In vivo monoamine oxidase inhibition by d-amphetamine

In vitro, d- and l-amphetamine (AMPH) are reversible monoamine oxidase (MAO) type A inhibitors, the d-form being approximately five times more potent. Experiments were conducted in rats to determine whether MAO inhibition occurs in vivo. d-AMPH was more effective than l-AMPH at decreasing striatal 3,4-dihydroxyphenylacetic acid (DOPAC). However, assays of striatal MAO activity following administration of AMPH in vivo failed to show MAO inhibition. In other experiments, rats were treated with d-AMPH (zero time) followed by phcnelzine (1 hr), an irreversible MAO inhibitor, and were killed at 25 hr. MAO activity was determined in vitro for the striatum and the rest of the brain using serotonin (MAO-A) and phenylethylaminc (MAO-B) as substrates. d-AMPH provided significant protection against MAO-A inhibition by phenelzine, whereas l-AMPH and cocaine (used instead of AMPH) were without effect. d-AMPH failed to protect against MAO-B inhibition by phenelzine. Thus, d-AMPH appears to inhibit reversibly MAO type A in vivo. However, using the same ‘protection protocol’, d-AMPH failed to oppose phenelzine-induced lowering of striatal DOPAC. Experiments were undertaken to determine whether the protective effect of d-AMPH on MAO type A would influence striatal dopamine depletion by RO4-1284, a rapidly acting reserpine-like agent. RO4-1284-induced depletion of dopamine was inhibited by phenelzine. Prior treatment with d-AMPH reduced significantly the protective effect of phenelzine, suggesting reversible, intraneuronal MAO inhibition by d-AMPH in vivo. The possible neuronal mechanisms for these events are discussed.     

A comparison of cardiac and vascular clorgyline-resistant amine oxidase and monoamine oxidase: Inhibition by amphetamine,mexiletine and other drugs

Clorgyline-resistant amine oxidase (CRAO) and monoamine oxidase (MAO) were studied in homogenates of rat heart and aorta, using benzylamine and tyramine as substrates. In heart, benzylamine at 0.001 mM was deaminated solely by CRAO. With higher concentrations of benzylamine (0.01, 0.1 and 1.OmM), an increasing involvement of MAO-A and MAO-B became apparent in the deamination of benzylamine such that, at 1.0 mM benzylamine, deaminated products resulted equally from MAO-A, MAO-B and CRAO. In aorta, benzylamine was deaminated solely by CRAO irrespective of the concentration used. Tyramine (0.01, 0.1, 1.0 and 5.0 mM) was deaminated entirely by MAO-A in heart, whereas in the aorta both MAO-A and CRAO participated. In aorta the ratio of product formation from MAO-A and CRAO did not vary with changes in the concentration of tyramine, indicating similar K_m values for both enzymatic activities. Further studies with tyramine (0.1 mM) and clorgyline showed biphasic inhibition curves suggestive of two distinct MAO-A components in both heart and aorta. The two components showed different properties in the heart when compared with aorta. When homogenates of hearts were heated at 50° for 1 hr, their sensitivity to inhibition by clorgyline increased, while in homogenates of aorta sensitivity to clorgyline decreased. CRAO was investigated further with benzylamine as substrate. Kinetic studies gave similar K_m values for both heart and aorta (4–6 μM at pH 7.8), and these values were not altered by flushing the assay tubes with oxygen. However, flushing with nitrogen caused uncompetitive inhibition in the heart and noncompetitive inhibition in aorta. These results suggest a difference in the catalytic mechanism between CRAO of heart and aorta. In both heart and aorta, CRAO was inhibited by semicarbazide, (+)-amphetamine, phenelzine and (+)- and (?)-mexiletine, with the (+)-form being more potent. Straight-chain diamine and polyamine compounds failed to inhibit in concentrations up to 10^?4 M. Thus, CRAO is not a typical diamine or polyamine oxidase. The results show differences between heart and aortic CRAO and MAO-A, and the possibility exists for heterogeneity within each of these two distinct forms of amine oxidase. Additionally, drugs known to inhibit MAO-(+)-amphetamine, phenelzine and mexiletine also inhibit CRAO. However, the biological significance of since the physiological role of CRAO is unknown.     

