Semicarbazide-sensitive amine oxidase and monoamine oxidase in rat brain microvessels, meninges, retina and eye sclera

SSAO/VAP-1 in Cerebrovascular Disorders: A Potential Therapeutic Target for Stroke and Alzheimer's Disease

The semicarbazide-sensitive amine oxidase (SSAO), also known as vascular adhesion protein-1 (VAP-1) or primary amine oxidase (PrAO), is a deaminating enzyme highly expressed in vessels that generates harmful products as a result of its enzymatic activity. As a multifunctional enzyme, it is also involved in inflammation through its ability to bind and promote the transmigration of circulating leukocytes into inflamed tissues. Inflammation is present in different systemic and cerebral diseases, including stroke and Alzheimer’s disease (AD). These pathologies show important affectations on cerebral vessels, together with increased SSAO levels. This review summarizes the main roles of SSAO/VAP-1 in human physiology and pathophysiology and discusses the mechanisms by which it can affect the onset and progression of both stroke and AD. As there is an evident interrelationship between stroke and AD, basically through the vascular system dysfunction, the possibility that SSAO/VAP-1 could be involved in the transition between these two pathologies is suggested. Hence, its inhibition is proposed to be an interesting therapeutical approach to the brain damage induced in these both cerebral pathologies.

A Fluorogenic Probe for Ultrafast and Reversible Detection of Formaldehyde in Neurovascular Tissues

Formaldehyde (FA) is endogenously produced in live systems and has been implicated in a diverse array of pathophysiological processes. To disentangle the detailed molecular mechanisms of FA biology, a reliable method for monitoring FA changes in live cells would be indispensable. Although there have been several fluorescent probes reported to detect FA, most are limited by the slow detection kinetics and the intrinsic disadvantage of detecting FA in an irreversible manner which may disturb endogenous FA homeostasis. Herein we developed a coumarin-hydrazonate based fluorogenic probe (PFM) based on a finely-tailored stereoelectronic effect. PFM could respond to FA swiftly and reversibly. This, together with its desirable specificity and sensitivity, endows us to track endogenous FA in live neurovascular cells with excellent temporal and spatial resolution. Further study in the brain tissue imaging showed the first direct observation of aberrant FA accumulation in cortex and hippocampus of Alzheimer's mouse model, indicating the potential of PFM as a diagnostic tool.

Ocular non-P450 oxidative, reductive, hydrolytic, and conjugative drug metabolizing enzymes

Metabolism in the eye for any species, laboratory animals or human, is gaining rapid interest as pharmaceutical scientists aim to treat a wide range of so-called incurable ocular diseases. Over a period of decades, reports of metabolic activity toward various drugs and biochemical markers have emerged in select ocular tissues of animals and humans. Ocular cytochrome P450 (P450) enzymes and transporters have been recently reviewed. However, there is a dearth of collated information on non-P450 drug metabolizing enzymes in eyes of various preclinical species and humans in health and disease. In an effort to complement ocular P450s and transporters, which have been well reviewed in the literature, this review is aimed at presenting collective information on non-P450 oxidative, hydrolytic, and conjugative ocular drug metabolizing enzymes. Herein, we also present a list of xenobiotics or drugs that have been reported to be metabolized in the eye.

Osmolytic Effect of Sucrose on Thermal Denaturation of Pea Seedling Copper Amine Oxidase

Protein stability is a subject of interest by many researchers. One of the common methods to increase the protein stability is using the osmolytes. Many studies and theories analyzed and explained osmolytic effect by equilibrium thermodynamic while most proteins undergo an irreversible denaturation. In current study we investigated the effect of sucrose as an osmolyte on the thermal denaturation of pea seedlings amine oxidase by the enzyme activity, fluorescence spectroscopy, circular dichroism, and differential scanning calorimetry. All experiments are in agreement that pea seedlings amine oxidase denaturation is controlled kinetically and its kinetic stability is increased in presence of sucrose. Differential scanning calorimetry experiments at different scanning rates showed that pea seedlings amine oxidase unfolding obeys two-state irreversible model. Fitting the differential scanning calorimetry data to two-state irreversible model showed that unfolding enthalpy and T *, temperature at which rate constant equals unit per minute, are increased while activation energy is not affected by increase in sucrose concentration. We concluded that osmolytes decrease the molecular oscillation of irreversible proteins which leads to decline in unfolding rate constant.

