Semicarbazide-sensitive amine oxidase: current status and perspectives

Semicarbazide-sensitive amine-oxidase (SSAO) is present in various human tissues and in plasma. Oxidative deamination of short-chain aliphatic amines is catalyzed by this enzyme to afford the corresponding aldehydes, ammonia and hydrogen peroxide. Methylamine and aminoacetone have been recognized to be physiological substrates for SSAO. There are several pathological states where increased serum SSAO activity have been found, such as diabetes mellitus, congestive heart failure, multiple types of cerebral infarction, uraemia, and hepatic cirrhosis. The role of SSAO in pathophysiology of diabetes has been most extensively investigated. The elevated formation of the potentially cytotoxic products of the enzyme may contribute to the endothelial injury of blood vessels, resulting in the early development of severe atherosclerosis; it may also contribute to the pathogenesis of diabetic angiopathy. It is now suggested that SSAO inhibitors may prevent the development of atherosclerosis and diabetic complications as well. Inhibitors can be conveniently subdivided into the main groups of hydrazine derivatives, arylalkylamines, propenyl- and propargylamines, oxazolidinones, and haloalkylamines. Of them, aryl(alkyl)hydrazines, and 3-halo-2-phenylallylamines are generally very strong SSAO inhibitors. Most of these inhibitors of SSAO have been originally developed for other purposes, or they are simple chemical reagents with highly reactive structural element(s); these compounds have not been able to fulfil all criteria of high potency, selectivity, and acceptable toxicity. New potent compounds with selectivity and low toxicity are needed, which may prove useful tools for understanding the roles and function of SSAO, or they may even be valuable substances for treatment of various diseases.     

Semicarbazide-sensitive amine oxidase (SSAO): present and future

Although the existence of plasma and tissue-bound semicarbazide-sensitive amine oxidases (SSAOs) has been recognised for a long time, the physiological relevance of these enzymes still remains uncertain. The ability of SSAO to metabolise various aliphatic and aromatic monoamines differs between species, which limits the predictive value of the animal studies for human tissues. SSAO plays a protective role because the oxidative deamination of monoamines reduces their pharmacological activities. However, the products of deamination may be toxic. Several observations indicated that the plasma and tissue SSAO activities differ in certain disease states. It is proposed that selective inhibitors, of low toxicity, might be protective, through inhibiting the formation of the toxic products and the countering the disease-related elevation of SSAO activity. We reported earlier that there was a significant correlation between the serum SSAO activity and severity of atherosclerosis, as well as the intima-media thickness and serum cholesterol levels. Thus SSAO activity might be a clinical marker in the prognostic evaluation of diabetic-vascular complications. Although molecular biological studies are providing more and more reliable knowledge about the enzyme structure, many more studies should be carried out in different disease states are necessary to discover the clinical meaning of the enzyme function.     

Semicarbazide-sensitive amine oxidase/vascular adhesion protein-1: a patent survey

INTRODUCTION: Vascular adhesion protein-1 (VAP-1)/semicarbazide-sensitive amine oxidase (SSAO) is an adhesion protein involved in leukocyte trafficking and inflammatory processes, with a special amine oxidase activity. Inhibitors have been mainly developed for treating chronic inflammatory disorders. The utility of inhibitors as antiangiogenic agents in ophthalmological and oncological diseases is currently under evaluation. SSAO substrates may mimic several insulin effects, although their utility for the treatment of diabetes is still far from being fully understood. AREAS COVERED: This paper reviews the patent literature of SSAO/VAP-1 inhibitors and substrates, for the period of 1990 - 2010. The current stage of SSAO/VAP-1-interacting agents published in patents is described, along with their chemical structures and pharmacological uses. EXPERT OPINION: SSAO/VAP-1 is a promising anti-inflammatory target. Another important field for therapeutic application of these inhibitors may be ophthalmology, due to their antiangiogenic effects. SSAO substrates might also be of therapeutic value in the treatment of diabetes; however, more extensive research has to be undertaken to validate this approach.     

Semicarbazide-sensitive amine oxidase activity (SSAO) of rat epididymal white adipose tissue

An amine oxidase activity distinguishable from MAO, which is inhibited by carbonyl reagents is present in rat epididymal WAT. This enzyme, referred to as semicarbazide-sensitive amine oxidase (SSAO), appears concentrated in adipose cells. Close homologies between WAT SSAO and the circulating plasma BAO are discussed.     

Semicarbazide-sensitive amine oxidase from the smooth muscles of dog aorta and trachea: activation by the MAO-A inhibitor clorgyline.

Semicarbazide-sensitive amine oxidase (SSAO) has been identified in the dog trachea and aorta smooth muscles. The dog SSAO is blocked by hydrazine inhibitors. SSAOs from several different vascular smooth muscle sources, such as the rat and bovine aorta, and human umbilical artery, as well as the bovine plasma, are insensitive to the MAO-A inhibitor clorgyline; the dog SSAO on the other hand is significantly activated by clorgyline. Two methods, i.e. radioenzymatic and fluorometric methods, have been applied to substantiate this clorgyline-induced activation. The activation was detected with respect to the deamination of different substrates, such as benzylamine, beta-phenylethylamine and longer carbon chain aliphatic amines, but not with respect to methylamine. The clorgyline effect is reversible, non-competitive and time-independent; it depends on electrostatic and hydrophobic interactions between clorgyline and hydrophobic regions of the dog SSAO enzyme.     

