p53 phosphorylation is involved in vascular cell death induced by the catalytic activity of membrane-bound SSAO/VAP-1

Semicarbazide sensitive amine oxidase (SSAO) is a multifunctional enzyme present mainly in adipocytes, endothelial and smooth muscle cells. It metabolizes primary aliphatic and aromatic amines generating products able to contribute to cellular oxidative stress. SSAO is expressed in a membrane-bound form and is also present as a soluble enzyme in plasma. Both isoforms are increased in several pathologies, and the catalytic products generated by the soluble enzymatic activity can induce cytotoxicity of vascular cells in culture. We have analyzed whether the transmembrane form of the enzyme is able to produce a cytotoxic effect through methylamine oxidation. Since cells in culture lose the expression of this enzyme, we used an SSAO stably transfected smooth muscle cell line. Herein we report that cell treatment with the substrate methylamine induced a dose and time dependent cytotoxic effect. The tumor suppressor protein p53 played an important role in the molecular pathway involved in this cell death. Moreover, we also observed the induction of PUMA-alpha expression with mitochondrial Bcl-2 family proteins being affected, and final effector caspases being activated.

CYP-Dependent Metabolism of PF9601N, A New Monoamine Oxidase-B Inhibitor, by C57BL / 6 Mouse and Human Liver Microsomes

Purpose. The selective monoamine oxidase-B (MAO-B) inhibitor, l-deprenyl, is still used for treating Parkinson's patients, however, a disadvantage of its use lies in the formation of l-amphetamine and l-methamphetamine. Subsequently, this has promoted the design of a novel, more potent, MAO-B inhibitor PF9601N, which also has neuroprotective and antioxidant properties. The aim of this work was to investigate the effect of treatment with PF9601N on its own phase I hepatic metabolism. Kinetic parameters of PF9601N CYP-dependent N-dealkylation reaction was also studied and compared with those of l-deprenyl. Methods. C57BL/6 mice were treated with PF9601N for 4 days. After CYP content and related monooxygenase activities were assayed in liver microsomes of control and treated animals. Results. CYP activities, cytochrome b5 content, NADPH-cytochrome P450 reductase and various monooxygenase activities were unaffected by in vivo PF9601N treatment. With microsomes from both control and treated mice, the PF9601N-dealkylation product, FA72, was the only detected metabolite with its formation rate following an hyperbolic, Michaelis-Menten curve. Among various inhibitors, only ketoconazole inhibited the FA72 formation rate, indicating a major involvement for CYP3A. Apparent Km and Vmax values generated by human liver microsomes were similar to those found with mouse microsomes. Ketoconazole inhibition indicates that CYP3A is one of the major enzymes involved in PF9601N metabolism also by human liver microsomes. In mouse liver microsomes, the intrinsic clearance of PF9601N was significantly lower than that of l-deprenyl suggestive of an improved bioavailability for the former. Conclusion. The observed favourable metabolic profile may suggest suitability of PF9601N for clinical use.

Sodium Bicarbonate Enhances Membrane-bound and Soluble Human Semicarbazide-sensitive Amine Oxidase Activity In Vitro

Semicarbazide-sensitive amine oxidase (SSAO) is a multifunctional enzyme with different biological roles that depend on the tissue where it is expressed. Because SSAO activity is altered in several pathological conditions, we were interested in studying the possible regulation of the human enzyme activity. It has been previously reported that SSAO activity is increased in the presence of Dulbecco's modified Eagle medium (DMEM) in vitro. The aim of the present work was to investigate the effects of the different constituents of DMEM on human SSAO activity. We found that sodium bicarbonate was the only component able to mimic the enhancement of both human aorta and plasma SSAO activity in vitro, suggesting a possible physiological role of bicarbonate as an intrinsic modulator of the human enzyme. Failure to take this activating effect into account could also result in inaccuracies in the reported tissue activities of this enzyme.

Prolonged treatment with aminoguanidine strongly inhibits adipocyte semicarbazide-sensitive amine oxidase and slightly reduces fat deposition in obese Zucker rats

Beneficial effects of aminoguanidine (AG) on diabetic vascular complications result from prevention of protein glycation, inhibition of inductible NO synthase, and inhibition of vascular semicarbazide-sensitive amine oxidase (SSAO). However, influence of AG on adipose tissue deposition has been poorly investigated in obesity. Considering that SSAO is highly expressed in fat cells, and that a SSAO blocker has been recently reported to reduce body weight gain in obese mice, this work aimed to investigate the influence of AG on adipose tissue functions. First, AG was shown to directly inhibit SSAO activity in cultured adipocytes. Although AG did not directly alter lipolytic activity in human adipocytes, it inhibited benzylamine-induced antilipolysis via SSAO (but not NO synthase) inhibition. When AG was i.p. administered to obese Zucker rats (270 micromol kg(-1)day(-1) for 3 weeks), treated rats lost their capacity to oxidize benzylamine in a SSAO-dependent manner in adipose tissues and in cerebral vessels. Monoamine oxidase activity was unmodified in liver, skeletal muscles or adipose tissues and tended to increase in brain vessels. AG-treatment did not change body weight gain or hyperinsulinemic state of obese rats but slightly reduced subcutaneous fat deposition. AG did not modify insulin responsiveness in adipocytes but impaired the effects of SSAO substrates, such as glucose transport activation and lipolysis inhibition by methylamine or benzylamine plus vanadate. These results show that complete impairment of SSAO activity produced by AG-treatment in obese rats was likely responsible for a weak limitation of fat deposition. Previously proposed for prophylaxis in diabetes, AG may be useful for treating obesity via its SSAO blocking properties.

Variations in activity and inhibition with pH: the protonated amine is the substrate for monoamine oxidase, but uncharged inhibitors bind better

It has been accepted that, as required mechanistically, the neutral form of the amine is the substrate for monoamine oxidase, despite the amine pK (a) of above 9.5. The pH dependence of the kinetic parameters for kynuramine oxidation by purified human MAO-A and for phenylethylamine oxidation by MAO-B in granulocytes at pH values from 5 to 10 was consistent with the protonated amine being used. Deprotonation of a group of pK (a) = 7.1 in MAO-B and pK (a) = 7.5 +/- 0.1 (n = 4) in MAO-A was important for efficient catalysis. The K(i) values for two oxazolidinone inhibitors of MAO-A gave opposite pH-dependence indicating that the uncharged form of each inhibitor bound better than the charged form. Decreased pH induced a blue shift in the spectral maximum of MAO-A indicative of a more hydrophobic environment around the flavin, and also influenced the redox properties of the flavin.

Semicarbazide-sensitive amine oxidase (SSAO) and its possible contribution to vascular damage in Alzheimer's disease

One of the key pathological features of the progressive neurodegenerative disorder Alzheimer's disease (AD) is cerebral amyloid angiopathy (CAA). CAA is present in most cases of AD, and it is characterized by the deposition of beta-amyloid (Abeta) in brain vessels, inducing the degeneration of vascular smooth muscle cells and endothelial cells. Herein we report that semicarbazide-sensitive amine oxidase (SSAO) is overexpressed in cerebrovascular tissue of patients with AD-CAA, and that it colocalizes with beta-amyloid deposits. This over-expression correlates with high SSAO activity in plasma of severe AD patients. In addition, we have observed that the catalytic activity of SSAO is able to induce apoptosis in smooth muscle cells in vitro. Taken together, these results allow us to postulate that SSAO may contribute to the vascular damage associated to AD.