Inhibition of monoamine oxidase activity by cannabinoids

Brain monoamines are involved in many of the same processes affected by neuropsychiatric disorders and psychotropic drugs, including cannabinoids. This study investigated in vitro effects of cannabinoids on the activity of monoamine oxidase (MAO), the enzyme responsible for metabolism of monoamine neurotransmitters and affecting brain development and function. The effects of the phytocannabinoid Δ⁹-tetrahydrocannabinol (THC), the endocannabinoid anandamide (N-arachidonoylethanolamide [AEA]), and the synthetic cannabinoid receptor agonist WIN 55,212-2 (WIN) on the activity of MAO were measured in a crude mitochondrial fraction isolated from pig brain cortex. Monoamine oxidase activity was inhibited by the cannabinoids; however, higher half maximal inhibitory concentrations (IC₅₀) of cannabinoids were required compared to the known MAO inhibitor iproniazid. The IC₅₀ was 24.7 μmol/l for THC, 751 μmol/l for AEA, and 17.9 μmol/l for WIN when serotonin was used as substrate (MAO-A), and 22.6 μmol/l for THC, 1,668 μmol/l for AEA, and 21.2 μmol/l for WIN when phenylethylamine was used as substrate (MAO-B). The inhibition of MAOs by THC was noncompetitive. N-Arachidonoylethanolamide was a competitive inhibitor of MAO-A and a noncompetitive inhibitor of MAO-B. WIN was a noncompetitive inhibitor of MAO-A and an uncompetitive inhibitor of MAO-B. Monoamine oxidase activity is affected by cannabinoids at relatively high drug concentrations, and this effect is inhibitory. Decrease of MAO activity may play a role in some effects of cannabinoids on serotonergic, noradrenergic, and dopaminergic neurotransmission.     

Inhibition of monoamine oxidase A-form and semicarbazide-sensitive amine oxidase by selective and reversible monoamine oxidase-A inhibitors, amiflamine and FLA 788(+)

In vitro studies demonstrated that two selective monoamine oxidase (MAO)-A inhibitors, amiflamine and FLA 788(+), have been shown to inhibit semicarbazide-sensitive amine oxidase (SSAO) in rat testis and lung homogenates in a concentration-dependent way. The inhibition was not greatly influenced by pretreatment of the preparations with either clorgyline (10(-3) mol/l), l-deprenyl (10(-3) mol/l) or SKF 525A (10(-4) mol/l). The two compounds showed a time-dependent inhibition of SSAO, and for the initial phase of the inhibition, amiflamine is a competitive inhibitor with a Kislope of 135 mumol/l, but FLA 788(+) is a noncompetitive inhibitor with a Ki value of 180 mumol/l. After preincubation for 60 min at 37 degrees C, however, inhibition by amiflamine was found to be essentially irreversible whereas that produced by FLA 788(+) was still noncompetitive and reversible. These two compounds also reversibly and competitively inhibited rat testis MAO-A with FLA 788(+) being much more selective towards this MAO (Kislope = 0.26 mumol/l for FLA 788(+) and 7 mumol/l for amiflamine, respectively). The present results indicate that both MAO-A-selective inhibitors also inhibit SSAO in vitro, but their properties as SSAO inhibitors differ from those as MAO-A inhibitors.     

A benzylamine oxidase distinct from monoamine oxidase B—Widespread distribution in man and rat

Benzylamine oxidase (BzAO) and monoamine oxidase type B(MAO-B) both selectively catalyse the oxidative deamination of benzylamine (Bz). We define the former as that benzylamine-metabolizing activity insensitive to 4 × 10^?4 M deprenyl, a concentration which totally inhibits all forms of MAO. Although both enzymes are widespread in human and rat tissues, their organ distribution differs. Liver and brain show highest MAO-B activity, whilst BzAO activity predominates in aorta and lung. Relatively low BzAO and no MAO-B activity is present in plasma. In the rat, phenylethylamine (PEA) and dopamine (DA) are both substrates for a deprenyl-resistant enzyme with a distribution similar to BzAO, but in man these amines are solely oxidized by MAO. At pH 7.2 the K_m of BzAO for benzylamine is 2.2 × ^?4 M in the rat; μn man, it is 1.1 × 10^?4M. The K_m of MAO-B for benzylamine is 1.0 × 10^?4M in the rat and 5 × 10^?5 in man. Semicarbazide, procarbazine and carbidopa are potent inhibitors of BzAO and inhibit it selectively, leaving MAO substantially unaffected.     

