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Two Alzheimer's patients with disabling depression failed to respond to standard antidepressants but improved with a monoamine oxidase inhibitor. The author points out that the cholinergic system is involved in Alzheimer's symptoms and that demented patients have high monoamine oxidase levels.
Nitrobenzoid, nitroheterocyclic and cyanobenzoid compounds inhibit type B monoamine oxidase. A partially purified enzyme preparation from rabbit liver mitochondria, oxidizing rho-dimethyl-aminobenzylamine as the substrate, was competitively inhibited by nitrobenzoid compounds with K1 values in the range of 0.28 muM for rho-dinitrobenzene to 0.56 muM for rho-nitrobenzoic acid. The potencies of nitrobenzoid compounds were positively correlated with the Hammett sigma value for each substituent on nitrobenzene. Dinitro derivatives were slightly more potent than the corresponding mononitro compounds but not as potent as would be expected from their sigma values. For the nitroheterocyclic compounds, inhibition was also competitive; the lowest K1 was 1.3 muM for 5-nitrofurfural semicarbazone (nitrofurazone). Compounds with cyano groups in place of nitro groups were also inhibitory; the most potent was rho-acetobenzonitrile with a K1 of 1.3 muM. The results of this study indicate that, in addition to nitrobenzoid compounds, other compounds with planar, electron-deficient nuclei are effective inhibitors of type B monoamine oxidase. Although hydrophobic and steric parameters may play some role in inhibition, the predominant factor is the electron-withdrawing power of the ring substituents.
Monoamine oxidase activity was measured in isolated rat liver mitochondria using the radiochemical assay with [14C]tyramine as substrate. With toluene as the extracting solvent the apparent activity in the resting state (State 4) was much higher than in the active state (State 3) in agreement with Smith and Reid (Smith, G.S. and Reid, R.A. (1978) Biochem. J. 176, 1011-1014). However, with ethyl acetate or diethyl ether as extracting solvents, the activity in both states was almost identical and several times higher than that measured with toluene. p-Hydroxyphenylacetaldehyde, p-hydroxyphenylacetalcohol and p-hydroxyphenylacetic acid were identified as final reaction products, the latter one being hardly extractable with toluene. It is concluded that monoamine oxidase activity is not influenced by the respiratory state of mitochondria and that differences found by Smith and Reid are due to different extractability of secondary reaction products. NADPH-dependent aldehyde reductase was tentatively identified in rat liver mitochondria, its specific activity amounting to about one fourth of that in the cytosol.
Monoamine oxidase (MAO) is an enzyme that oxidizes various physiologically and pathologically important monoamine neurotransmitters and hormones such as dopamine, noradrenaline, adrenaline, and serotonin. Two types of MAO, i.e. type A (MAO-A) and type B (MAO-B), were first discovered pharmacologically. MAO-A is inhibited by clorgyline; and MAO-B, by deprenyl. cDNAs MAO-A and MAO-B were cloned and their structures determined. MAO-A and MAO-B are made of similar but different polypeptides and encoded by different nuclear genes located on the X chromosome (Xp11.23). MAO-A and MAO-B genes consist of 15 exons with identical intron-exon organization, suggesting that they were derived from a common ancestral gene. Both enzymes require a flavin cofactor, flavin adenine dinucleotide (FAD), which binds to the cysteine residue of a pentapeptide sequence (Ser-Gly-Gly-Cys-Tyr). Both enzymes exist on the outer membrane of mitochondria of various types of cells in various tissues including the brain. In humans, MAO-A is abundant in the brain and liver, whereas the liver, lungs and intestine are rich in MAO-B. MAO-A oxidizes noradrenaline and serotonin; and MAO-B, mainly beta-phenylethylamine. In the human brain, MAO-A exists in catecholaminergic neurons, but MAO-B is found in serotonergic neurons and glial cells. MAO-A knockout mice exhibit increased serotonin levels and aggressive behavior, whereas MAO-B knockout mice show little behavioral change. The gene knockout mice of MAO-A or MAO-B, together with the observation that some humans lack MAO-A, MAO-B, or both have contributed to our understanding of the function of MAO-A and MAO-B in health and disease. MAO-A and MAO-B may be closely related to various neuropsychiatric disorders such as depression and Parkinson's disease, and inhibitors of them are the subject of drug development for such diseases.
