Physiological and pathological influences on serum monoamine oxidase level. Effect of age, sex, contraceptive steroids and diabetes mellitus

Chemical probes for drug-induced liver injury imaging

Drug-induced liver injury (DILI) has been a long-standing concern of modern medicine, and the single most frequent reason for drug nonapprovals and postapproval restrictions or withdrawals. Chemical probes for early diagnosis of DILI has triggered a tremendous interest in the field of molecular imaging. In this review, we make a brief summary of the recently developed chemical probes and their applications in DILI imaging with special attention to the design of chemical probes, mechanism of their actions and their performances in DILI imaging.

A "Double-Locked" and enzyme-activated molecular probe for accurate bioimaging and hepatopathy differentiation

Molecular probes activated by a single enzyme have been extensively used in bioimaging and disease diagnosis; however, imaging and identification in an accurate manner remains a challenge for such probes. Here, based on the specificity of enzyme recognition, we engineered a "double-locked" and enzyme-activated molecular probe (NML) for accurate bioimaging and hepatopathy differentiation. Triggered by the successive reactions with leucine aminopeptidase (LAP, first "key") and monoamine oxidase (MAO, second "key"), the emissive fluorophore (NF) was released. NML can be activated only in the presence of both LAP and MAO and can be silenced when either enzyme is inhibited. Benefiting from the "double-locked" strategy, NML showed higher accuracy for imaging of drug-induced liver injury (DILI) than the "single-locked" probe. With serum testing, NML showed significant differences in mouse models of both CCl₄-induced liver cirrhosis and DILI. Significantly, NML can be applied to accurately distinguish serum samples from clinical patients with different hepatopathies. Our smart molecular probe may hold great potential for hepatopathy diagnosis and clinical transformation.

Semicarbazide-sensitive amine oxidase and kidney disease

With better understanding of the molecular mechanisms underpinning chronic kidney disease, the roles of inflammation and fibrosis are becoming increasingly inseparable. The progression of renal disease is characterized by pathomorphological changes that consist of early inflammatory responses followed by tubulointerstitial fibrosis, tubular atrophy, and glomerular and vascular sclerosis. Currently available therapies that reduce hypertension, proteinuria, hyperglycemia, and interruption of the renin-angiotensin-aldosterone system are at best only partially effective. Hence, there remains a need to explore agents targeting nonrenin-angiotensin-aldosterone system pathways. In this review, we discuss mechanistic aspects in the physiological and pathological role of semicarbazide-sensitive amine oxidase, a protein enzyme involved in cellular trafficking and inflammation, with respect to the kidney. We explore the evidence for the use of semicarbazide-sensitive amine oxidase inhibitors as potential agents in renal fibrosis to delay the onset and progression of chronic kidney disease.

Soluble vascular adhesion protein-1 correlates with cardiovascular risk factors and early atherosclerotic manifestations

OBJECTIVE

Vascular adhesion protein-1 is an endothelial enzyme that regulates leukocyte traffic and contributes to vascular damage in animal models. The relations of soluble vascular adhesion protein-1 (sVAP-1) with cardiovascular risk factors and markers of subclinical atherosclerosis at a population level have not been studied.

METHODS AND RESULTS

We developed a new high-throughput method and measured sVAP-1 activities in serum of 2183 persons (The Cardiovascular Risk in Young Finns Study). In women, sVAP-1 activity correlated indirectly with body mass index (r=-0.15, P<0.0001), triglycerides (r=-0.13, P<0.0001), C-reactive protein (r=-0.23; P<0.0001), and brachial artery flow-mediated vasodilatation (r=-0.076, P=0.0089) and directly with carotid plaques (r=0.066, P=0.023). None of these correlations was significant in men. In women, all these univariate correlations remained significant after adjustment for body mass index, and direct correlations with LDL-cholesterol (r=0.094, P=0.0014) and carotid intima-media thickness (r=0.075, P=0.010) became evident. In men, sVAP-1 activity associated directly with glucose (r=0.074, P=0.020), intima-media thickness (r=0.072, P=0.025), metabolic syndrome (P=0.016), and type 1 (P=0.0002) and type 2 (P<0.0001) diabetes. In multivariable analyses, sVAP-1 activity was an independent determinant of carotid intima-media thickness (P=0.0072) and plaques [odds ratio 1.71 (95% confidence interval 1.07-2.72, P=0.025] in women, but not in men.