Role of benzylamine oxidase in the cytotoxicity of allylamine toward aortic smooth muscle cells

In this study we demonstrate that by inhibiting benzylamine oxidase (BzAO) with either semicarbazide or phenelzine, aortic smooth muscle cells (ASMCs) are protected from cytolethal injury by the cardiovascular toxin allylamine. We find that although both semicarbazide and phenelzine inhibit BzAO or ASMCs grown in vitro, phenelzine is the more effective inhibitor. We further demonstrate that although semicarbazide--at concentrations inhibiting BzAO--protects ASMCs from cytolethal concentrations of allylamine, it does not fully protect ASMCs from sublethal injury as assessed by [3H]uridine uptake. In contrast, phenelzine appears to afford complete protection of ASMCs from allylamine injury. Although semicarbazide and phenelzine pretreatment does not interfere with [14C]allylamine uptake by ASMCs, retention time of the 14C-moiety from radiolabeled allylamine is less in pretreated ASMCs. Subcellular distribution studies of ASMCs exposed to [14C]allylamine demonstrate that inhibiting BzAO activity in ASMCs results in marked derangement of the distribution pattern of 14C-moiety in subcellular fractions of ASMCs, with 14C-moiety not localized to mitochondrial/endoplasmic reticulum enriched fractions.     

Inhibition of monoamine oxidase by N-phenacyl-cyclopropylamine

N-phenacyl-cyclopropylamine hydrobromide (54761) was evaluated in vitro and in vivo as a monoamine oxidase (MAO) inhibitor in rats. In contrast to 51641, which has an o-chlorophenoxy group in place of the phenacyl group and which is a highly selective inhibitor of type A MAO, 54761 showed a slight preference as a type B MAO inhibitor, since it inhibited phenylethylamine oxidation at slightly lower concentrations than were required to inhibit serotonin oxidation in vitro by rat liver MAO. Twelve analogs of 54761 with various substituents on the phenyl ring were also studied, but none was substantially more selective than 54761 as a type B inhibitor and most were preferential type A inhibitors. When 51641 and 54761 were injected into rats and MAO activity was assayed in tissue homogenates, the oxidation of serotonin in brain, heart and liver was inhibited more by 51641 than by 54761. In contrast, the oxidation of phenylethylamine was inhibited more by 54761 than by 51641 in brain and liver. In heart, however, 51641 was a more effective inhibitor of phenylethylamine oxidation than was 54761, supporting earlier evidence that phenylethylamine is destroyed in heart mainly by type A MAO. The oxidation of exogenous [¹⁴C]phenylethylamine was inhibited in vivo more effectively by 54761, whereas the oxidation of endogenous serotonin in brain was inhibited more by 51641. Although 54761 is not as selective an inhibitor of type B MAO as some other compounds such as deprenyl, it illustrates that a large range of selectivity in MAO inhibition can exist within the N-cyclopropylamine series. Further, selective type B inhibition could be achieved in vivo 24 hr after injection of 54761 by co-administration of harmaline. Harmaline selectively protected against the inactivation of type A MAO by 54761 but permitted the inactivation of type B MAO to occur.     

Cytochrome c oxidase activity in tissues of rats exposed to polyurethane pyrolysis fumes.

Adult male rats were exposed by inhalation to thermal degradation products from a rigid polyurethane foam degraded at 500°C. All rats survived a 5-min exposure. Cytochrome c oxidase activity of heart and brain was noncompetitively inhibited as a result of exposure. The degree of inhibition was correlated with the concentration of blood cyanide within the concentration range of 0.1–1.1 μg/ml. The mean blood cyanide concentration corresponding to 50% inhibition of the brain and heart enzymes was 0.28 μg/ml. Liver cytochrome c oxidase activity showed a mild noncompetitive activation after a 5-min exposure. After a lethal exposure of approximately 8 min duration, liver cytochrome c oxidase activity was moderately inhibited in some but not in all animals.     