Oxidative Stress Plays an Important Role in the Pathogenesis of Drug-Induced Retinopathy

Several pharmaceutical agents have been associated with rare but serious retinopathies, some resulting in blindness. Little is known of the mechanism(s) that produce these injuries. Mechanisms proposed thus far have not been embraced by the medical and scientific communities. However, preclinical and clinical data indicate that oxidative stress may contribute substantially to iatrogenic retinal disease. Retinal oxidative stress may be precipitated by the interaction of putative retinal toxins with the ocular redox system. The retina, replete with cytochromes P450 and myeloperoxidase, may serve to activate xenobiotics to oxidants, resulting in ocular injury. These activated agents may directly form retinal adducts or may diminish ocular reduced glutathione concentrations. Data are reviewed that suggest that indomethacin, tamoxifen, thioridazine, and chloroquine all produce retinopathies via a common mechanism—they produce ocular oxidative stress.

Formaldehyde Stress Responses in Bacterial Pathogens

Formaldehyde is the simplest of all aldehydes and is highly cytotoxic. Its use and associated dangers from environmental exposure have been well documented. Detoxification systems for formaldehyde are found throughout the biological world and they are especially important in methylotrophic bacteria, which generate this compound as part of their metabolism of methanol. Formaldehyde metabolizing systems can be divided into those dependent upon pterin cofactors, sugar phosphates and those dependent upon glutathione. The more prevalent thiol-dependent formaldehyde detoxification system is found in many bacterial pathogens, almost all of which do not metabolize methane or methanol. This review describes the endogenous and exogenous sources of formaldehyde, its toxic effects and mechanisms of detoxification. The methods of formaldehyde sensing are also described with a focus on the formaldehyde responsive transcription factors HxlR, FrmR, and NmlR. Finally, the physiological relevance of detoxification systems for formaldehyde in bacterial pathogens is discussed.

Monoamine oxidase inhibitors and neuroprotection: a review

Monoamine oxidase inhibitors have been available for more than 50 years, initially developed as antidepressants but currently used in a variety of psychiatric and neurological conditions. There has been a recent surge of interest in monoamine oxidase inhibitors because of their reported neuroprotective and/or neurorescue properties. Interestingly, it seems that often these properties are independent of their ability to inhibit monoamine oxidase. This review article presents an overview of the neuroprotective/neurorescue properties of these multifaceted drugs and focuses on phenelzine, (-)-deprenyl, rasagiline, ladostigil, tranylcypromine, moclobemide, and clorgyline and their possible neuroprotective mechanisms.

Formate generated by cellular oxidation of formaldehyde accelerates the glycolytic flux in cultured astrocytes

Formaldehyde is a neurotoxic compound that can be endogenously generated in the brain. Because astrocytes play a key role in metabolism and detoxification processes in brain, we have investigated the capacity of these cells to metabolize formaldehyde using primary astrocyte-rich cultures as a model system. Application of formaldehyde to these cultures resulted in the appearance of formate in cells and in a time-, concentration- and temperature-dependent disappearance of formaldehyde from the medium that was accompanied by a matching extracellular accumulation of formate. This formaldehyde-oxidizing capacity of astrocyte cultures is likely to be catalyzed by alcohol dehydrogenase 3 and aldehyde dehydrogenase 2, because the cells of the cultures contain the mRNAs of these formaldehyde-oxidizing enzymes. In addition, exposure to formaldehyde increased both glucose consumption and lactate production by the cells. Both the strong increase in the cellular formate content and the increase in glycolytic flux were only observed after application of formaldehyde to the cells, but not after treatment with exogenous methanol or formate. The accelerated lactate production was not additive to that obtained for azide, a known inhibitor of complex IV of the respiratory chain, and persisted after removal of formaldehyde after a formaldehyde exposure for 1.5 h. These data demonstrate that cultured astrocytes efficiently oxidize formaldehyde to formate, which subsequently enhances glycolytic flux, most likely by inhibition of mitochondrial respiration.