CONCLUSIONS

sVAP-1 activity correlates directly with intima-media thickness and carotid plaques in general population and may play a role in the pathophysiology of preclinical atherosclerosis.

Origins of serum semicarbazide-sensitive amine oxidase

Semicarbazide-sensitive amine oxidases (SSAO) are enzymes that are capable of deaminating primary amines to produce aldehyde, ammonia, and hydrogen peroxide. This activity has been associated with vascular adhesion protein-1 (VAP-1) and is found in the serum, endothelium, adipose, and smooth muscle of mammals. Circulating SSAO activity is increased in congestive heart failure, diabetes, and inflammatory liver diseases. To investigate the origin of circulating SSAO activity, two transgenic mouse models were created with full-length human VAP-1 (hVAP-1) expressed on either endothelial (mTIEhVAP-1) or adipose tissues (aP2hVAP-1), with tie-1 and adipocyte P2 promoters, respectively. Under normal conditions a circulating form of hVAP-1 was found at high levels in the serum of mice with endothelium-specific expression and at low levels in the serum of mice with adipose specific expression. The level of circulating hVAP-1 in the transgenic mice varied with gender, transgene zygosity, diabetes, and fasting. Serum SSAO activity was absent from VAP-1 knockout mice and endothelial cell-specific expression of human VAP-1 restored SSAO activity to the serum of VAP-1 knockout mice. Together, these experiments show that in the mouse VAP-1 is the only source of serum SSAO, that under physiological conditions vascular endothelial cells can be a major source of circulating VAP-1 protein and SSAO, and that serum VAP-1 can originate from both endothelial cells and adipocytes during experimental diabetes. An increased endothelial cell capacity for lymphocyte binding and altered expression of redox-sensitive proteins was also associated with the mTIEhVAP-1 transgene.

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.

Plasma semicarbazide-sensitive amine oxidase in human (patho)physiology

Semicarbazide-sensitive amine oxidases (SSAO) are widely distributed enzymes, with as yet not fully elucidated functions and roles, present in many tissues but also circulating in plasma. The enzyme also functions as an adhesion molecule, the vascular adhesion protein-1. In healthy humans, plasma SSAO activity is constant from birth until 16 years of age, when it drops to lower values, gradually increasing again at advanced ages. When measuring SSAO activity, care should be taken to ensure proper preparation and storage conditions, and it should be realized that quite a few drugs unintentionally are good inhibitors, and sometimes even substrates, of SSAO. Under normal conditions SSAO activity is constant and inter-individual variation is small. In various pathophysiological conditions plasma SSAO activities are increased, most notably in diabetes mellitus (both type I and type II), in congestive heart failure and in cirrhotic liver inflammation. In patients with other vascular and inflammatory diseases plasma SSAO is normal, while it is low in children with congenital lung diseases. Interpretation of these changes is speculative, since source and regulation of plasma SSAO are as yet unknown. However, in two situations where the disease-causing process was ended (transplantation, delivery), plasma SSAO returned to normal. Many questions remain to be answered.

Follow-up of plasma semicarbazide-sensitive amine oxidase activity and retinopathy in Type 2 diabetes mellitus

Plasma activity of the enzyme semicarbazide-sensitive amine oxidase (SSAO) is high in diabetes. Production of angiotoxic substances (an aldehyde, hydrogen peroxide, and ammonia) in vessel walls is catalysed by SSAO, suggesting a role for SSAO in the development of complications of diabetes. The objective of the present study was to follow up plasma SSAO activity (measured radiometrically), HbA(1c) (using ion exchange chromatography), and retinopathy (by fundus photography) after 2.8 years, in 34 patients with Type 2 diabetes. We also measured urinary levels of an SSAO substrate, methylamine, by fluorometric high-performance liquid chromatography (HPLC). As at baseline, plasma SSAO activity was now higher in subjects with retinopathy (mean 19.5) than in subjects without retinopathy (mean 16.0), 95% confidence interval (CI) for difference 0.6-6.3 nmol benzylamine ml(-1) plasma h(-1). SSAO activity had not changed significantly since baseline, mean difference -1.65 and 95% CI for difference -3.76 to 0.46 nmol benzylamine ml(-1) plasma h(-1). Mean HbA(1c) level remained higher for patients with retinopathy (now 7.9%) compared to those without retinopathy (6.1%), 95% CI for difference 0.6-3.0%. Comparing baseline and the present study, retinopathy was nonproliferative; level had worsened for five and improved for two patients. Urinary methylamine/creatinine ratio was lower in the group of patients with retinopathy (mean 0.99) than in those without retinopathy (mean 1.78), 95% CI for difference 0.1-1.5 microg mg(-1). The results of the present study are compatible with a role for SSAO in the development of diabetic retinopathy.