Characterization of A7r5 cell line transfected in a stable form by hSSAO/VAP-1 gene (A7r5 hSSAO/VAP-1 cell line)

A smooth muscle cell line (A7r5) was stably transfected with the human SSAO/VAP-1 (hSSAO/VAP-1) gene. The expressed protein was located solely in the membrane fraction of the cell. However it was also shown to be released into the cell-culture medium. Both the membrane-bound and released, soluble, forms had SSAO enzyme activity. Although MAO-A is present in wild-type A7r5 cells, it was undetectable in the transfected cells.

Protective effect of N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine (PF9601N) on mitochondrial permeability transition

PF9601N, N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine, an monoamine oxidase (MAO) B inhibitor, has shown neuroprotective properties against dopaminergic toxins. To elucidate the mechanisms involved in this protection, the effect of PF9601N on mitochondria was assessed. PF9601N prevents mitochondrial swelling, drop in the electrical potential and oxidation of sulfhydryl groups, glutathione and pyridine nucleotides induced by Ca(2+). These observations demonstrate the protective effect of PF9601N on the induction of mitochondrial permeability transition. This protection is due to the interaction of the secondary protonated amino group in the molecule with pore-forming structures and to its antioxidant property, rather than to inhibition of MAO B activity. PF9601N also prevents the release of cytochrome c from mitochondria, suggesting its potential inhibitory effect on mitochondria-mediated apoptosis. The low IC(50) value for this inhibition, in comparison with deprenyl, make it a more efficient compound than propargylamines and other amines in protecting the bioenergetic functions of mitochondria.

The decrease of NAD(P)H has a prominent role in dopamine toxicity

We characterized dopamine toxicity in human neuroblastoma SH-SY5Y cells as a direct effect of dopamine on cell reductive power, measured as NADH and NADPH cell content. In cell incubations with 100 or 500 microM dopamine, the accumulation of dopamine inside the cell reached a maximum after 6 h. The decrease in cell viability was 40% and 75%, respectively, after 24 h, and was not altered by MAO inhibition with tranylcypromine. Dopamine was metabolized to DOPAC by mitochondrial MAO and, at 500 microM concentration, significantly reduced mitochondrial potential and oxygen consumption. This DA concentration caused only a slight increase in cell peroxidation in the absence of Fe(III), but a dramatic decrease in NADH and NADPH cell content and a concomitant decrease in total cell NAD(P)H/NAD(P)+ and GSH/GSSG and in mitochondrial NADH/NAD+ ratios. Dopaminechrome, a product of dopamine oxidation, was found to be a MAO-A inhibitor and a strong oxidizer of NADH and NADPH in a cell-free system. We conclude that dopamine may affect NADH and NADPH oxidation directly. When the intracellular concentrations of NAD(P)H and oxidized dopamine are similar, NAD(P)H triggers a redox cycle with dopamine that leads to its own consumption. The time-course of NADH and NADPH oxidation by dopamine was assessed in cell-free assays: NAD(P)H concentration decreased at the same time as dopamine oxidation advanced. The break in cell redox equilibrium, not excluding the involvement of free oxygen radicals, could be sufficient to explain the toxicity of dopamine in dopaminergic neurons.

Soluble Semicarbazide Sensitive Amine Oxidase (SSAO) catalysis induces apoptosis in vascular smooth muscle cells

Semicarbazide sensitive amine oxidase (SSAO) metabolizes oxidative deamination of primary aromatic and aliphatic amines. It is selectively expressed in vascular cells of blood vessels, but it is also circulating in blood plasma. SSAO activity in plasma is increased in some diseases associated with vascular complications and its catalytic products may cause tissue damage. We examined the effect of the oxidation of the SSAO substrate, methylamine, on cultured smooth muscle cells. Cell incubation with methylamine plus soluble SSAO, contained in bovine serum, resulted toxic to rat aorta A7r5 and human aortic smooth muscle cells, as measured by MTT reduction. This effect was completely reverted by specific SSAO inhibitors, indicating that the toxicity was mediated by the end products generated. Moreover, SSAO-mediated deamination of methylamine induced apoptosis in A7r5 cells, detected by chromatin condensation, Caspase-3 activation, PARP cleavage and cytochrome c release to cytosol. Formaldehyde, rather than H2O2, resulted to be a strong apoptotic inducer to A7r5 cells. Taken together, the results suggest that increased plasma SSAO activity in pathological conditions, could contribute to apoptosis in smooth muscle cells, leading to vascular tissue damage.

Human plasma semicarbazide sensitive amine oxidase (SSAO), β-amyloid protein and aging

Semicarbazide sensitive amine oxidase (SSAO) metabolizes oxidative deamination of primary aromatic and aliphatic amines. The final products of its catalysis, ammonia, hydrogen peroxide (H2O2) and the corresponding aldehyde, may contribute to diseases involving vascular degeneration. SSAO is selectively expressed in blood vessels in the brain, but is also present in blood plasma. We have previously reported that membrane-bound SSAO is overexpressed in the cerebrovascular tissue of Alzheimer's disease (AD) patients. The aim of the present work is to study whether the circulating SSAO is also altered in this neurodegenerative disease. SSAO activity was determined in plasma of control cases (n = 23) and patients suffering sporadic Alzheimer dementia, distributed according to the Global Deterioration Scale (GDS): mild (n = 33), moderate (n = 14), moderate-severe (n = 15) and severe dementia (n = 19). Results show a clear increase of plasma SSAO activity (p < 0.001) in moderate-severe and severe AD patients, with patient age being an independent correlative factor. However, plasma SSAO activity was not altered in AD patients with mild or moderate dementia compared to controls. beta-Amyloid (Abeta) (40-42) immunoreactivity in plasma samples was also determined, and no correlation was observed between Abeta 40-42 levels and the severity of the dementia or the plasma SSAO activity. Our results suggest that an increase in circulating SSAO activity could contribute to oxidative stress and vascular damage in advanced Alzheimer's disease.

Indolalkylamines derivatives as antioxidant and neuroprotective agents in an experimental model of Parkinson's disease

BACKGROUND

The neuroprotective effect of N-(2-propynyl)2-(5-benzyloxy-indol)methylamine (PF 9601N), a novel MAO B inhibitor, and its metabolite FA 72 on the human neuroblastoma SHSY5Y cell line lesioned with (300 microM) dopamine was assessed and compared with that of 1-deprenyl assayed at identical experimental conditions.

MATERIAL/METHODS

Using this experimental model, PF 961N showed a neuroprotective effect in a dose-dependent manner, and at a concentration of 10 pM a 20% recovery of cell viability was observed. However, the metabolite FA72 assayed under the same experimental conditions showed an increase in cell viability of nearly 50%. In the case of l-deprenyl, a concentration of 100 microM was necessary to recover only 10% of cell viability.

RESULTS

This neuroprotective effect could be explained in terms of the antioxidant capacity of PF 9601N. In this context, the antioxidant capacities of the novel series of MAO inhibitors, PF 9601N and its analogues, were evaluated by their inhibition of the auto-oxidation of dopamine to melanin and by the dichlorofluorescein and 2-deoxyribose methods.