Monoamine oxidase inhibition and brain amine metabolism after oral treatment with toloxatone in the rat

Rats were administered toloxatone 100 mg kg^?1 p.o., and killed 0·5 to 8 hours later. Toloxatone reversibly inhibited type A MAO, but did not affect the activity of type B MAO in whole brain. Cerebral concentrations of noradrenaline, dopamine and 5-hydroxytryptamine were increased after toloxatone, while their metabolite concentrations were reduced. Synaptosomal uptake processes of these amines were not altered by toloxatone.     

Monoamine oxidase inhibition and brain amine metabolism after oral treatment with toloxatone in the rat.

Rats were administered toloxatone 100 mg kg-1 p.o., and killed 0.5 to 8 hours later. Toloxatone reversibly inhibited type A MAO, but did not affect the activity of type B MAO in whole brain. Cerebral concentrations of noradrenaline, dopamine and 5-hydroxytryptamine were increased after toloxatone, while their metabolite concentrations were reduced. Synaptosomal uptake processes of these amines were not altered by tolaxatone.     

Semicarbazide-sensitive amine oxidase/vascular adhesion protein 1: recent developments concerning substrates and inhibitors of a promising therapeutic target

SSAO/VAP-1 is not only involved in the metabolism of biogenic and xenobiotic primary amines and in the production of metabolites with cytotoxic effects or certain physiological actions, but also plays a role, for example, as an adhesion molecule, in leukocyte trafficking, in regulating glucose uptake and in adipocyte homeostasis. Interest in the enzyme has been stimulated by the findings that the activities of the SSAOs are altered (mostly increased) in various human disorders, including diabetes, congestive heart failure, liver cirrhosis, Alzheimer's disease and several inflammatory diseases, although the underlying causes are often unknown. On the basis of their insulin-mimicking effect, SSAO substrates are possibly capable of ameliorating metabolic changes in diabetes, while SSAO inhibitors (somewhat of a contradiction) are of potential benefit in preventing diabetes complications, atherosclerosis and oxidative stress contributing to several disorders or modulating inflammation, and hence may be of substantial therapeutic value. Great efforts have been made to develop novel compounds which may lead to future drugs useful in therapy, based on their effects on SSAO/VAP-1, and some of the results relating to novel substrates and inhibitors are surveyed in the present review.     

Developmental vasculotoxicity associated with inhibition of semicarbazide-sensitive amine oxidase

The endogenous substrate(s) and physiological function(s) of semicarbazide-sensitive amine oxidase (SSAO), a group of enzymes exhibiting highest activity in vascular smooth muscle cells of the mammalian aortic wall, remain undetermined. This study examines the pathophysiological effects in the thoracic aortic wall resulting from specific in vivo SSAO inhibition. Weanling Sprague-Dawley rats were treated acutely or chronically with either semicarbazide hydrochloride or the allylamine derivatives MDL-72274 or MDL-72145 (Marion Merrell Dow Research Institute, Cincinnati, OH). Treatment with these compounds produced acute (6 and 24 h) and chronic (21 day) lowering of SSAO activity in aorta and lung with little effect on the activity of the vital matrix-forming enzyme, lysyl oxidase, in aortas of chronically treated animals. Chronic SSAO inhibition produced lesions consisting of striking disorganization of elastin architecture within the aortic media accompanied by degenerative medial changes and metaplastic changes in vascular smooth muscle cells. No significant difference in the total weight of dry, lipid-extracted aortic elastin and collagen components were observed between chronically SSAO inhibited and control animals. However, the amount of mature elastin was lowered and mature collagen was raised in the aortas of animals treated chronically with semicarbazide. Descending thoracic aortic rings isolated from chronically SSAO-inhibited animals had larger cross-sectional diameters (i.e., exhibited dilation) when compared to corresponding rings from control animals. This study demonstrates that developmental toxicity, characterized by striking vascular lesions and dilated thoracic aortas, can result from specific in vivo SSAO inhibition, suggesting a role for SSAO in connective tissue matrix development and maintenance, and specifically in the development of normal elastin.     