Michaelis-Menten analysis of bovine plasma amine oxidase by capillary electrophoresis using electrophoretically mediated microanalysis in a partially filled capillary

A method for determining bovine plasma amine oxidase (PAO; EC 1.4.3.6) activity with benzylamine (Bz) as substrate is described. Electrophoretically mediated microanalysis (EMMA) combined with micellar electrokinetic capillary chromatography (MEKC) was used to perform an on-capillary enzymatic reaction and to separate the generated benzaldehyde from the other reaction products. The capillary was only partially filled with the separation solution, since the enzyme was unstable in the presence of the applied surfactant. The initial reaction velocity of the enzyme-catalyzed reaction was estimated from the peak area of the enzyme product, benzaldehyde. An amplification step was introduced by means of an on-capillary incubation of 15 min, in order to accumulate enough reaction product to detect spectrophotometrically at 254 nm. This set-up resulted in a fully automated assay, which can be carried out in less then 35 min. Using the Lineweaver-Burk equation, an average Michaelis constant (K(M)) for PAO was calculated to be 0.74 mM +/- 0.05 mM, which is consistent with previously reported values.

Variation in semicarbazide-sensitive amine oxidase activity in plasma and tissues of mammals

Semicarbazide-sensitive amine oxidase (SSAO) (E.C. 1.4.3.6) is a group of enzymes with as yet poorly understood function which is widely present in nature. The variation in methodology for determination of activity, differences in substrates used and in nomenclature have made it difficult to compare SSAO in different species and tissues. Since SSAO is implicated in the pathophysiology of diabetes mellitus and congestive heart failure, our aim was to analyse the importance and abundance of SSAO in human plasma and tissues compared to other mammals. In plasma of ten different mammals, Vmax values were found to vary more than 10,000-fold, while KM differed much less; in human plasma SSAO activity is relatively low. In some species more than one SSAO entity was present in plasma. SSAO activity was ubiquitous in tissues of human, rat and pig, but varied considerably, both between species and between tissues. In human tissues, SSAO activity is higher than in tissues from rat and pig. Relative to monoamine oxidase-B there is also wide variation in SSAO, with much higher relative activities in human than in rat and pig tissues. We conclude that in plasma, SSAO activity is highest in ruminants, while in tissues, SSAO activity is more prominently present in human than in rat and pig.

Assessment of the deamination of aminoacetone, an endogenous substrate for semicarbazide-sensitive amine oxidase

Methylglyoxal, a toxic aldehyde, has been reported to be increased in diabetes and has been claimed to be related to diabetic complications. Aminoacetone, an intermediate in the metabolism of threonine and glycine, has been proposed to be an endogenous substrate for semicarbazide-sensitive amine oxidase (SSAO). Methylglyoxal is the product. An HPLC procedure for the determination of SSAO activity toward aminoacetone in vitro is described. It was observed in previous assays that methylglyoxal formed via deamination of aminoacetone was quite unstable and led to erroneous results. o-Phenylenediamine (o-PD) was therefore employed for derivatization of methylglyoxal. o-PD does not affect SSAO activity and can be included in the enzyme reaction mixture for continuous trapping of methylglyoxal. This can avoid the loss of methylglyoxal during incubation. Deamination of aminoacetone by human umbilical artery SSAO was confirmed with this improved assay. The values of Km and Vmax, are 125.9 +/- 20.5 microM and 332.2 +/- 11.7 nmol/h/mg protein, respectively. Deamination of aminoacetone was nearly completely inhibited by 1 mM semicarbazide and 1 microM MDL-72974A, a potent selective SSAO inhibitor, whereas MAO inhibitors clorgyline (1 mM) and deprenyl (1 mM) had no inhibitory effect.