CONCLUSIONS

All of these compounds have the basic structure of an indole ring in common, but show different substituents at different positions in it. The corresponding structure-activity relationship studies allowed us to conclude that the presence of a benzyloxy group, or a hydroxy or methoxy group, at position 5 of the indol ring enhanced these antioxidant characteristics, presenting a decreasing order of antioxidant activity of the primary > secondary > tertiary amines. The antioxidant properties of PF 9601 N would explain its neuroprotective effect observed in SHSY5Y cells lesioned with dopamine.

Exploring the binding mode of semicarbazide-sensitive amine oxidase/VAP-1: identification of novel substrates with insulin-like activity

We previously reported that substrates of semicarbazide-sensitive amine oxidase in combination with low concentrations of vanadate exert potent insulin-like effects. Here we performed homology modeling of the catalytic domain of mouse SSAO/VAP-1 and searched through chemical databases to identify novel SSAO substrates. The modeling of the catalytic domain revealed that aromatic residues Tyr384, Phe389, and Tyr394 define a pocket of stable size that may participate in the binding of apolar substrates. We identified a number of amines as substrates of human, rat, and mouse SSAO. The compounds PD0119035, 2,3-dimethoxy-benzylamine, and C-naphthalen-1-yl-methylamine showed high affinity as substrates of rat SSAO. C-Naphthalen-1-yl-methylamine was the only substrate that showed high affinity for human SSAO. C-Naphthalen-1-yl-methylamine and 4-aminomethyl-benzenesulfonamide showed the highest capacity to stimulate glucose transport in isolated rat adipocytes. The impact of these findings on the development of new treatments for diabetes is discussed.

Localization of monoamine oxidase A and B and semicarbazide-sensitive amine oxidase in human peripheral tissues

Monoamine oxidase (MAO) A and B and semicarbazide-sensitive amine oxidase (SSAO) localizations in peripheral human tissues were compared by immunohistochemistry. The primary antibodies used were mouse monoclonal anti-human MAO-A (6G11/E1) and anti-human MAO-B (3F12/G10/2E3) and a rabbit polyclonal anti-bovine SSAO antibody. Immunoreactivities of the samples, obtained from 6 routine autopsy cases, showed different distributions in the tissues studied (heart, lung, duodenum, liver, pancreas, spleen, thyroid gland, adrenal gland and kidney). The relative MAO-A, MAO-B and SSAO distributions indicated a widespread distribution of these enzymes in the human body that is characterized by a matching cellular pattern in only few tissues. These differences suggest that each amine oxidase may play a specific function in, at least some, peripheral tissues.

Semicarbazide-sensitive amine oxidases: enzymes with quite a lot to do

The semicarbazide-sensitive amine oxidases (SSAO) (EC 1.4.3.6) were believed to be detoxifying enzymes, primarily involved in the oxidative deamination of endogenous amines, such as methylamine and aminoacetone, together with some xenobiotic amines. However, it appears that the reaction products may have important signalling functions in the regulation of cell development and glucose homeostasis. Furthermore, enzyme, from some sources, behaves as a cellular adhesion protein under inflammatory and it may also be involved in lipid transport. This review considers what is known about the activities and potential functions of this hardworking protein.

Synthesis of 4-methyl-thio-phenyl-propylamine and the evaluation of its interaction with different amine oxidases

A new molecule, the 4-methyl-thio-phenyl-propylamine (PrNH(2)) was synthesized and its biological interaction with different amine oxidases such as semicarbazide sensitive amine oxidase (SSAO) [E.C.1.4.3.6], and monoamine oxidase [E.C.1.4.3.4] under its two isoforms, MAO A and MAO B, has been assessed. The substrate specifities of MAO and SSAO overlap to some extent. In this context, the search of new molecules, able to discriminate between these different amine oxidases is very important as it will allow greater elucidation of the SSAO's role in physiological and pathological conditions. We report for the first time, the synthesis and evaluation of a new molecule which has a high affinity towards the SSAO family of enzymes, more so than previously described and furthermore an ability to discriminate between the different amine oxidases.

Activation of human lung semicarbazide sensitive amine oxidase by a low molecular weight component present in human plasma

Semicarbazide-sensitive amine oxidase (SSAO) encodes a wide family of enzymes named E.C.1.4.3.6 [amine:oxygen oxidoreductase (deaminating) (copper containing)] that metabolises primary aliphatic and aromatic amines. It is present in almost all vascularised and nonvascularised mammalian tissues, and it is also present in soluble form in plasma. SSAO appears to show different functions depending on the tissue where it is expressed. Here we describe, for the first time, the activation of the SSAO from human lung by human plasma. The extent of activation was greater when the human plasma came from diabetic and heart infarcted patients. A kinetic mechanism of such effect is proposed. The activation was lost after the plasma was dialysed, indicating a low molecular weight component (MW <3800 Da) to be responsible. The activator component is heat stable and resistant to proteolysis by chymotrypsin and trypsin and also resistant to perchloric acid treatment. However, treatment with 35% formic acid, completely abolished activation, suggesting involvement of lipid material. The possibility of that lysophosphatidylcholine (LPC), an amphiphilic phospholipid derived from the phosphatidylcholine, the major component in plasma accumulated in pathological conditions, was studied. LPC was shown to behave as a "competitive activator" of human lung SSAO at concentrations below its critical micellar concentration (CMC value=50 microM). Thus LPC may be a component of the SSAO activatory material present in human plasma.

Protective effect of N-(2-propynyl)-2-(5-benzyloxyindolyl) methylamine (PF 9601N), a novel MAO-B inhibitor, on dopamine-lesioned PC12 cultured cells

Oxidative stress may play a role in the pathogenesis of Parkinson's disease. We have standardised a new model of dopaminergic-cell toxicity that uses dopamine, which is able to generate free radicals, as a toxin. The effect of this catecholamine on cell viability (MTT staining) was dose-dependent, reaching 65% of cell loss at a dopamine concentration of 300 microM. In this model, the protective effect of a novel MAO-B inhibitor, N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine (PF 9601N), was studied and compared with the effect of L-deprenyl assayed under the same experimental conditions. Whereas PF 9601N (50 microM and 100 microM) showed a significant protective effect, this was not the case with L-deprenyl. This different behaviour could be explained in terms of difference in antioxidant capacity. The toxicity induced in PC12 cells by 300 microM dopamine was partially reversed by incubating it in the presence of GBR-12909, a dopamine-transporter blocker. The results indicated that, besides the intracellular toxicity effect of dopamine, another non-specific extracellular mechanism could be involved.

The inhibition of semicarbazide-sensitive amine oxidase by aminohexoses

Semicarbazide-sensitive amine oxidase (EC 1.4.3.6; amine:oxygen oxidoreductase (deaminating) (copper-containing); SSAO) is a multifunctional protein. It acts under inflammatory conditions as a vascular-adhesion protein (VAP-1), mediating the adhesion of lymphocytes to vascular endothelial cells. The relationships, if any, between this adhesion function and the enzymatic functions (amine-substrate specificity and catalysis) of SSAO have not yet been defined. Since cell surface amino sugars and their derivatives are known to be involved in cell-to-cell recognition, we have investigated their possible effects on the enzyme activity of SSAO. The aminohexoses galactosamine, glucosamine and mannosamine were not oxidatively deaminated by SSAO. However, their presence during the assay of benzylamine oxidation resulted in a time-dependent inhibition. This inhibition was shown to follow saturation kinetics with respect to hexosamine concentration. Although time-dependent, the inhibition of SSAO activity was found to be reversible by dilution. In contrast, there is no such inhibition when the N-acetylamino sugar derivatives or the parent sugars (galactose, glucose and mannose) replaced the amino sugars in the reaction mixture. These results suggest that the interactions between SSAO and aminohexoses are specific and, therefore, that the cell-adhesion functions and amine-recognition functions of VAP-1/SSAO may be interlinked.