Semicarbazide-sensitive amine oxidase (SSAO) of the rat aorta. Interactions with some naturally occurring amines and their structural analogues

The influence of a number of naturally occurring amines and their structural analogues has been examined on the metabolism of radiolabelled benzylamine (BZ) by the membrane bound semicarbazide-sensitive amine oxidase (SSAO) of the rat aorta. Only primary monoamines were effective in reducing the deamination of BZ. In the phenylethylamine series, addition of hydroxyl groups to the benzene ring decreased their potency as inhibitors while addition of a hydroxyl group at the beta position increased the inhibitory potency. Stereoselectivity of action was shown with octopamine, the L-isomer being the more active form. Kinetic analysis of these interactions showed predominantly competitive inhibition and kynuramine had the lowest Ki of 5.4 microM. The aliphatic monoamines, isoamylamine and isobutylamine both competed with BZ. 5-Hydroxytryptamine (5-HT) was the only amine that inhibited non-competitively. Direct evidence for metabolism by SSAO of some of the competing amines such as isoamylamine, phenylethylamine, tyramine and tryptamine was obtained by fluorimetric or radiochemical assays. The inhibitors clorgyline and (E)-2-(3',4'-dimethoxyphenyl)-3-fluoroallylamine (MDL 72145) were used to characterise the amine oxidase activity responsible for the deamination. Octopamine and phenylethanolamine (PeOH) were not SSAO substrates and inhibited BZ metabolism in the fluorimetric assay. It is possible that the activity of SSAO is controlled by octopamine released from sympathetic nerve endings or 5-HT released from platelets.     

Substances inhibiting amine oxidases. III. Synthesis and amine oxidase inhibition of imidazoquinoline derivatives

The activity of copper and FAD dependent amine oxidases was tested with some derivatives of 3H-imidazo[4,5-h]quinoline and its isomers 3H-imidazo[4,5-f]quinoline, the chemistry of which is described in the literature (1), and Ki calculated. The methyl derivative of 3H-imidazo[4,5-f]quinoline was found to activate the copper bovine serum enzyme, but inhibits the FAD mitochondrial enzyme.     

Enhancement of hydralazine hypotension by low doses of isoniazid. Possible role of semicarbazide-sensitive amine oxidase inhibition

The influence of pretreatment with 1 through 300 mg/kg ip of isoniazid (ISO) on blood pressure and heart rate responses to 0.1 mg/kg iv of hydralazine (HYD) was assessed in rats anesthetized with chloralose--urethane. HYD hypotension was significantly enhanced by ISO at doses between 3 and 300 mg/kg ip. Heart rate was not influenced by HYD in control or pretreated animals. Depressor responses to 0.2 mg/kg iv of pinacidil (PIN) were also potentiated by ISO at 100 and 300, but not at 30 mg/kg. Similarly, ISO decreased cerebral gamma-aminobutyric acid (GABA) at the two highest doses; 30 mg/kg was without effect. Pretreatment of rats with ISO at 1 through 300 mg/kg failed to influence HYD-induced relaxation of aortic rings. These results were interpreted as indicating that potentiation of HYD hypotension by high doses of ISO is not specific for that vasodilator and is related to decreased cerebral GABA, as postulated previously. Lower doses could specifically potentiate the HYD-induced hypotensive effect by inhibition of semicarbazide-sensitive amine oxidase (SSAO), since both ISO and HYD are potent inhibitors of this enzyme. In support of this hypothesis, the SSAO inhibitors, benserazide (100 mg/kg ip) and mexiletine (50 mg/kg ip), were also found to enhance HYD hypotension.     

The inhibition of amine oxidase and the central stimulating action of the stereoisomeric amphetamines and 1-phenylethylamines

The stereoisomers of amphetamine and 1-phenylethylamine have been studied in the rat both as central stimulants and as inhibitors of amine oxidase from brain, liver, and kidney. There was no correlation between these two effects; thus it is unlikely that the central stimulating action of amphetamine is due to inhibition of amine oxidase.     

Further evidence for suicide inhibition of semicarbazide-sensitive amine oxidase in guinea pig lung by 2-bromoethylamine

The inhibitory effects of 2-bromoethylamine (2-BEA), a derivative of ethylamine, on guinea pig lung semicarbazide-sensitive amine oxidase (SAO) have been studied. Preincubation with 2-BEA time-dependently inhibited SSAO activity. The mode of the initial phase of inhibition was competitive, with a Ki value of 52 microM. After preincubation at 37 degrees C for 2 h, the inhibition was noncompetitive and irreversible, as there was no recovery of SSAO activity by dilution of the inhibited samples. Kinetic analyses confirmed previous results with rat lung SSAO that 2-BEA is a suicide SSAO inactivator with a dissociation constant of 42 microM. This latter value is similar to that of the Ki value (52 microM) for the reversible phase of inhibition by 2-BEA. Addition of the nucleophilic compound 2-mercaptoethanol could not reduce the SSAO inhibition, indicating that inactivation could not be prevented by trapping the enzymatic reaction product from 2-BEA. This finding clearly indicates that the reaction product should not diffuse away from its site of genesis and agrees with one of the characteristics of suicide inhibitors. This conclusively excludes the possibility of an affinity-labeling mechanism.     

Age-related changes in monoamine oxidase and semicarbazide-sensitive amine oxidase activities of rat aorta

Aorta MAO-A and SSAO activities were measured on young (3 months) and old (23-26 months) rats. A significant decrease (30-40%) in SSAO activity was found with benzylamine as substrate and the decrease was due to a reduction in Vmax. No significant changes in MAO-A activity were found in the aorta of old rats. beta-PEA is oxidized mainly by SSAO in rat aorta. However, the significance of this is unclear since the physiological role of that enzyme remains unknown.