Elevated serum semicarbazide-sensitive amine oxidase activity in non-insulin-dependent diabetes mellitus: correlation with body mass index and serum triglyceride

Previous clinical studies reported elevated semicarbazide-sensitive amine oxidase (SSAO) activity in insulin-dependent diabetes mellitus (IDDM), but there are not sufficient data about SSAO in non-insulin-dependent diabetes mellitus (NIDDM). The present study was conducted to investigate serum SSAO activity in NIDDM patients compared with nondiabetic and IDDM patients. Serum SSAO activity in 61 patients with diabetes (n = 34 NIDDM and n = 27 IDDM) and 36 controls was determined using 14C-benzylamine as a substrate. NIDDM and IDDM patients exhibited higher SSAO activity compared with controls ([mean +/- SD] NIDDM, 164.60+/-69.43 pmol/mg protein/h, P<.0001; IDDM, 143.91+/-72.45 pmol/mg protein/h, P<.002; control, 91.46+/-28.11 pmol/mg protein/h). There was a significant positive correlation between serum SSAO activity and the body mass index (BMI), body weight, hemoglobin A1c (HbA1c), fasting plasma glucose, and triglycerides. Within the control group, SSAO correlated with total cholesterol levels. The progression and severity of diabetic complications such as angiopathy may be exacerbated by cytotoxic metabolites (e.g., formaldehyde and hydrogen peroxide) formed by SSAO. These results reveal the possibility that elevated serum SSAO activity in association with obesity and hyperlipidemia may be a cardiovascular risk factor in diabetes mellitus.

Deamination of methylamine and aminoacetone increases aldehydes and oxidative stress in rats

Semicarbazide-sensitive amine oxidase (SSAO)-mediated deamination of methylamine and aminoacetone in vitro produces carbonyl compounds, such as formaldehyde and methylglyoxal, which have been proposed to be cytotoxic and may be responsible for some pathological conditions. An HPLC procedure was developed to assess different aldehydes, which were derivatized with 2,4-dinitrophenylhydrazine (DNPH). We have demonstrated in vivo deamination of methylamine and aminoacetone by examining the excretion of formaldehyde and methylglyoxal, respectively, in rats. Following chronic administration of methylamine, the urinary level of malondialdehyde (MDA), an end product of lipid peroxidation, was also found to be substantially increased. A selective SSAO inhibitor blocked the increase of MDA. The results support the idea that increased SSAO-mediated deamination of methylamine and aminoacetone can be a potential cytotoxic risk factor.

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.

Autoradiographic imaging of formaldehyde adducts in mice: possible relevance for vascular damage in diabetes

The activity of semicarbazide-sensitive amine oxidase (SSAO) has been reported to be elevated in blood from diabetic patients. It has been suggested that the enzyme is involved in the development of complications such as retinopathies, nephropathies and neuropathies, which are associated with advanced diabetes, possibly by the formation of toxic metabolites. Under the influence of SSAO, methylamine is deaminated to formaldehyde which is known to react with various macromolecules. It has therefore been proposed that specific inhibition of SSAO could be of therapeutic value for treatment of diabetic patients. The present results provide evidence that treatment with an SSAO inhibitor potently reduces the levels of irreversible adducts. In this study, 14C-methylamine was given intraperitoneally to NMRI mice, and the tissue distribution of irreversibly bound methylamine metabolites was estimated by an autoradiographic method. Such radioactive residues occurred in high concentrations in the intestinal wall, brown adipose tissue, spleen and bone marrow. By inhibiting SSAO irreversibly with hydralazine before giving 14C-methylamine to the mice, it was possible to determine the resynthesis rate of SSAO in different tissues. A complete recovery of SSAO activity was seen in the intestinal wall after 6 days, whereas only about 60% was recovered in adipose tissue after 14 days. This suggests that factors controlling the synthesis of SSAO differ in these tissues, or that these tissues express different forms of enzymes.