Skin fibroblast cultures were prepared from 21 men, and found to contain types A and B activity of monoamine oxidase, with a possible slight predominance of type A, as evaluated by substrate preferences and differential inhibition by clorgyline and deprenyl. Three women had similar activities. There was a close correlation of activities with different substrates, but there was no quantitative correlation between fibroblast and blood platelet enzyme (type B) activities. The fibroblasts also contained catechol-O-methyltransferase activity exceeding, but poorly correlated with, that in erythrocytes. Fibroblasts may be advantageous in studies of monoamine oxidase in man by providing both types of enzyme as found, for example, in the central nervous system, and by providing a means of removing many in vivo chemical influences from the cells in culture. Nevertheless, great caution must be exercised in generalizing results of this "model" to other tissues, since activities of both enzymes correlated poorly with those in blood cells of the same individuals.
The evidence that there are two forms of monoamine oxidase (MAO) enzymes is reviewed. On the basis of various substrate affinities, differential inhibition by deprenyl and clorgyline, and heat inactivation, there appear to be a type A and a type B MAO in various tissues. The enzymatic oxidation of telemethylhistamine (tMH) is an example of an endogenous substrate for type B MAO.
The ability of liver lipid exchange proteins to introduce foreign phospholipids into intact mitochondria was used for a study of the lipid dependence of monoamine oxidase. Introduction of exogenous phosphatidylcholine into rat hepatoma mitochondria, in which both the monoamine oxidase activity and the phosphatidylcholine content are comparatively low, leads to considerable activation of the enzyme. The introduction of exogeneous phosphatidylserine, phosphatidylethanolamine and cardiolipin has no activating effect. This indicates that the decreased activity of monoamine oxidase in the hepatoma may be due to a low amount of phosphatidylcholine. The method described allows the study in situ of the lipid dependence of non-solubilized membrane-bound enzymes.
Experimental and theoretical physico-chemical methods were used to investigate the interaction between several reversible monoamine oxidase A inhibitors in the oxazolidinone series and the active site of the enzyme. Phenyloxazolidinones include toloxatone and analogues, among which befloxatone was selected as drug candidate for the treatment of depression. Identification of the forces responsible for the crystal cohesion of befloxatone reveals functional groups that could interact with monoamine oxidase. Calculation of electronic properties of those compounds using ab initio molecular orbital methods lead to a description of the mode of interaction between befloxatone and the cofactor of the enzyme. Electronic absorption spectroscopy measurements confirm the hypothesis of a privileged interaction of phenyloxazolidinone-type inhibitors with the flavin cofactor of MAO. Additional sites of interaction with the protein core of MAO A are also examined with regard to the primary structure of the enzyme. As a result of this work, a model is proposed for the reversible inhibition of MAO A by befloxatone via long distance, reversible interactions with the flavin adenine dinucleotide (FAD) cofactor of the enzyme and with specific amino acids of the active site. This model is partially corroborated by experimental evidence and should be helpful in designing new potent inhibitors of monoamine oxidase.
Monoamine oxidase (MAO) A and B catalyze the oxidative deamination of neuroactive and dietary monoamines such as serotonin, tyramine, and phenylethylamine. Here we show that MAO B, but not MAO A, gene expression was induced during Caco-2 cell differentiation; thus this cell line was used as a model system to study the gene regulation unique for MAO B. Luciferase and gel shift assays showed that transcription factors Sp1 and Sp3 binding to -246 and -99 bp were responsible for the observed gene activation. Overexpression of Sp3 inhibited the induction of MAO B gene by Sp1, and the expression of Sp3 was decreased during Caco-2 cell differentiation. Computer analysis revealed a putative CpG island containing 22 potential CpG methylation sites between -261 and -58 bp. In vitro methylation of MAO B promoter with 5-aza-2'-deoxycytidine, a DNA methyltransferase inhibitor, up-regulated MAO B gene expression in both HeLa and Caco-2 cells. Sodium bisulfite sequencing showed a gradually reduced methylation of the CpG sites during Caco-2 cell differentiation. These results suggested that MAO B gene expression is selectively induced by a decreased Sp3/Sp1 ratio and reduced DNA methylation. This new information may provide insights on the tissue-specific expression of these two isoenzymes.
The specific activity of human platelet monoamine oxidase from control subjects undergoing glucose tolerance tests is reduced drastically. Three hours after intake of 100 g of glucose only 25%-30% of the MAO-baseline activity was measured with tryptamine. beta-phenylethylamine and p-tyramine as substrates. At about 5 hr, platelet MAO activity has increased again. Inhibition was not due to small molecular weight inhibitors or other diffusible factors. Studies of other platelet enzymes, including succinate dehydrogenase and isocitrate dehydrogenase (NADP+ dependent) showed no parallel reductions; hGH, insulin, blood glucose and platelet glycogen concentrations did not correlate with platelet MAO activity. The changes of MAO activity in respect with p-tyramine and tryptamine as substrates 24 hr after glucose ingestion suggest changes of the lipid microenvironment of this enzyme of the outer mitochondrial membrane.