Semicarbazide-sensitive amine oxidases in pig dental pulp

The behaviour of semicarbazide-sensitive amine oxidase (SSAO; E.C. 1.4.3.6) in dental pulp has been studied, with particular reference to the metabolism of 5-hydroxytryptamine (5-HT; serotonin). Kinetic studies using radioactively labelled substrates have confirmed benzylamine, 2-phenylethylamine (PEA) and 5-HT to be substrates for microsomal SSAO from porcine dental pulp. Kinetic substrate-competition studies indicated the presence of two forms of SSAO in dental pulp; one that oxidises benzylamine and PEA but not 5-HT and a second that oxidises 5-HT and PEA but not benzylamine. These two forms also differ in their thermostabilities at 60 and 70 degrees C, although this thermal inactivation is partly reversible.

PF 9601N [N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine], a new MAO-B inhibitor, attenuates MPTP-induced depletion of striatal dopamine levels in C57/BL6 mice

Monoamine oxidase isoform B (MAO-B) is involved in Parkinson's disease (PD) induced by the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxin (MPTP) in human and non-human-primate. MAO-B inhibitors, such as L-deprenyl have shown to prevent against MPTP-toxicity in different species, and it has been used in Parkinson therapy, however, the fact that it is metabolized to (-)-methamphetamine and (-)-amphetamine highlights the need to find out new MAO-B inhibitors without a structural amphetaminic moiety. In this context we herein report, for the first time, anywhere a novel non-amphetamine-like MAO-B inhibitor, PF 9601N, N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine. This attenuates the MPTP-induced striatal dopamine depletion in young-adult and adult-old C57/BL mice, using different schedules of administration, and which behave "ex vivo" as a slightly more potent and selective MAO-B inhibitor than L-deprenyl, assayed for comparative purposes in the same experimental conditions. The MAO-B ID(50) values were calculated from the total MAO-B activity measured against [14C] phenylethylamine (22 microM) as substrate, at each inhibitor concentration. The MAO-B ID(50) values resulted to be 381 and 577 nmol/kg for PF 9601N and L-deprenyl, respectively. The intraperitoneally (i.p.) co-administration to young-adult C57/BL6 mice of MPTP (30 mg/kg), with different concentrations of PF 9601N or L-deprenyl (29.5-0.357 micromol/kg) showed a dose-dependent protective effect against striatal dopamine depletion, measuring the dopamine contents and its metabolites by HPLC. The ED(50) value proved to be 3.07 micromol/kg without any significant differences between either MAO-B inhibitor. Nevertheless, lower doses of PF 9601N (1.5 micromol/kg) were necessary to get almost total protection, without any change in the DOPAC and HVA content, when administered 2 h before MPTP (30 mg/kg), whereas partial protection (45%) against dopamine depletion was observed in the case of L-deprenyl. In both cases, MAO-B inhibition was a necessary condition in order to observe the protective effect. When adult-old (8-10 months) C57/BL6 mice were used, MPTP (25 mg/kg) administration induced 25 days later, an irreversible dopamine depletion. In these conditions, chronic administration with 0.15 micromol/kg of PF 9601N, before the toxin, every 24 h for 10 days, rendered almost total protection of dopamine depletion, whereas L-deprenyl yielded only 50% protection of the dopamine content, assayed in the same conditions. It is worth remarking, that in both cases MAO-B was not affected. From these results, it can be concluded that PF 9601N attenuates MPTP neurotoxicity "in vivo" better than L-deprenyl through different mechanisms, with special relevance to the protective effect, independent of MAO-B inhibition, observed in the irreversibly MPTP-lesioned adult-old mice. Therefore, this novel non-amphetamine MAO-B inhibitor could be potentially effective in PD therapy.

Neuroprotective effect of the monoamine oxidase inhibitor PF 9601N [N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine] on rat nigral neurons after 6-hydroxydopamine-striatal lesion

Monoamine oxidase B (MAO-B) inhibitors are potentially useful in the therapeutic treatment of Parkinson's disease. L-Deprenyl has been shown to slow nigrostriatal tract degeneration in human idiopathic Parkinsonism and to be an effective neuroprotector in experimental 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity models. However, L-amphetamine and (-)methamphetamine, the metabolites generated by L-deprenyl, can have adverse and severe side-effects. Therefore, the search for new MAO-B inhibitors without potential amphetamine-like properties is a matter of great therapeutic interest. The present report is the first to describe the neuroprotective effect--following chronic intraperitoneal (i.p.) treatment--of a novel and non-amphetaminic MAO-B inhibitor, [N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine] (PF 9601N), on the neurodegeneration of nigral dopaminergic neurons caused by administration of intrastriatal 6-hydroxydopamine (6-OHDA). Two groups of six animals were unilaterally injected with 6-OHDA in the right striatum. One group was treated daily with 60 mg/kg PF 9601N i.p., starting before stereotaxic lesion and continuing for 18 days thereafter. The other group was treated with vehicle solution. Coronal slabs including the substantia nigra pars compacta (SNpc) were processed for tyrosine hydroxylase immunohistochemistry (TH). The number of TH positive (TH+) neurons in the SNpc was 60% lower in 6-OHDA lesioned rats. However, the loss of TH+ neurons in the SNpc was only 30% in PF 9601N i.p.-treated animals. Therefore, treatment with the specific MAO-B inhibitor significantly reduced the 6-OHDA-induced degeneration to about 50%.

PP1/PP2A phosphatases inhibitors okadaic acid and calyculin A block ERK5 activation by growth factors and oxidative stress

Okadaic acid is an inhibitor of the protein Ser/Thr phosphatases PP1 and PP2A, which blocks the activation of extracellular signal-regulated protein kinase 5 (ERK5), a member of the MAP kinase family activated by growth factors and several types of stressors. The blocking of ERK5 activation by okadaic acid was observed in HeLa cells exposed to epidermal growth factor and H(2)O(2) as well as in PC12 cells stimulated by nerve growth factor and H(2)O(2). Calyculin A, another PP1 and PP2A inhibitor, behaved similarly although these compounds are not structurally related. This suggests that either PP1 or PP2A or both are necessary for ERK5 activation. Protein kinase C (PKC) acts as a negative regulator of the ERK5 activation pathway, however our data suggest that the effects of PKC and the phosphatase are unrelated.

Overexpression of semicarbazide sensitive amine oxidase in the cerebral blood vessels in patients with Alzheimer's disease and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy

Semicarbazide sensitive amine oxidase (SSAO) metabolizes oxidative deamination of primary aromatic and aliphatic amines, and, in the brain, it is selectively expressed in blood vessels. SSAO expression is examined, by immunohistochemistry with a purified polyclonal antibody to SSAO from bovine lung, in the brains of subjects with Alzheimer disease (AD; n=10), cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL; n=2), and age-matched controls (n=8). SSAO immunoreactivity is restricted to meningeal and parenchymal blood vessels in control and diseased brains. Yet, a marked and selective increase in SSAO immunoreactivity occurs in association with betaA4 vascular amyloid deposits in patients with AD, and in the vicinity of the typical granular deposits in the blood vessels of gray and white matter in patients with CADASIL. Oxidative deamination of primary aromatic and aliphatic amines by SSAO produces ammonia, hydrogen peroxide and the corresponding aldehyde. Moreover, increased SSAO immunoreactivity is associated with increased Cu/Zn superoxide dismutase 1 expression restricted to abnormal blood vessels in diseased brains. Therefore, it is suggested that increased SSAO expression is a source of oxidative stress in the blood vessel wall in AD and CADASIL.