Effect of phenelzine on the metabolism and membranal transport of glucose in brain

A colorimetric method for the estimation of phenelzine (β-phenylethylhydrazine) based on its reaction with p-dimethylaminobenzaldehyde is presented. Using this method it was shown that in vivo phenelzine rapidly penetrated all regions of the brain. In vitro with glucose as substrate the drug inhibited the respiration of tissue slices from all regions of the brain; with pyruvate as substrate the inhibition was much weaker and slower to develop. It was concluded that the inhibition of respiration was due to the drug's reducing the permeability of the cell membrane to glucose, since the respiration of brain homogenates was not inhibited and that of slices was more resistant to the action of the drug when the glucose concentration in the medium was increased. It was also shown that phenelzine did not inhibit any of the enzymes of glycolysis nor did it react to any significant extent with any of the glycolytic intermediates.     

Inhibition by clorgyline and deprenyl of the different forms of monoamine oxidase in rat liver mitochondria

The type of inhibition of the different forms of monoamine oxidase by clorgyline (preferentially ‘A’ form-inhibiting) and deprenyl (preferentially ‘B’ form-inhibiting) has been investigated. For both inhibitors a reversible phase of inhibition was found to precede the irreversible reaction. When incubated at 25° for 20 min, clorgyline inhibited the ‘A’ form in an irreversible fashion, while 4 hr at 37° was needed to inhibit the ‘B’ form irreversibly. In contrast, deprenyl inhibited the ‘A’ form reversibly even when incubated at 37° for 4 hr, whereas the ‘B’ form was irreversibly inhibited under these conditions. The selectivity of both inhibitors was considerably lower with longer incubation times. The implications of the results for the interpretation of previous findings on multiple forms of monoamine oxidase (an ‘A’ and a ‘B’ form) and for chronic treatment with the inhibitors in vivo is discussed.     

The isolated perfused rat brain preparation in the study of monoamine oxidase and benzylamine oxidase: Lack of selective brain perfusion

Monoamine oxidase (MAO) is present in brain blood vessels, and a different amine oxidase, benzylamine oxidase (BzAO), is claimed to exist in porcine cerebral vessels. The object of the present investigation was to evaluate these deaminating activities in the structurally intact brain, utilizing the isolated perfused rat brain preparation (IPRB). [¹⁴C]Benzylamine (10 μM), a substrate for both BzAO and MAO, was perfused via the internal carotid arteries for 5 min, and deaminated metabolites were measured. BzAO and MAO activities were distinguished by the use of the selective inhibitors semi-carbazide and pargyline. Both BzAO and MAO deaminated benzylamine (10 μm), but over 90 per cent of the total deaminated products resulted from BzAO. This was due to the much lower K_m, value of benzylamine for BzAO (2.8μM) than for MAO (169 μM). In vitro assays, however, revealed that the brain contained no measurable BzAO activity, whereas all other head structures (skull, mandible, skin, skeletal muscle, eyes, periocular tissues and tongue) contained BzAO activity. MAO was present both within and outside the brain. These results suggest that the IPRB preparation is not specific for brain perfusion. Experiments with technetium-labeled microspheres showed that, although the brain is preferentially perfused on a per gram basis, 52 per cent of the total perfusate passed through the skull and 13 per cent through extracranial structures. Only 35 per cent was specific for the brain. Other experiments showed that perfusion of the intact rat head produced greater deamination of benzylamine than when the skin and 70 per cent of the muscle were removed. Additionally, perfusion via the pterygopalatine arteries, to bypass the brain, resulted in increased deamination. It is concluded that the IPRB preparation is not specific for brain perfusion and that BzAO activity is present in all head structures other than the brain. The presence of BzAO in bone, skin, and muscle is consistent with suggestions for a physiological function of the enzyme in connective tissue.     

Inhibition of hepatic gluconeogenesis by phenethylhydrazine (phenelzine).