Formaldehyde stimulates Mrp1-mediated glutathione deprivation of cultured astrocytes

Formaldehyde (Fal) is an environmental neurotoxin that is also endogenously produced in brain. Since the tripeptide glutathione (GSH) plays an important role in detoxification processes in brain cells, we have investigated the consequences of a Fal exposure on the GSH metabolism of brain cells, using astrocyte-rich primary cultures as model system. Treatment of these cultures with Fal resulted in a rapid time- and concentration-dependent depletion of cellular GSH and a matching increase in the extracellular GSH content. Exposure of astrocytes to 1mm Fal for 3h did not compromise cell viability but almost completely deprived the cells of GSH. Half-maximal deprivation of cellular GSH was observed after application of 0.3mm Fal. This effect was rather specific for Fal, since methanol, formate or acetaldehyde did not affect cellular GSH levels. The Fal-stimulated GSH loss from viable astrocytes was completely prevented by semicarbazide-mediated chemical removal of Fal or by the application of MK571, an inhibitor of the multidrug resistance protein 1. These data demonstrate that Fal deprives astrocytes of cellular GSH by a multidrug resistance protein 1-mediated process.

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.

Potential implications of endogenous aldehydes in ?-amyloid misfolding, oligomerization and fibrillogenesis

Aldehydes are capable of inducing protein cross-linkage. An increase in aldehydes has been found in Alzheimer's disease. Formaldehyde and methylglyoxal are produced via deamination of, respectively, methylamine and aminoacetone catalyzed by semicarbazide-sensitive amine oxidase (SSAO, EC 1.4.3.6. The enzyme is located on the outer surface of the vasculature, where amyloidosis is often initiated. A high SSAO level has been identified as a risk factor for vascular disorders. Serum SSAO activity has been found to be increased in Alzheimer's patients. Malondialdehyde and 4-hydroxynonenal are derived from lipid peroxidation under oxidative stress, which is also associated with Alzheimer's disease. Aldehydes may potentially play roles in beta-amyloid aggregation related to the pathology of Alzheimer's disease. In the present study, thioflavin-T fluorometry, dynamic light scattering, circular dichroism spectroscopy and atomic force microscopy were employed to reveal the effect of endogenous aldehydes on beta-amyloid at different stages, i.e. beta-sheet formation, oligomerization and fibrillogenesis. Formaldehyde, methylglyoxal and malondialdehyde and, to a lesser extent, 4-hydroxynonenal are not only capable of enhancing the rate of formation of beta-amyloid beta-sheets, oligomers and protofibrils but also of increasing the size of the aggregates. The possible relevance to Alzheimer's disease of the effects of these aldehydes on beta-amyloid deposition is discussed.

Diabetes and semicarbazide-sensitive amine oxidase (SSAO) activity: A review

The enzyme of semicarbazide-sensitive amine oxidase (SSAO) activity has been reported to be elevated in blood from diabetic patients. SSAO are widely distributed in plasma membranes of various tissues and blood plasma. SSAO-mediated production of toxic aldehydes has been proposed to be related to pathophysiological conditions. Cytotoxic metabolites by SSAO may cause endothelial injury and subsequently induce atherosclerosis. The precise physiological functions of SSAO could play an important role in the control of energy balance in adipose tissue. It is possible that the increased SSAO activity in diabetes may be a result of up-regulation due to increase of SSAO substrates, such as methylamine or aminoacetone. SSAO could play an important role in the regulation of adipocyte homeostasis. Inhibition of SSAO could be of therapeutic value for treatment of diabetic patient.

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.