Involvement of Ca2+ in dopamine release in striatal rat slices by PF9601N and L-deprenyl

Rat striatal slices were incubated 1 hour in Krebs buffer with carbogen by continuous perfusion. When both MAO-B inhibitors PF9601N and l-deprenyl were added in the incubation medium, at 200 microM concentration, a diminution in the dopamine content was observed. The decrease in dopamine content was partially calcium dependent and showed a different mechanism between both compounds. When 1 mM concentration EGTA was added at the Krebs incubation medium without calcium, dopamine content was partially recovered, being not affected in case of l-deprenyl. When 50 microM dantrolene (a calcium vesicular release inhibitor) was added in the Krebs incubation medium without calcium, dopamine was partially recovered for l-deprenyl, being not affected in case of PF9601N. These data indicate that dopamine release is extracellular calcium dependent in case of PF9601N, whereas l-deprenyl depends on calcium intracellular origin.

Neuroprotective aspects of a novel MAO-B inhibitor PF9601N

PF9601N is an acetylenic tryptamine derivative devoid of amphetamine-like properties, that behaves as suicide MAO-B inhibitor more potent than l-deprenyl. It is highly selective towards MAO-B and it neuroprotects from the neurotoxicity induced in C57Bl/6 adult mice by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). PF9601N also shows in vitro antioxidant properties by inhibiting the dopamine autoxidation. A potential therapeutic use in Parkinson's disease treatment is proposed for this compound.

The novel type B MAO inhibitor PF9601N enhances the duration of L-DOPA-induced contralateral turning in 6-hydroxydopamine lesioned rats

The present study examined the effect of the highly potent and selective MAO B inhibitor PF9601N on L-DOPA-induced rotational behavior in unilateral nigrostriatal 6-hydroxydopamine lesioned rats. Three doses of PF9601N (20, 40 and 60mg/kg) were administered 30 min before an injection of L-DOPA (25mg/kg), and both contralateral and ipsilateral rotational behavior was measured. In addition, we also studied the effect produced by another MAO B inhibitor, deprenyl (20mg/kg), the MAO A inhibitor, clorgyline (20mg/kg), and the dopamine reuptake inhibitor, GBR2909 (7.5 mg/kg) on L-DOPA-induced rotational behavior. The results showed that PF9601N plus L-DOPA significantly enhanced the duration of contralateral rotational behavior with respect to L-DOPA plus vehicle in a dose-related manner. At the dose of 40 and 60mg/kg, PF9601N produced significantly more overall contralateral turning than L-DOPA plus vehicle, and at the dose of 60mg/kg, PF9601N produced significantly more turning behavior than L-DOPA plus deprenyl. These results suggest that PF9601N may be used as a novel tool in the treatment of Parkinson's disease.

Semicarbazide-sensitive amine oxidase substrates stimulate glucose transport and inhibit lipolysis in human adipocytes

Semicarbazide-sensitive amine oxidases (SSAO) are widely distributed enzymes scavenging biogenic or exogenous amines and generating hydrogen peroxide. We asked whether human adipose tissue could express SSAO. Since hydrogen peroxide exhibits pharmacological insulin-like effects, we also tested whether its endogenous production by SSAO could mimic several insulin effects on adipocytes, such as stimulation of glucose uptake and inhibition of lipolysis. The benzylamine oxidation by human adipose tissue was inhibited by semicarbazide or hydralazine and resistant to pargyline or selegiline. It was due to an SSAO activity localized in adipocyte membranes. A protein of 100-kDa and a 4-kb mRNA corresponding to SSAO were identified in either mammary or abdominal subcutaneous fat depots. In isolated adipocytes, SSAO oxidized similarly benzylamine and methylamine that dose dependently stimulated glucose transport in a semicarbazide-sensitive manner. Antioxidants also inhibited the benzylamine and methylamine effects. Moreover, the ability of diverse substrates to be oxidized by adipocytes was correlated to their effect on glucose transport. Benzylamine and methylamine exerted antilipolytic effects with a maximum attained at 1 mM. These results show that human adipocytes express a membrane-bound SSAO that not only readily oxidizes exogenous amines and generates H(2)O(2), but that also interplays with glucose and lipid metabolism by exerting insulin-like actions. Based on these results and the fact that variations in plasma levels of the soluble form of SSAO have been previously reported in diabetes, we propose that determination of adipocyte SSAO, feasible on subcutaneous microbiopsies, could bring relevant information in pathologies such as obesity or diabetes.

On the primary structure of membrane-bound semicarbazide-sensitive amine oxidase (SSAO)

Mammalian semicarbazide-sensitive amine oxidase (SSAO) activities are a diverse group of copper dependent enzymes within the classification EC 1.4.3.6. [amine:oxygen oxidoreductase (deaminating) (copper-containing)]. They include plasma amine oxidase and the membrane-bound enzyme. Although soluble plasma SSAO from several species have been successfully purified and cloned, relatively little work has been carried out on the molecular properties of the tissue-bound enzyme. At present there is not conclusive evidence that allows to conclude whether or not plasma SSAO is released from the tissue-bound enzyme. This review focuses upon recent progress made in determining the primary structure of the membrane-bound SSAO.

Tissue activity and cellular localization of human semicarbazide-sensitive amine oxidase

Semicarbazide-sensitive amine oxidase (SSAO), widely distributed in highly vascularized mammalian tissues, metabolizes endogenous and xenobiotic aromatic and aliphatic monoamines. To assess whether its physiological role in humans is restricted to oxidation, we used an immunohistochemical approach to examine the cellular localization of SSAO in human peripheral tissues (adrenal gland, duodenum, heart, kidney, lung, liver, pancreas, spleen, thyroid gland, and blood vessels) and also analyzed its subcellular localization. The results are in agreement with the specific activities also determined in the same samples and are discussed with reference to the tissue distribution of monoamine oxidase A and B. Together with the oxidative deamination of monoamines, SSAO cellular localization indicates that, in most human peripheral tissues, it might participate in the regulation of physiological processes via H(2)O(2) generation. (J Histochem Cytochem 49:209-217, 2001)

Time-dependent activation of the semicarbazide-sensitive amine oxidase (SSAO) from ox lung microsomes

The activity of ox lung microsomal semicarbazide-sensitive amine oxidase (EC 1.4.3.6; SSAO) towards benzylamine increased 20-fold during incubation at 37 degrees C. After an initial lag-period, activation was first-order with time and complete after approx. 20 h. No significant changes in activity towards methylamine, histamine or 2-phenylethylamine were observed, although mixed-substrate experiments were consistent with the same enzyme being involved in the oxidation of all these substrates, both before and after time-dependent activation. The enzyme-tryptophan fluorescence increased on incubation at 37 degrees C in parallel with the increase in activity towards benzylamine. Treatment of the activated-enzyme preparation with 6 M guanidinium chloride followed by dialysis, caused both the activity towards benzylamine and the fluorescence to fall to that occurring before activation. However, incubation of this preparation at 37 degrees C resulted in increases in fluorescence and activity similar to those seen with the unactivated enzyme. Benzylamine oxidation was inhibited, uncompetitively with respect to oxygen, by high substrate concentrations but no such inhibition was observed with the other amines. Activation resulted in an increase in V(max) for benzylamine oxidation, with no significant alterations in the K(m) or the K(si) for high-substrate inhibition. Kinetic studies were consistent with sequential mechanisms being followed for the oxidation of both benzylamine and methylamine but the dependence on oxygen concentration was complex. These results might indicate that benzylamine follows a different reaction pathway from the other substrates, with substrate-specific activation involving a reaction step that is rate-limiting for benzylamine oxidation but not for the others.

