine oxidation to inhibition by Tris-HCl was not changed after lipid-depletion. The results are consistent with the hypothesis that the mitochondrial membrane lipids, while essential for the activity of the -A form of the enzyme in rat liver, play a more subtle modulatory role in the activity of the -B form.

Titration of human brain type-B monoamine oxidase

The interaction of the substrate-selective irreversible inhibitor J-508 [N-methyl-N-propargyl-(1-indanyl)-ammonium hydrochloride] with the B form of human brain monoamine oxidase has been investigated, and the conditions necessary for this inhibitor to "titrate" the concentration of this enzyme form determined. It was found that the concentration of monoamine oxidase-B determined in this way was the same when either benzylamine or beta-phenethylamine was used to assay for activity, which would indicate that this enzyme form is not heterogeneous. Furthermore, the variation in activity from sample to sample was found to be due to a variation in the concentration of available monoamine oxidase-B active centers, raher than due to a variation in the molecular turnover numbers of this enzyme form towards its amine substrates.

Monoamines and related enzymes in cerebral cortex and basal ganglia following transient ischemia in gerbils

The post-ischemic effects on cerebral cortex and basal ganglia monoamine levels and monoamine oxidase (MAO A and B) and catechol-O-methyl transferase (COMT) activities were evaluated in Mongolian gerbils (Meriones unguiculatus) subjected to bilateral common carotid arteries of occlusion for 15 min and reflow for 7 days. Disorders of monoamine metabolism was found in ischemic brain which persisted during the long-term post-ischemia. A rebound increase of norepinephrine and serotonin appeared in early stages (up to 1 h) of post-ischemia both in cerebral cortex and basal ganglia; a rebound increase of dopamine as found only in cerebral cortex. Thereafter, the serotonin level ws enhanced over the control level during the whole post-ischemic period whereas the levels of catecholamines were reduced particularly in basal ganglia. With respect to monoamine content and activities of monoamine degraded enzymes an oscillatory behavior was observed in the post-ischemia. Disorder of the monoamine metabolism found during post-ischemic period possibly contributes to neurological dysfunction after an ischemic insult.