Physiological and pathological implications of semicarbazide-sensitive amine oxidase

Semicarbazide-sensitive amine oxidase (SSAO) catalyzes the deamination of primary amines. Such deamination has been shown capable of regulating glucose transport in adipose cells. It has been independently discovered that the primary structure of vascular adhesion protein-1 (VAP-1) is identical to SSAO. VAP-1 regulates leukocyte migration and is related to inflammation. Increased serum SSAO activities have been found in patients with diabetic mellitus, vascular disorders and Alzheimer's disease. The SSAO-catalyzed deamination of endogenous substrates, that is, methylamine and aminoacetone, led to production of toxic formaldehyde and methylglyoxal, hydrogen peroxide and ammonia, respectively. These highly reactive aldehydes have been shown to initiate protein cross-linkage, exacerbate advanced glycation of proteins and cause endothelial injury. Hydrogen peroxide contributes to oxidative stress. 14C-methylamine is converted to 14C-formaldehyde, which then forms labeled long-lasting protein adduct in rodents. Chronic methylamine treatment increased the excretion of malondialdehyde and microalbuminuria, and enhanced the formation of fatty streaks in C57BL/6 mice fed with an atherogenic diet. Treatment with selective SSAO inhibitor reduces atherogenesis in KKAy diabetic mice fed with high-cholesterol diet. Aminoguanidine, which blocks advanced glycation and reduces nephropathy in animals, is in fact more potent at inhibiting SSAO than its effect on glycation. It suggests that SSAO is involved in vascular disorders under certain pathological conditions. Although SSAO has been known for several decades, its physiological and pathological implications are just beginning to be recognized.

Semicarbazide-sensitive amine oxidase in aortic smooth muscle cells mediates synthesis of a methylglyoxal-AGE: implications for vascular complications in diabetes

Semicarbazide-sensitive amine oxidase (SSAO) catalyzes formation of methylglyoxal (MG) from aminoacetone; MG then reacts with proteins to form advanced glycation end products or AGEs. Because of its potential to generate MG, SSAO may contribute to AGE-associated vascular complications of aging and diabetes. We developed a method to measure SSAO activity in bovine aortic smooth muscle cells (BASMC) based on the oxidation of 2',7'-dichlorofluorescin by hydrogen peroxide and horseradish peroxidase. The SSAO activity was completely inhibited by 10 mM semicarbazide. Argpyrimidine is a readily detectable fluorescent product of the reaction between MG and arginine. Cell lysates incubated with aminoacetone formed argpyrimidine in a reaction that was inhibited by 20 mM semicarbazide. Immunostaining of tissue sections showed that aminoacetone-treated rats (normal as well as diabetic) formed more argpyrimidine in aortic smooth muscle than untreated controls. We believe that SSAO can enhance AGE synthesis in the macrovasculature of diabetic individuals by production of MG.

Immunolocalization of semicarbazide-sensitive amine oxidase in human dental pulp and its activity towards serotonin

The semicarbazide-sensitive amine oxidase (EC 1.4.3.6; SSAO) from crude homogenates of human dental pulp was shown to catalyse the oxidative deamination of 5-hydroxytryptamine (serotonin; 5-HT) with a K(m) of 318+/-52 microM. In this respect the human enzyme resembles that in pig dental pulp, but differs from SSAO in all other tissues studied, which are inactive towards 5-HT. A method is described for obtaining intact dental pulp in which the anatomical details are preserved. Extracted teeth are frozen in dry ice and later defrosted rapidly before being fractured in a mechanical vice, facilitating pulp removal. Immunohistochemistry showed SSAO in the odontoblast layer, nerve fibres and blood vessels. The presence of SSAO in nerves in dental pulp appears to be unique. Tryptophan hydroxylase, a key enzyme in 5-HT synthesis, was also demonstrated in nerves and the odontoblast layer of human dental pulp.