A spectrophotometric method for determining the oxidative deamination of methylamine by the amine oxidases

Previously published studies on the oxidative deamination of methylamine by the amine oxidases have determined the formation of radioactively labeled formaldehyde from [(14)C]methylamine. The present work describes a coupled spectrophotometric assay, using formaldehyde dehydrogenase, for the continuous determination of the oxidative deamination of methylamine by semicarbazide-sensitive amine oxidase (SSAO) and its potential use for determining methylamine concentrations in plasma. In this assay, the formaldehyde produced by methylamine deamination is further oxidized to formate, with the reduction of NAD(+), by formaldehyde dehydrogenase. The NADH generated is monitored continuously at 340 nm. Interference from the presence of a rotenone-insensitive NADH oxidase activity in crude tissue homogenates and microsomal fractions can be minimized by pretreating samples with Triton X-100 or substituting NAD(+) by APAD(+) in the coupled assay. This relatively inexpensive and reproducible assay procedure avoids the use of radioactively labeled material.

Molecular determinants of MAO selectivity in a series of indolylmethylamine derivatives: biological activities, 3D-QSAR/CoMFA analysis, and computational simulation of ligand recognition

A series of indolylmethylamine derivatives were assayed toward MAO-A and MAO-B inhibition. The K(i) values of these compounds are in the range from 0.8 to >10(6) nM for MAO-A or from 0.75 to 476000 nM for MAO-B. The most selective MAO-A or MAO-B inhibitors elicit a ratio of K(i) in the order of 1500 or 1000, respectively. Comparison of MAO-A and MAO-B CoMFA models showed that both the steric and electrostatic properties at the 5 position of the indole ring are determinant for MAO selectivity. Computational simulations of the complex between this part of the ligand and Phe-208 of MAO-A or Ile-199 of MAO-B, experimentally identified as responsible for substrate selectivity, allowed us to further characterize the nature of these enzyme-inhibitor interactions.

Presence of SSAO in human and bovine meninges and microvessels

In spite that SSAO enzyme is widely distributed in almost all tissues, specially in vascularized ones, its presence in brain microvessels is still controversy. Our results resolve this question showing that both human and bovine cerebrovascular tissues do contain the SSAO enzyme. This was achieved biochemically, using benzylamine and methylamine as substrates, and by immunoblot analysis, using polyclonal antibodies anti-SSAO that recognized a 100 kDa single band in tissue homogenates.

FA-70, a novel selective and irreversible monoamine oxidase-A inhibitor: effect on monoamine metabolism in mouse cerebral cortex

A series of indolealkylamine derivatives has been previously designed and evaluated with the aim of finding the most potent and selective novel monoamine oxidase (MAO) inhibitors to be used in the therapy of neurological and affective disorders. Among them, FA70, a 5-hydroxy-indolealkylamine derivative, has been characterized in vitro as a potent, irreversible, and mechanism-based inhibitor of the MAO-A isoform. The comparison with clorgyline, analyzed under the same experimental conditions, confirmed FA70 as the most potent MAO-A inhibitor. The ex vivo effect of FA70 on MAO activity in mouse cerebral cortex was similar to that observed in vitro, showing more efficacy than in peripheral tissues. The ex vivo effect of FA70 on amine metabolism also was evaluated after acute and chronic treatment, and the results showed that between both MAO isoforms, MAO-A is the only one responsible for monoamine metabolism in this region of the brain. The ex vivo effect of FA70 on dopamine content was correlated with the activation effect on tyrosine hydroxylase activity, the enzyme responsible for the regulation of the limiting step in catecholamine synthesis.

Relevance of benzyloxy group in 2-indolyl methylamines in the selective MAO-B inhibition

1. Previous studies with indolyl derivatives as monoamine oxidase (MAO) inhibitors have shown the relevance of the indole structure for recognition by the active site of this enzyme. We now report a new series of molecules with structural features which determine the selectivity of MAO inhibition. 2. A benzyloxy group attached at position 5 of the indole ring is critical for this selective behaviour. Amongst all of these benzyloxy-indolyl methylamines, N-(2-propynyl)-2-(5-benzyloxyindol)methylamine FA-73 was the most potent MAO-B 'suicide' inhibitor studied. 3. The Ki values for MAO-A and MAO-B were 800+/-60 and 0.75+/-0.15 nM, respectively. These data represent a selectivity value of 1066 for MAO-B, being 48 times more selective than L-deprenyl (Ki values of 376+/-0.032 and 16.8+/-0.1 nM for MAO A and MAO-B, respectively). The IC50 values for dopamine uptake in striatal synaptosomal fractions from rats were 150+/-8 microM for FA-73 and 68 +/- 10 microM for L-deprenyl whereas in human caudate tissue the IC50 values were 0.36+/-0.015 microM for FA-73 and 0.10+/-0.007 microM for L-deprenyl. Moreover, mouse brain MAO-B activity was 90% ex vivo inhibited by both compounds 1 h after 4 mg kg(-1) administration, MAO-A activity was not affected. 4. These novel molecules should provide a better understanding of the active site of monoamine oxidase and could be the starting point for the design of further selective, non-amphetamine-like MAO-B inhibitors with therapeutic potential for the treatment of neurological disorders.

Semicarbazide-sensitive amine oxidase (SSAO) from human and bovine cerebrovascular tissues: biochemical and immunohistological characterization

Semicarbazide-sensitive amine oxidase (SSAO) is widely distributed in almost all tissues, especially in vascularized ones. However, its presence in brain microvessels is still controversial. We have investigated the presence of SSAO in human and bovine brain microvessels by biochemical and immunohistological techniques, and we have compared it with SSAO present in meninges from the same species. SSAO metabolizes benzylamine and methylamine in all tissues tested and possibly dopamine and octopamine as well, as shown in competition studies. Kynuramine inhibited the metabolism of benzylamine by SSAO with high affinity in a non-competitive manner. Western-blot analysis rendered a positive staining of a 100 kDa band, in tissues from both species. These results were confirmed by immunohistological studies: the tunica media and intima of the meninges from both species were positively stained, and so was the endothelial layer of microvessels. SSAO was absent in brain parenchyma. These results definitively confirm the presence of SSAO in human and bovine cerebrovascular tissues and they demonstrate for the first time, the presence of this amine oxidase in endothelial cells from microvessels, through biochemical and immunological approaches.