The hepatic effects of phenethylhydrazine (phenelzine), an antidepressive drug occasionally causing hypoglycemia, were examined. Phenelzine (1 mM) inhibits gluconeogenesis from l-lactate, pyruvate and propionate, but not from fructose in experiments with isolated perfused rat livers. Ketogenesis from endogenous sources as well as from hexanoate or fructose is likewise inhibited. Gluconeogenesis, urea formation, and net formation of l-lactate + pyruvate from l-alanine are inhibited in experiments with isolated hepatocytes. Phenelzine leads to a reduction of the cytoplasmic, but not of the mitochondrial NAD⁺/NADH system.Cross-over plots of intrahepatic metabolites revealed forward cross-overs between l-malate and phosphoenolpyruvate (PEP) and fructose-1,6-bisphosphate (FDP) and fructose-6-phosphate (F-6-P) in the presence of phenelzine, when l-lactate was glucogenic precursor. With pyruvate as substrate forward crossover points were between pyruvate and l-malate and between FDP and F-6-P. The concentrations of l-glutamate, l-aspartate, and α-oxoglutarate changed in the presence of phenelzine in a way compatible with an inhibition of the aspartate aminotransferase reaction. The overall concentrations of acetyl-CoA decreased in the presence of phenelzine, when pyruvate was substrate.PEP-car?ykinase was inhibited in vitro by phenelzine, due to trapping of oxaloacetate by phenethyl-hydrazone formation. The same mechanism was found for aspartate aminotransferase when tested in the direction of l-aspartate formation. In the direction of oxaloacetate formation a competitive inhibition was observed (K_iapp= 7.2 · 10^?4M), probably due to an interaction of phenelzine with the enzyme linked pyridoxal-5'-phosphate (PLP) as indicated by aldimine formation of phenelzine with PLP in vitro. Phenelzine (1mM) inhibited significantly the incorporation of carbon from the C-1 and from the C-2 position of the lactate-pyruvate pool into CO₂, glucose, and (C-2 only) fatty acids, whereas the incorporation into the glyceride-glycerol fraction increased. The incorporation of hydrogen from ³H₂O into total lipids and glyceride-glycerol was strongly, that into fatty acids completely, inhibited under the same conditions. Phenelzine did not inhibit acetoacetate reduction by isolated rat liver mitochondria with succinate. citrate or isocitrate as substrate, but it inhibited strongly when pyruvate, and slightly when l-malate, was the substrate.The ability of phenelzine to form hydrazones with 2-keto acids increased in the sequence α-oxoglutarate < pyruvate < oxaloacetate. No inhibition of aspartate aminotransferase was observed in the presence of 2-phenethylhydrazonopentanoate or 2-phenethylhydrazonopropionate. Gluconeogenesis from l-lactate, but not from pyruvate, was inhibited by 0.1 mM 2-phenethylhydrazonopropionate.It is concluded that phenelzine, if at all, affects gluconeogenesis only partly via its hydrazone derivatives. It acts mainly by restricting oxalacetate formation in the cytosol due to an inhibition of aspartate aminotransferase. In addition, phenelzine inhibits pyruvate oxidation. This effect is mainly responsible for the observed inhibition of fatty acid synthesis from carbohydrates. The mechanism of action precludes the use of this or similar drugs in the treatment of diabetes.     

Tyramine infusions and selective monoamine oxidase inhibitor treatment

The relationship between changes in IV tyramine pressor sensitivity accompanying selective monoamine oxidase (MAO) inhibitor treatment and estimates of MAO-A and MAO-B inhibition in vivo were studied. Reductions in platelet MAO activity provided an index of MAO-B inhibition, while changes in plasma 3-methoxy-4-hydroxyphenethylene glycol (MHPG) were used as an hypothesized reflection of MAO-A inhibition. Chronic treatment with the MAO-A inhibitor clorgyline and the MAO-B inhibitor pargyline showed significant inhibition of the alternate MAO enzyme as well, although this crossover effect was greater for pargyline than clorgyline. The MAO-B inhibitor deprenyl appeared to maintain the greatest degree of MAO inhibition selectivity in vivo. Tyramine pressor sensitivity changes accompanying administration of the MAO inhibitors were highly correlated with decreases in plasma MHPG (r=0.92), supporting our previous data indicating the rank order of clorgyline > pargyline > deprenyl for enhancement of tyramine pressor sensitivity and, thus, suggesting that tyramin potentiation is primarily a function of MAO-A rather than MAO-B inhibition. Changes in plasma MHPG are suggested to provide a potentially useful clinical index of in vivo MAO-A inhibition.Presently with the Biological Psychiatry Branch, NIMH     

The effect of monoamine oxidase inhibitors on ‘first-pass’ metabolism of tyramine in dog intestine