Semicarbazide-sensitive amine oxidase (SSAO) in the brain

Semicarbazide-sensitive amine oxidase (SSAO) is widely distributed in almost tissues. However, its presence in brain microvessels is still controversial. The affinity of SSAO towards benzylamine (Bz) is considerably higher than that of monoamine oxidase (MAO). SSAO plays a role in the toxicity of several environmental and endogenous amines. SSAO-mediated production of toxic aldehydes has been proposed to be related to pathophysiological conditions. The most potent of inhibition of SSAO in monkey brain was observed by tricyclic antidepressant drug imipramine, as compared to tetracyclic drug maprotiline or non-cyclic drug nomifensine. An endogenous SSAO modulator in rat brain cytosol after immobilization stress (IMMO) was found and that this inhibitor could be induced by IMMO. SSAO activity in rat brain might be regulated by the level of this inhibitor. Semicarbazide, a SSAO inhibitor, enhances the formation of .OH products of efflux/oxidation due to 1-methyl-4-phenylpyridinium ion (MPP+). The precise physiological functions of SSAO could play an important role in the control of energy balance in adipose tissue. SSAO could play an important role in the regulation of adipocyte homeostasis.

Comparative effects of dehydropirlindole and other compounds on rat brain monoamine oxidase type A

Dehydropirlindole (DHP) is the dehydroderivative of pirlindole, a short-acting inhibitor of monoamine oxidase type A (MAO-A). DHP would be formed in vivo from oxidation of pirlindole by MAO-A. The aim of this work is to compare the inhibitory potency of DHP with three reference compounds: harmaline, befloxatone and clorgyline; the two former are reversible inhibitors and the later is an irreversible inhibitor of MAO-A. Both in vitro and ex vivo assays were performed on rat brain homogenates, and IC50 and ID50 were calculated by a fluorometric method with octopamine as selective MAO-A substrate. In vitro clorgyline and befloxatone were more potent inhibitors than DHP and harmaline with IC50 values of 1.6 and 7.7 nM vs. 40 and 55 nM; ex vivo ID50 values were 1.5 and 32 micromol/kg vs. 41 and 49 micromol/kg. Befloxatone had an ID50/IC50 ratio four to five times higher than DHP and harmaline. Preincubation time experiments did not distinguish befloxatone from DHP and harmaline. In conclusion, this study shows that DHP behaves as a reversible MAO-A inhibitor whose potency is situated between that of befloxatone and harmaline.

Involvement of cerebrovascular semicarbazide-sensitive amine oxidase in the pathogenesis of Alzheimer's disease and vascular dementia

Fibrillary tangles and senile plaques resulting from advanced aggregation of beta-amyloid and other proteins are pathological characteristics of Alzheimer's disease (AD). Cerebral amyloid angiopathy is quite common in AD. In fact, amyloid fibrils fuse to and emanate from the vascular basement membrane. Semicarbazide-sensitive amine oxidase (SSAO), located in outer membranes of vascular smooth muscles and endothelia, catalyzes deamination of methylamine-producing formaldehyde and hydrogen peroxide. SSAO is also involved in lymphocyte adhesion and is up-regulated in response to inflammation. SSAO-mediated generation of formaldehyde can induce protein (i.e. beta-amyloid) cross-linkage, deposition and subsequently plaque formation in the compartment adjacent to the cerebrovessels. Formaldehyde may cause cytotoxicity, which induces inflammation and release of more SSAO, producing a cascade of toxic cycle. Increased SSAO-mediated reaction may be chronically involved in the pathogenesis of vascular dementia and AD.

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.

Endogenous formaldehyde as a potential factor of vulnerability of atherosclerosis: involvement of semicarbazide-sensitive amine oxidase-mediated methylamine turnover