New 2-[(5-methoxy-1-methylindolyl)]-alkylamine derivatives: the effect of branching and elongation of the side chain on MAO inhibition

A novel series of previously synthesised 2-[(5-methoxy-1-methylindolyl)]alkylamine derivatives were irreversible and time-dependent mechanism-based inhibitors of MAO. The effect of branching and elongation of the side chain was evaluated on the inhibitory potency towards MAO-A and MAO-B activities. The KI of the reversible step and the kinact of the irreversible one were determined in each case. The results obtained lead to the conclusion that neither the elongation nor the branching of the side chain improve the potency of the compounds as MAO inhibitors.

"In vitro" effect of some 5-hydroxy-indolalkylamine derivatives on monoamine uptake system

Three different indolalkylamine derivatives (FA 102, FA 69, FA 70) having in common an -OH group at 5 position of the indole ring and differing in the presence of a methyl group at the N or the acetylenic group of the side chain, have been synthesized and assayed as monoamine oxidase-A (MAO-A) [E.C.1.4.3.4] inhibitors. They were effective inhibitors with, in some cases, similar potencies to clorgyline. "In vitro" experiments were performed on rat brain synaptosomes to investigate whether these MAO-A inhibitors had any effect on noradrenaline (NA), dopamine (DA) and 5-hydroxytryptamine (5-HT) transport systems in different rat brain regions. The effect of these drugs were compared with those of clorgyline and 1-deprenyl. FA 102, FA 69, FA 70 behaved as inhibitors of 3H-monoamine uptake with similar rank of order of potency for amine uptake inhibition: 5-HT > DA > NA. The IC50 values for FA 102, FA 69, FA 70, respectively, were: 17 microM, 60 microM, 18 microM for 5HT uptake in cortex and 37 microM, 55 microM and 20 microM in hippocampus; 70 microM, 385 microM for NA uptake in cortex and 315 microM, 255 microM and 600 microM in hypothalamus; 270 microM, 160 microM, 40 microM for DA uptake in striatum. 1-Deprenyl was a very poor inhibitor of monoamine uptake, whereas clorgyline behaved similarly to these indolalkylamine derivatives. Comparing these results with the IC50 values of citalopram, nisoxetine and GBR12909, specific and selective inhibitors of 5-HT, NA and DA transport systems respectively, indicated that these indolalkylamine derivatives interact more strongly with the 5HT uptake system.

Studies on the time-dependent activation of microsomal semicarbazide-sensitive amine oxidase

The semicarbazide-sensitive amine oxidase (SSAO) from bovine lung microsomes was activated in a temperature- and time-dependent process. This behaviour was observed when the enzyme was preincubated at 25 degrees C, 37 degrees C and 50 degrees C but not at 4 degrees C. This activation was only observed when benzylamine was used as substrate but not when methylamine, histamine or 2-phenylethylamine were used. The activation was independent of pH, ionic strength and the nature of the buffer used. At 37 degrees C the specific activity had risen to a value that was about 7 times higher than that of the starting material after 120 min. This process affected only the maximum velocity of the reaction with the Km value remaining essentially unchanged. Treatment of SSAO with phospholipases and detergents did not affect this behaviour. Incubation of the enzyme with serine proteases, metal chelating agents, reducing agents or protease inhibitors, had no effect on the activation. The fact that both forms of the enzyme (activated and non-activated), showed the same Mr values on gel filtration chromatography excluded the possibility of an enzyme aggregation and/or degradation being involved in this process.

Role of semicarbazide-sensitive amine oxidase on glucose transport and GLUT4 recruitment to the cell surface in adipose cells

The previous characterization of an abundant population of non-adrenergic imidazoline-I2 binding sites in adipocytes and the recent demonstration of the interplay between these binding sites and amine oxidases led us to analyze the amine oxidase activity in membranes from isolated rat adipocytes. Adipocyte membranes had substantial levels of semicarbazide-sensitive amine oxidase (SSAO). SSAO activity and immunoreactive SSAO protein were maximal in plasma membranes, and they were also detectable in intracellular membranes. Vesicle immunoisolation analysis indicated that GLUT4-containing vesicles from rat adipocytes contain substantial levels of SSAO activity and immunoreactive SSAO protein. Immunotitration of intracellular GLUT4 vesicles indicated that GLUT4 and SSAO colocalize in an endosomal compartment in rat adipocytes. SSAO activity was also found in GLUT4 vesicles from 3T3-L1 adipocytes and rat skeletal muscle. Benzylamine, a substrate of SSAO activity, caused a marked stimulation of glucose transport in isolated rat adipocytes in the presence of very low vanadate concentrations that by themselves were ineffective in exerting insulin-like effects. This synergistic effect of benzylamine and vanadate on glucose transport was totally abolished in the presence of semicarbazide, a specific inhibitor of SSAO. Subcellular membrane fractionation revealed that the combination of benzylamine and vanadate caused a recruitment of GLUT4 to the plasma membrane of adipose cells. The stimulatory effects of benzylamine and vanadate on glucose transport were blocked by catalase, suggesting that hydrogen peroxide production coupled to SSAO activity plays a crucial regulatory role. Based on these results we propose that SSAO activity might contribute through hydrogen peroxide production to the in vivo regulation of GLUT4 trafficking in adipose cells.

Purification and characterization of membrane-bound semicarbazide-sensitive amine oxidase (SSAO) from bovine lung

Semicarbazide-sensitive amine oxidase (SSAO) has been purified from bovine lung microsomes in a form which is catalytically active and stable to storage. The enzyme, an integral membrane protein, was solubilized with Triton X-100 and purification was achieved, in the presence of detergent, by chromatography with Cibacron Blue 3GA-agarose, hydroxylapatite, Lens culinaris-agarose, Resource Q-FPLC and gel filtration on Superdex 200 HR-FPLC. This is the first reported procedure for the extensive purification of a membrane-bound SSAO. The purified enzyme had an apparent Mr of 400000 but exhibited microheterogeneity with SDS/PAGE and isoelectric focusing, probably as a result of its glycoprotein nature. It behaved as a tetramer with subunits with apparent Mr values of 100. Antibodies raised towards the purified enzyme cross-reacted with the enzymes from human lung and bovine plasma. Redox-cycling staining and reaction with carbonyl reagents were consistent with the presence of a quinone cofactor, possibly topa quinone. The enzyme was also shown to contain two mol of Cu/mol of enzyme and removal of half of this bound copper resulted essentially in complete inhibition of enzyme activity. In contrast to the reported behaviour of the SSAO enzymes from plasma, the bovine lung enzyme was relatively insensitive to inhibition by cyanide, copper-chelating agents and amiloride. The specificity of the bovine lung enzyme was also narrower than reported for soluble SSAO. It catalysed the oxidative deamination of benzylamine, methylamine, 2-phenylethylamine and histamine but had no significant activity towards dopamine, 5-hydroxytryptamine, tryptamine or tyramine.