Increase of formation of methylamine and formaldehyde in vivo after administration of nicotine and the potential cytotoxicity

Methylamine is a constituent of cigarette smoke and the major end product of nicotine metabolism. Smoking or nicotine can induce the release of adrenaline, which is in turn deaminated by monoamine oxidase, also producing methylamine. We found that the urinary level of methylamine was significantly elevated following administration of nicotine (25 mg/Kg, i.p.). Semicarbazide-sensitive amine oxidase (SSAO) inhibitors further increased the excretion of methylamine induced by nicotine. Following administration of L-(-)-[N-methyl-3H]nicotine long-lasting irreversible radioactive adducts were detected in different mouse tissues and such adduct formation could be blocked by selective SSAO inhibitors. These adducts are probably cross-linked oligoprotein complexes cross-linked by formaldehyde. The findings support the idea that nicotine can enhance SSAO/methylamine-mediated increase of formaldehyde and oxidative stress and this could in part contribute the adverse effect of health associated with smoking.

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.

Aminoguanidine inhibits semicarbazide-sensitive amine oxidase activity: implications for advanced glycation and diabetic complications

Aminoguanidine, a nucleophilic hydrazine, has been shown to be capable of blocking the formation of advanced glycation end products. It reduces the development of atherosclerotic plaques and prevents experimental diabetic nephropathy. We have found that aminoguanidine is also quite potent at inhibiting semicarbazide-sensitive amine oxidase (SSAO) both in vitro and in vivo. The inhibition is irreversible. This enzyme catalyses the deamination of methylamine and aminoacetone, which leads to the production of cytotoxic formaldehyde and methylglyoxal, respectively. Serum SSAO activity was reported to be increased in diabetic patients and positively correlated with the amount of plasma glycated haemoglobin. Increased SSAO has also been demonstrated in diabetic animal models. Urinary excretion of methylamine is substantially increased in the rats following acute or chronic treatment with aminoguanidine. Urinary methylamine levels were substantially increased in streptozotocin (STZ)-induced diabetic rats following administration of aminoguanidine. The non-hydrazine SSAO inhibitor (E)-2-(4-fluorophenethyl)-3-fluoroallylamine hydrochloride (MDL-72974A) has been shown to reduce urinary excretion of lactate dehydrogenase (an indicator of nephropathy) in STZ-induced diabetic rats. Formaldehyde not only induces protein crosslinking, but also enhances the advanced glycation of proteins in vitro. The results support the hypothesis that increased SSAO-mediated deamination may be involved in structural modification of proteins and contribute to advanced glycation in diabetes. The clinical implications for the use of aminoguanidine to prevent glycoxidation have been discussed.

Age and sex related alterations in serum and platelet monoamine oxidase

The process of aging presents itself with various alterations in physiological events. Although the turnover of catecholamines increases with aging, there is a lack of response to catecholamines in target tissues. One of the key enzymes in catecholamine metabolism is monoamine oxidase. It has been suggested that tissue and serum monoamine oxidase activities show pathological alterations in various diseases while physiological fluctuations can also be detected in normals. The aim of this study is to determine the sex and age related changes of platelet and serum monoamine oxidase in healthy volunteers. In this study, 75 healthy volunteers of different ages (21-80 a) and sexes (40 females, 35 males) were included. Serum and platelet monoamine oxidase determinations were performed spectrophotofluorometrically by Tufvesson's (Scand J Clin Lab Invest 1970; 26:151-4) and Kraml's (Biochem Pharmacol 1965; 14:1684-6) modified methods, respectively. While there was no significant difference in serum monoamine oxidase activities related to age and sex, platelet monoamine oxidase manifested a significant increase in females compared to males (p < 0.05) and the mean values in both sexes showed an increase with age (p < 0.001). The results of this study imply that platelet monoamine oxidase shows an age related increase which is more prominent in females.