The mouse is known to be highly resistant to atherosclerosis. However, some inbred mouse strains are vulnerable to atherosclerosis when they are fed a high-cholesterol, high-fat diet. Increased deamination of methylamine (MA) and the subsequent production of formaldehyde has been recently shown to be a potential risk factor of atherosclerosis. In the present study semicarbazide-sensitive amine oxidase (SSAO)-mediated MA turnover in C57BL/6 mouse, a strain very susceptible to atherosclerosis, has been assessed in comparison to a moderate, i.e. BALB/c, and resistant, i.e. CD1, mouse strains. Kidney and aorta SSAO activities were found to be significantly increased in C57BL/6 in comparison to BALB/c and CD1 mice. A significant increase of urinary MA and formaldehyde were detected in C57BL/6. [14C]MA following intravenous injection would be quickly metabolized by SSAO. The labeled formaldehyde product would cross link with proteins. C57BL/6 exhibits significantly higher labeled protein adducts than BALB/c and CD1 in response to [14C]MA. The results indicated that mice vulnerable to atherosclerosis possess an increased SSAO-mediated MA turnover. The increase of production of formaldehyde, possibly other aldehydes, may induce endothelial injury or be chronically involved in protein cross-linking and subsequent angiopathy.

Deamination of methylamine and angiopathy; toxicity of formaldehyde, oxidative stress and relevance to protein glycoxidation in diabetes

Semicarbazide-sensitive amine oxidase (SSAO) is located in the vascular smooth muscles, retina, kidney and the cartilage tissues, and it circulates in the blood. The enzyme activity has been found to be significantly increased in blood and tissues in diabetic patients and animals. Methylamine and aminoacetone are endogenous substrates for SSAO. The deaminated products are formaldehyde and methylglyoxal respectively, as well as H2O2 and ammonia, which are all potentially cytotoxic. Formaldehyde and methylglyoxal are cytotoxic towards endothelial cells. Excessive SSAO-mediated deamination may directly initiate endothelial injury and plaque formation, increase oxidative stress, which can potentiate oxidative glycation, and/or LDL oxidation and damage vascular systems. Formaldehyde is also capable of exacerbating advanced glycation, and thus increase the complexity of protein cross-linking. Uncontrolled SSAO-mediated deamination may be involved in the acceleration of the clinical complications in diabetes.

Formation of formaldehyde from adrenaline in vivo; a potential risk factor for stress-related angiopathy

Cardiovascular and cerebrovascular disorders are well known to be associated with stress related behaviors. Stress enhances excretion of adrenaline, which is deaminated by monoamine oxidase and methylamine is formed. This product can be further deaminated by semicarbazide-sensitive amine oxidase (SSAO) and converted to toxic formaldehyde, hydrogen peroxide and ammonia. SSAO is located in the cardiovascular smooth muscles and circulated in the blood. We investigated whether formaldehyde can be derived from adrenaline in vivo. Methylamine was confirmed to be a product of adrenaline catalyzed by type A monoamine oxidase (MAO-A). Irreversible and long-lasting radioactive residual activity was detected in different tissues following administration of 1-[N-methyl-3H]-adrenaline. Such irreversible linkage could be blocked by selective MAO-A or SSAO inhibitors. Endothelial cells are quite sensitive to formaldehyde and relatively resistant to hydrogen peroxide. It is possible that stimulation of adrenaline excretion by chronic stress could increase the levels of circulatory formaldehyde. Such chronic "formaldehyde" stress may be involved in the initiation of endothelial injury and subsequently angiopathy.

Formaldehyde produced endogenously via deamination of methylamine. A potential risk factor for initiation of endothelial injury

Methylamine can be converted by semicarbazide-sensitive amine oxidase (SSAO) to formaldehyde and hydrogen peroxide, which have been proven to be toxic towards cultured endothelial cells. We investigated whether or not these deaminated products from methylamine can exert potentially hazardous toxic effects in vivo. Long lasting residual radioactivity in different tissues was detected following administration of [14C]-methylamine in the mouse. Approximately 10% of the total administered radioactivity could even be detected 5 days after injection of [14C]-methylamine. Eighty percent of the formation of irreversible adducts can be blocked by a highly selective SSAO inhibitor, (E)-2-(4-fluorophenethyl)-3-fluoroallylamine hydrochloride (MDL-72974A). The residual radioactivity was primarily associated with the insoluble tissue components and the soluble macromolecules. Radioactively labelled macromolecules were fragmented following enzymatic proteolysis. Results suggest that the formaldehyde derived from methylamine interacts with proteins in vivo. In the streptozotocin-induced diabetic mice, both SSAO activity and the formation of residual radioactivity were found to be significantly increased in the kidney. Chronic administration of methylamine enhances blood prorenin level, which strongly suggests that uncontrolled deamination of methylamine may be a risk factor for initiation of endothelial injury, and subsequent genesis of atherosclerosis.