The role of the intestine in metabolic inactivation of tyramine (TYR) has been studied in an isolated intestinal loop preparation in anaesthetized dogs. In control animals there was extensive metabolism of tyramine on passage through the intestinal wall and p-hydroxyphenylacetic acid (p-OHPA) was the only metabolite found in venous plasma from the loop (mean ratio p-OHPA/TYR = 5). Oral or intravenous pretreatment with the monoamine oxidase (MAO) inhibitors tranylcypromine or MD780515 significantly lowered the ratio of p-OHPA/TYR (range = 0.2–1.1) measured 3hr after the last dose. Twenty-four hours after the last dose of MAO inhibitor p-OHPA/TYR ratios in dogs pretreated orally with MD780515 had returned to control levels while ratios in dogs pretreated orally with tranylcypromine remained low (mean = 1.6). In vitro rates of deamination of the substrates 5-hydroxytryptamine (selective for the A form of MAO) and β-phenylethylamine (selective for the B form of MAO) in homogenates of intestine paralleled the in vivo findings in most cases. Tranylcypromine produced a nonselective irreversible inhibition of both MAO-A and MAO-B whereas MD780515 was found to be a selective inhibitor of MAO-A and also appeared to be reversible.     

The effect of lipophilic compounds upon the activity of rat liver mitochondrial monoamine oxidase-A and -B

The effect of various lipophilic compounds on the activity of monoamine oxidase (MAO) was determined. The local anaesthetics procaine, procainamide, tetracaine and lignocaine were all MAO-A selective inhibitors, whereas benzyl alcohol, butan-l-ol, hexan-l-ol and octan-l-ol inhibited MAO-B selectively. Procaine was found to be a competitive inhibitor of the deamination of 5-hydroxy-tryptamine (5-HT), tyramine, β-phenethylamine and benzylamine. Benzyl alcohol was competitive towards β-phenethylamine and benzylamine, but a mixed-type inhibitor towards 5-HT and tyramine. The same patterns of inhibition for both drugs were found when the activity was assayed under atmospheres of either oxygen or air. The inhibition produced by both compounds was fully reversible. Triton X-100 appeared to inhibit the activity of MAO-A selectively when preincubated with the enzyme for 30 min at 30°. This selectivity was lost when the preincubation temperature was raised to 37°. The inhibition of MAO activity by Triton X-100 after preincubation at 37° was found to be irreversible. Sodium deoxycholate and SDS were also found to inhibit the activity of MAO after preincubation with the enzyme at 37°. The significance of these results is discussed.     

Substrate and inhibitor selectivity of human heart monoamine oxidase.

The subcellular distribution, inhibitor sensitivity, thermostability and pH profiles of monoamine oxidase (MAO) from samples of human heart obtained at post mortem have been investigated with several substrates. A simple subcellular fractionation showed that, with either tyramine or benzylamine as substrate, about 50 per cent of the MAO activity was found in the mitochondrial fraction, with negligible quantities in the high speed supernatant. From the use of clorgyline, it appears that 5-HT is a substrate for MAO-A, benzylamine and β-phenethylamine are substrates for MAO-B, while tyramine and dopamine are substrates for both forms of the enzyme, d-Amphetamine was shown to be a selective competitive inhibitor of MAO-A, of similar potency to that observed with MAO from rat liver. No significant difference between the thermostability at 50° of the MAO activity towards 5-HT and benzylamine was observed. Preliminary results for the effect of pH on human heart MAO are presented. The results are discussed with respect to similar data obtained for MAO from other human and animal tissues.     

In vitro effects of acetylcholinesterase reactivators on monoamine oxidase activity

Administration of acetylcholinesterase (AChE) reactivators (oximes) is usually used in order to counteract the poisoning effects of nerve agents. The possibility was suggested that oximes may show some therapeutic and/or adverse effects through their action in central nervous system. There are no sufficient data about interaction of oximes with monoaminergic neurotransmitter's systems in the brain. Oxime-type AChE reactivators pralidoxime, obidoxime, trimedoxime, methoxime and HI-6 were tested for their potential to affect the activity of monoamine oxidase of type A (MAO-A) and type B (MAO-B) in crude mitochondrial fraction of pig brains. The compounds were found to inhibit fully MAO-A with half maximal inhibitory concentration (IC₅₀) of 0.375 mmol/l (pralidoxime), 1.53 mmol/l (HI-6), 2.31 mmol/l (methoxime), 2.42 mmol/l (obidoxime) and 4.98 mmol/l (trimedoxime). Activity of MAO-B was fully inhibited by HI-6 and pralidoxime only with IC₅₀ 4.81 mmol/l and 11.01 mmol/l, respectively. Methoxime, obidoxime and trimedoxime displayed non-monotonic concentration dependent effect on MAO-B activity. Because oximes concentrations effective for MAO inhibition could not be achieved in vivo at the cerebral level, we suppose that oximes investigated do not interfere with brain MAO at therapeutically relevant concentrations.     