Monoamine oxidase (MAO) activity and its forms were measured in normal human thyroid glands (39 samples from necropsy and four from perinodular tissues) and in pathological thyroid glands (13 multinodular goitres, three carcinomas, seven chronic thyroiditis, five Graves' disease and seven follicular, seven foetal and five embryonal adenomas). MAO activity in carcinomas was lower than in normal perinodular thyroid control tissue; in Graves' disease, foetal and embryonal adenomas it was significantly higher than in the normal control. The use of clorgyline (a selective MAO A inhibitor) and deprenyl (selective MAO B inhibitor) allowed estimation of the relative proportion of MAO forms. A high proportion of MAO A (more than 90%) was observed, both in normal and in pathological thyroid tissues. Changes in total thyroid MAO activity, but not in its forms, were found in different pathologies of the thyroid gland.
The present study investigated the effect of monoamine oxidase inhibitors, nialamide which is a non specific monoamine oxidase inhibitor (MAOI), toloxatone which is a A type MAOI and L-deprenyl which is a B type MAOI compared with classical tricyclic antidepressants (clomipramine, desipramine and imipramine), on the escape deficit induced by inescapable shocks (learned helplessness paradigm). Rats were first exposed to inescapable shock pretreatment (60 shocks, 15 sec duration, 0.8 mA, every minute + 15 sec) and 48 h later, shuttle box training (30 trials/day, 15 min.) was initiated in order to evaluate interference effect. Rats with inescapable shocks exhibited escape and avoidance deficits when tested for subsequent responding in a shuttle-box. Daily i.p. injections of nialamide (8 and 16 mg/kg), toloxatone (16 and 32 mg/kg), L-deprenyl (32 and 64 mg/kg) and tricyclic antidepressants (clomipramine: 16 and 32 mg/kg, desipramine: 16 and 24 mg/kg, imipramine: 16 and 32 mg/kg) eliminated escape deficits. In rat exposed to inescapable shocks and treated with L-deprenyl (16 mg/kg/day), nialamide (32 mg/kg/day) or toloxatone (64 mg/kg/day), avoidance responses are significantly increased as compared with non drugged rats preexposed to inescapable shocks. These data extend previous results bearing on the similarity of action of MAOI and tricyclic antidepressants in learned helplessness paradigm (Sherman et al., 1982) and are in agreement with data obtained in other animal models of depression.
The effect of 1,4-benzoquinone (BQ) on type A and B monoamine oxidase (MAO-A and -B) in human brain synaptosomes was examined. MAO-A was found to be markedly inhibited by BQ competitively with the substrate, kynuramine, while MAO-B was less sensitive to this inhibitor and the inhibition was non-competitive with the substrate. The Ki value of MAO-A (9.62 +/- 0.35 microM) was much smaller than the Km value of the enzyme with the substrate (56.3 +/- 3.5 microM) or the Ki value of MAO-B with BQ (20.3 +/- 0.9 microM). The inhibition of MAO-A by BQ was also confirmed by the use of platelet mitochondria and clonal rat pheochromocytoma PC12h cells as enzyme sources. The inhibition of MAO activity by BQ proved to be reversible: the inhibited enzyme activity could be recovered by column chromatography on Sephadex G-25.
The endogenous monoamine oxidase (MAO) inhibitory activity, termed tribulin, contains several components. We have previously identified one of them, isatin, which is a selective inhibitor of MAO B. In the present study we have purified several further components of human urinary tribulin which act as selective inhibitors of MAO A. They have been identified by gas chromatography-mass spectrometry (GC-MS) as ethyl indole-3-acetate (and/or methyl indole-3-propionate), methyl indole-3-acetate and ethyl 4-hydroxyphenylacetate. IC50 values for MAO A were found to be 44 microM (105 microM for methyl indole-3-propionate), 88 microM and 120 microM, respectively, whilst those for MAO B were each greater than 1 mM. The artificial formation of these esters was excluded by carrying the parent acids, from which they are presumably synthesized, through the purification procedure. As tribulin output is increased during stress or anxiety, these results point to a possible role for tryptamine and tyramine pathways in such disorders.