Semicarbazide-sensitive amine oxidases: some biochemical properties and general considerations

Semicarbazide-sensitive amine oxidases with a high affinity for benzylamine (Bz.SSAO) (E.C.1.4.3.6) have been biochemically described in many mammalian tissues (adipose tissue, lung, heart, blood vessels). The enzymic activity appears to be expressed by mesenchymal cells (fibroblasts, adipocytes, smooth muscles). Although the physiological role of this enzymic activity is still unclear, some possible physiological substrates such as histamine are discussed. Some enzymes of this class (SSAO) have been purified. They share many similarities, among which are that they contain copper and a carbonyl active site. The nature of the organic cofactor of these enzymes is discussed and data are presented which have identified pyridoxal in pig kidney diamine oxidase and in pig plasma benzylamine oxidase by gas chromatography-mass spectrometry.

Substrate-specificity of mammalian tissue-bound semicarbazide-sensitive amine oxidase

Although the existence of a membrane-bound (probably plasmalemmal) semicarbazide-sensitive amine oxidase (SSAO) is well established in various mammalian tissues, and especially within vascular smooth muscle, its importance and the possible consequences of its metabolism of certain physiological and xenobiotic amines in vivo are under continuing investigation. In this respect, there are major species-related differences in substrate specificity determined in vitro, not only towards the synthetic amine benzylamine, but also towards some other aromatic amines (e.g. tyramine, tryptamine, 2-phenylethylamine, dopamine, histamine) which are possible endogenous substrates. Inhibition of SSAO can potentiate the pharmacological activity of some amines in isolated tissue (e.g. blood vessel) preparations from some species. Recent evidence has accumulated that SSAO may also be involved in metabolizing endogenous aliphatic amines such as methylamine and aminoacetone, focussing attention on the fact that the aldehyde products (formaldehyde and methylglyoxal, respectively) are potentially cytotoxic agents. Indeed, SSAO has been implicated in experimental models of cardiovascular toxicity involving conversion of the industrial aliphatic amine allylamine to acrolein. In summary, metabolism by SSAO may reduce the physiological/pharmacological effects of some amines, but the resulting metabolites (aldehydes, H2O2) may also have important actions.

Some aspects of the pathophysiology of semicarbazide-sensitive amine oxidase enzymes

The widespread distribution of enzymes classed as semicarbazide-sensitive amine oxidases (SSAO enzymes) throughout a very wide range of eukaryotic as well as prokaryotic organisms encourages the aspirations of those who wish to demonstrate physiological, pathological or pharmacological importance. Such enzymes are found in several tissues of mammals, both freely soluble, as in blood plasma, and membrane-bound, for example, in smooth muscle and adipose tissue. While they are capable of deaminating many amines with the production of an aldehyde and hydrogen peroxide, doubt still surrounds the identity of the most important endogenous substrates for these enzymes. At present, methylamine and aminoacetone appear to head the list of candidates. The possibility that SSAO enzymes can convert amine substrates to highly toxic metabolites is illustrated by the production of acrolein from the xenobiotic amine, allylamine and formaldehyde and methylglyoxal from methylamine and aminoacetone, respectively. Activities of SSAO enzymes may be influenced by physiological changes, such as pregnancy or pathologically by disease states, including diabetes, tumours and burns. Increased deamination of aminoacetone by tissue and plasma SSAO enzymes as a result of its increased production from L-threonine in conditions such as exhaustion, starvation and diabetes mellitus may be harmful. Such dangers could be mitigated either physiologically by a compensatory reduction in SSAO activity or pharmacologically by treatment with inhibitors of SSAO.