1-methyl substituent and stereochemical effects of 2-phenylcyclopropylamines on the inhibition of rat brain mitochondrial monoamine oxidase A and B

(E)-2-Phenylcyclopropylamine ((E)-TCP), (Z)-2-phenylcyclopropylamine ((Z)-TCP), (E)-1-methyl-2-phenylcyclopropylamine ((E)-MTCP), and (Z)-1-methyl-2-phenylcyclopropylamine ((Z)-MTCP) were synthesized and used to determine to what extent 1-methyl substitution and stereochemistry of 2-phenylcyclopropylamines affect inhibition of monoamine oxidase (MAO). Inhibition of rat brain mitochondrial MAO-A and B by the compounds were measured using serotonin and benzylamine as the substrate, respectively and IC₅₀ values obtained with 95% confidence limits by the method of computation. For the inhibition of MAO-A, (E)-MTCP (IC₅₀=6.2×10^?8M) was found to be 37 times more potent than (Z)-MTCP (IC₅₀=2.3×10^?6 M), whereas the activity of (E)-TCP (IC₅₀=2.9×10^?7 M) was slightly less than that of (Z)-TCP (IC₅₀=2.3×10^?7 M). Similarly, for the inhibition of MAO-B, (E)-MTCP (IC₅₀=6.3×10^?8 M) was 7 times more potent than (Z)-MTCP (IC₅₀=4.7×10^?7 M) and (E)-TCP (IC₅₀=7.8×10^?8 M), 0.6 times as potent as (Z)-TCP (IC₅₀=4.4×10^?8 M). The results suggested that while without 1-methyl group, potency of a (Z)-isomer was comparable to that of (E)-isomer, the methyl group in its (Z)-position was very unfavorable to the inhibition of MAO and that in its (E)-position, the methyl group contributed positively to the potency as found by the fact that (E)-MTCP was 1–5 times more potent than (E)-TCP. In view of the selective inhibition of MAO-A or B, all compounds elicited 4–10 times higher preference for the inhibition of MAO-B over MAO-A and 1-methyl substitution as well as the stereochemical factors did not significantly influence the selectivity.     

Monoamine oxidase type a: differences in selectivity towards l-norepinephrine compared to serotonin

l-Norepinephirine and serotonin have been regarded as preferential substrates for monoamine oxidase (MAO) type A. A close comparative examination of a number of tissues from different species, however, indicated the following differences. Serotonin was a more selective substrate for MAO-A, being inhibited by low concentrations (< 10^-7M) of the irreversible MAO-A inhibitor, clorgyline, more consistently and to a greater extent (80–100%) than was l-norepinephrine (30–85%). These serotonin-norepinephrine differences were greater in humans and other primates than in rodents. Serotonin also had a 2- to 4-fold smaller apparent K_m for MAO-A than l-norepinephrine and was deaminated 2- to 5-fold more readily by MAO in vitro in most tissues. In contrast, the MAO-B in human platelets deaminated l-norepinephrine more readily than serotonin. Thus, l-norepinephrine, like dopamine, should be regarded as a substrate for both MAO-A and MAO-B in vitro. The prominent role of MAO-B in norepinephrine degradation in primates may need to be considered in interpreting laboratory and clinical studies of clorgyline selective MAO-inhibiting drugs.     

Human kidney diamine oxidase: inhibition studies

The oxidation of p-dimethylaminomethylbenzylamine by purified human kidney diamine oxidase was studied in the presence of substrate analogues such as dimethylsulphonium, trimethylammonium, isothiouronium and guaniclinium compounds of various chain lengths. The inhibition by mono- and bis-onium compounds is described. K_i values and ΔG° values are given and the inhibition due to monoamine oxidase inhibitors and certain time-dependent inhibitors is reported. It is concluded that purified diamine oxidase from human kidney resembles the diamine oxidases from hog kidney and human placenta in its inhibitor specificity. In particular, it is inhibited strongly by substrate analogues in the order isothiouronium ≈ guaniclinium > dimethylsulphonium > trimethylammonium suggesting a negatively charged substrate binding site. Also, it is weakly inhibited by some monoamine oxidase inhibitors and strongly inhibited in a time-dependent fashion by carbonyl-group reagents.