Monoamine oxidase B (MAO B) is an outer mitochondrial membrane enzyme that catalyzes the oxidation of arylalkylamine neurotransmitters. The crystal structures of MAO B in complex with four of the N-propargylaminoindan class of MAO covalent inhibitors (rasagiline, N-propargyl-1(S)-aminoindan, 6-hydroxy-N-propargyl-1(R)-aminoindan, and N-methyl-N-propargyl-1(R)-aminoindan) have been determined at a resolution of better than 2.1 A. Rasagiline, 6-hydroxy-N-propargyl-1(R)-aminoindan, and N-methyl-N-propargyl-1(R)-aminoindan adopt essentially the same conformation with the extended propargyl chain covalently bound to the flavin and the indan ring located in the rear of the substrate cavity. N-Propargyl-1(S)-aminoindan binds with the indan ring in a flipped conformation with respect to the other inhibitors, which causes a slight movement of the Tyr326 side chain. Four ordered water molecules are an integral part of the active site and establish H-bond interactions to the inhibitor atoms. These structural studies may guide future drug design to improve selectivity and efficacy by introducing appropriate substituents on the rasagiline molecular scaffold.
1. Rasagiline [N-propargyl-1R(+)-aminoindan], was examined for its monoamine oxidase (MAO) A and B inhibitor activities in rats together with its S(-)-enantiomer (TVP 1022) and the racemic compound (AGN-1135) and compared to selegiline (1-deprenyl). The tissues that were studied for MAO inhibition were the brain, liver and small intestine. 2. While rasagiline and AGN1135 are highly potent selective irreversible inhibitors of MAO in vitro and in vivo, the S(-) enantiomer is relatively inactive in the tissues examined. 3. The in vitro IC(50) values for inhibition of rat brain MAO activity by rasagiline are 4.43+/-0.92 nM (type B), and 412+/-123 nM (type A). The ED(50) values for ex vivo inhibition of MAO in the brain and liver by a single dose of rasagiline are 0.1+/-0.01, 0.042+/-0.0045 mg kg(-1) respectively for MAO-B, and 6.48+/-0.81, 2.38+/-0.35 mg kg(-1) respectively for MAO-A. 4. Selective MAO-B inhibition in the liver and brain was maintained on chronic (21 days) oral dosage with ED(50) values of 0.014+/-0.002 and 0.013+/-0.001 mg kg(-1) respectively. 5. The degree of selectivity of rasagiline for inhibition of MAO-B as opposed to MAO-A was similar to that of selegiline. Rasagiline was three to 15 times more potent than selegiline for inhibition of MAO-B in rat brain and liver in vivo on acute and chronic administration, but had similar potency in vitro. 6. These data together with lack of tyramine sympathomimetic potentiation by rasagiline, at selective MAO-B inhibitory dosage, indicate that this inhibitor like selegiline may be a useful agent in the treatment of Parkinson's disease in either symptomatic or L-DOPA adjunct therapy, but lack of amphetamine-like metabolites could present a therapeutic advantage for rasagiline.
Personality traits, as assessed by personality inventories, and platelet monoamine oxidase activities have been compared in the following two groups of normal subjects: (1) subjects who either have mountaineering as an active hobby or an interest in mountaineering and (2) subjects not interested in mountaineering. The group of subjects interested in mountaineering were found to be sensation seekers, as assessed by the Zuckerman Sensation Seeking Scale, and to have significantly lower platelet monoamine oxidase activities.
Previous studies have shown that a subpopulation of the catecholamine-degrading enzymes monoamine oxidase (MAO) A and B holds a previously unknown regulatory site, the I2-imidazoline binding site (I2BS). In the present work, we characterized the isoforms of monoamine oxidases expressed in the rabbit renal proximal tubule, defined their relationship with I2BS, and investigated the ability of I2BS ligands to inhibit enzyme activity in intact cells. Two findings indicate that MAO-B is the predominant isoform expressed in the renal proximal tubule cells: 1) Western blot performed with an anti-MAO-A/MAO-B polyclonal antiserum revealed a single 55-kDa band corresponding to MAO-B; 2) enzyme assays showed an elevated MAO-B activity ([14C]beta-phenylethylamine oxidation: Vmax = 1.31 +/- 0.41 nmol/min/mg protein), whereas MAO-A activity was only detectable ([14C]5-HT oxidation: Vmax = 80.3 +/- 19 pmol/min/mg protein). Photoaffinity labeling with the I2BS ligand [125I]2-(3-azido-4-iodophenoxy)-methylimidazoline revealed a single 55-kDa band, which indicates that MAO-B of the renal proximal tubule cells holds the I2 imidazoline binding site. [3H]Idazoxan binding studies and enzyme assays showed that, in intact cells, I2BS ligands bind to and inhibit MAO-B. Indeed, the increase in the accessibility of intracellular compartment by cell permeabilization did not enhance [3H]idazoxan binding, which indicates that, in intact cells, intracellular I2BS are fully occupied by imidazoline ligands. In addition, enzyme assays showed that incubation of proximal tubule cells with imidazoline ligands leads to a complete, dose-dependent inhibition of MAO activity. These data show the predominant expression of MAO-B in rabbit renal proximal tubule and its regulation by imidazoline ligands in intact cells.