Evidence of a coupled mechanism between monoamine oxidase and peroxidase in the metabolism of tyramine by rat intestinal mitochondria

The relationship between monoamine oxidase (EC 1.4.3.4; MAO) and peroxidase (EC 1.11.1.7; POD) in the metabolism of tyramine was investigated using the crude mitochondrial fraction of rat intestine. When tyramine was incubated with mitochondria, the formation of the peroxidase-catalysed oxidation product, 2,2'-dihydroxy-5,5'-bis(ethylamino)diphenyl (dityramine), identified by mass spectrometric analysis, was monitored spectrophotometrically. After an initial lag time, the formation rate of dityramine was linear up to 2 hr, amounting to 17 nmol x hr(-1) x mg protein(-1). A similar value was found for the oxidative deamination of tyramine catalysed by intestinal MAO. Either 10(-3) M clorgyline or 10(-3) M NaCN suppressed this reaction by completely inhibiting MAO or POD, respectively. In the former case, however, addition of H2O2 to the incubation mixture promptly started the reaction. Selective inhibition of MAO-A and MAO-B was achieved with 3 x 10(-7) M clorgyline and 3 x 10(-7) M deprenyl, respectively, and the formation rate of dityramine decreased in a corresponding manner. Preincubation with histamine or spermidine reduced the lag time without affecting the steady-state reaction rate. Higher levels of dityramine were also detected in vivo in rat intestine after oral administration of tyramine. These results indicate that the peroxidase-dependent metabolism of tyramine in the gut may be driven by H2O2 produced by MAO activities and that MAO-A is mainly responsible for this process, as well as for the oxidative deamination of tyramine.

Inhibition of monoamine oxidase A and B activities by imidazol(ine)/guanidine drugs, nature of the interaction and distinction from I2-imidazoline receptors in rat liver

1. I2-Imidazoline sites ([3H]-idazoxan binding) have been identified on monoamine oxidase (MAO) and proposed to modulate the activity of the enzyme through an allosteric inhibitory mechanism (Tesson et al., 1995). The main aim of this study was to assess the inhibitory effects and nature of the inhibition of imidazol(ine)/guanidine drugs on rat liver MAO-A and MAO-B isoforms and to compare their inhibitory potencies with their affinities for the sites labelled by [3H]-clonidine in the same tissue. 2. Competition for [3H]-clonidine binding in rat liver mitochondrial fractions by imidazol(ine)/guanidine compounds revealed that the pharmacological profile of the interaction (2-styryl-2-imidazoline, LSL 61112 > idazoxan > 2-benzofuranyl-2-imidazoline, 2-BFI = cirazoline > guanabenz > oxymetazoline > > clonidine) was typical of that for I2-sites. 3. Clonidine inhibited rat liver MAO-A and MAO-B activities with very low potency (IC50S: 700 microM and 6 mM, respectively) and displayed the typical pattern of competitive enzyme inhibition (lineweaver-Burk plots: increased K(m) and unchanged Vmax values). Other imidazol(ine)/guanidine drugs also were weak MAO inhibitors with the exception of guanabenz, 2-BFI and cirazoline on MAO-A (IC50S: 4-11 microM) and 2-benzofuranyl-2-imidazol (LSL 60101) on MAO-B (IC50: 16 microM). Idazoxan was a full inhibitor although with rather low potency, on both MAO-A and MAO-B isoenzymes (IC50S: 280 microM and 624 microM, respectively). Kinetic analyses of MAO-A inhibition by these drugs revealed that the interactions were competitive. For the same drugs acting on MAO-B the interactions were of the mixed type inhibition (increased K(m) and decreased Vmax values), although the greater inhibitory effects on the apparent value of Vmax/K(m) than on the Vmax value indicated that the competitive element of the MAO-B inhibition predominated. 4. Competition for [3H]-Ro 41-1049 binding to MAO-A or [3H]-Ro 19-6327 binding to MAO-B in rat liver mitochondrial fractions by imidazol(ine)/guanidine compounds revealed that the drug inhibition constants (Ki values) were similar to the IC50 values displayed for the inhibition of MAO-A or MAO-B activities In fact, very good correlations were obtained when the affinities of drugs at MAO-A or MAO-B catalytic sites were correlated with their potencies in inhibiting MAO-A (r = 0.92) or MAO-B (r = 0.99) activity. This further suggested a direct drug interaction with the catalytic sites of MAO-A and MAO-B isoforms. 5. No significant correlations were found when the potencies of imidazol(ine)/guanidine drugs at the high affinity site (pKiH, nanomolar range) or the low-affinity site (pKiL, micromolar range) of I2-imidazoline receptors labelled with [3H]-clonidine were correlated with the pIC50 values of the same drugs for inhibition of MAO-A or MAO-B activity. These discrepancies indicated that I2-imidazoline receptors are not directly related to the site of action of these drugs on MAO activity in rat liver mitochondrial fractions. 6. Although these studies cannot exclude the presence of additional binding sites on MAO that do not affect the activity of the enzyme, they would suggest that I2-imidazoline receptors represent molecular species that are distinct from MAO.

Inhibition of bovine lung semicarbazide-sensitive amine oxidase (SSAO) by some hydrazine derivatives

Microsomal semicarbazide-sensitive amine oxidase (SSAO) from bovine lung was shown to be inhibited by a number of hydrazine derivatives, but the mechanisms of inhibition were found to differ. Hydralazine behaved as an irreversible and partially time-dependent inhibitor with an IC50 value of 1 microM under the conditions used. Phenylhydrazine was found to be a potent irreversible inhibitor of SSAO (IC50 30 nM). Semicarbazide behaved as a specific irreversible inhibitor (active-site-directed irreversible inhibitor) in first forming a non-covalent enzyme-semicarbazide complex (with a Ki value of 85 microM), which then reacted to give an irreversibly inhibited enzyme species in a reaction defined by the first-order rate constant k2 = 0.065 min-1. Phenelzine behaved as a reversible inhibitor, but dialysis at 37 degrees C was found to be necessary to obtain full recovery of enzyme activity. The dependence of inhibition on phenelzine concentration was complex and consistent with multiple binding sites for this inhibitor. This diversity in the action of a family of compounds with the same functional group must be taken into account in attempts to design more specific inhibitors of this enzyme.

Kinetic studies of N-allenic analogues of tryptamine as monoamine oxidase inhibitors

A series of N-allenic analogues of tryptamine in which the side chain is located at the 2 position of the indole ring, but which differed in the ring and side-chain nitrogen substituents, were assayed kinetically as MAO A and MAO B inhibitors. All the compounds studied were mechanism-based inhibitors. The kinetic constants of each inhibition step Ki and ki, were determined for both MAO A and B. The data obtained indicated that these allenic derivatives show a greater selectivity and potency towards MAO A as inhibitors than the corresponding acetylenic derivatives.

Inhibition of monoamine oxidase from bovine retina by beta-carbolines

The behaviour of some beta-carboline derivatives as inhibitors of monoamine oxidase has been studied in bovine retina. Inhibition was found not to show any significant time dependence. Di- and tetrahydro-beta-carbolines were shown to behave as reversible and competitive inhibitors. In contrast, the fully unsaturated beta-carbolines harmane, harmine and harmaline, which showed deviation from linearity at high substrate concentrations, behaved as tight-binding inhibitors. In these cases, the concentration of the enzyme and the inhibitor were of the same order. This was confirmed by the Ki values for these compounds in the nanomolar concentration range. Consistent with this was that inhibition was only partly reversed by dialysis for 18 h at 4 degrees C, although complete reversal was observed after dialysis for the same period at 37 degrees C. Structure-activity relationships indicated that substitution of a methoxy group at the C7 position of the aromatic ring is determinant for this tight-binding behaviour; a substitution of this group at the C6 position greatly reduced inhibition. Since beta-carbolines have been reported to be formed endogenously, this suggests that they might have important physiological actions on monoamine oxidase activity in-vivo. In contrast, all the beta-carbolines investigated in this study had low potencies as inhibitors of monoamine oxidase B.