Mammalian plasma and tissue-bound semicarbazide-sensitive amine oxidases: biochemical, pharmacological and toxicological aspects

Mammalian plasma and tissues contain various soluble and membrane-bound enzymes which metabolize the synthetic amine benzylamine particularly well. The sensitivity of these enzymes to inhibition by semicarbazide and related compounds suggests that they contain a cofactor with a reactive carbonyl group, which has been proposed to be either pyridoxal phosphate, pyrroloquinoline quinone or (more recently) 6-hydroxydopa. It is not yet clear if all of these semicarbazide-sensitive amine oxidases (SSAOs) are copper-dependent enzymes. A variety of compounds have now been identified as relatively selective inhibitors to distinguish the SSAOs from other amine oxidases, in order to investigate the properties of SSAOs and their potential role in biogenic and xenobiotic amine metabolism in vivo. While plasma SSAO is soluble, most tissue SSAOs appear to be membrane-bound, probably plasmalemmal enzymes, which may be capable of metabolizing extracellular amines. Vascular (and non-vascular) smooth muscle cells have particularly high SSAO activity, although recently the enzyme has been found in other cell types (e.g. adipocytes, chondrocytes, odontoblasts) implying a functional importance not restricted solely to smooth muscle. The substrate specificity of plasma and tissue SSAOs shows considerable species-related variations. For example, while some endogenously-occurring aromatic amines such as tyramine and tryptamine are metabolized well by SSAO in homogenates of rat blood vessels, and also in vitro inhibition of SSAO can potentiate vasoconstrictor actions of these amines in rat vascular preparations, these amines are poor substrates for human SSAO, thus complicating attempts to generalize possible physiological roles for these enzymes. Vascular SSAO can metabolize the xenobiotic aliphatic amine, allylamine, to the cytotoxic aldehyde acrolein and this has been linked to the ability of allylamine administration to produce cardiovascular lesions in experimental animals, sometimes mimicking features of atherosclerotic disease. Recent studies showing that the endogenously-occurring aliphatic amines methylamine and aminoacetone are metabolized in vitro to formaldehyde and methylglyoxal, respectively, by SSAO in some animal (including human) tissues, suggest the possibility that toxicological consequences upon cellular function could result if such conversions occur in vivo.

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.

Determination of semicarbazide-sensitive amine oxidase activity in human plasma by high-performance liquid chromatography with fluorimetric detection

We report here a simple and sensitive method for the measurement of semicarbazide-sensitive amine oxidase (SSAO) activity in human plasma. Benzaldehyde, generated during a 1-h incubation of plasma with benzylamine, is derivatized with the specific aldehyde reagent dimedone after prior deproteinization. Quantitation of the derivatization product is done by automated injection onto an isocratic high-performance liquid chromatographic system with fluorimetric detection. The assay shows good linearity and reproducibility (intra-assay C.V. 7%). Detection limit is 25 mU/l (= pmol/ml/min). In 51 healthy controls (age 49 +/- 13 yr, 20 males) the measured SSAO activity was 352 +/- 102 mU/l (mean +/- S.D.). A large number of samples (70-80) can easily be processed in one day by one technician.

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.

Characterization of human serum and umbilical artery semicarbazide-sensitive amine oxidase (SSAO). Species heterogeneity and stereoisomeric specificity

Semicarbazide-sensitive amine oxidases (SSAOs) are located in cardiovascular smooth muscle, cartilage and brown adipose tissues of different species, including human. The enzyme is also present in blood, and its activity appears to be altered under certain pathological conditions. SSAOs from both human umbilical arteries and serum were partially purified, and some of their biochemical properties were investigated. Both human artery and blood SSAO exhibited very similar substrate preference, lack of stereospecificity catalyzing the deamination of pro-R and pro-S benzylamine-deuterated enantiomers, and were very sensitive towards (E)-2-(4-fluorophenethyl)-3-fluoroallylamine (MDL-72974A). It was concluded that circulating serum SSAO is identical to the SSAO from vascular tissues. Human SSAO exhibited distinctly different properties in comparison to bovine and rat SSAOs.

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

Monoamine oxidase-A and -B (MAO-A and MAO-B) and semicarbazide-sensitive amine oxidase (SSAO) activities were assessed in several rat micro-vascular tissues and eyes using selective substrates and inhibitors. In rat brain microvessels both MAO-A and MAO-B activities are relatively high and the levels of the two types of MAO's are comparable. Retina possesses a similar ratio of MAO-A and B but the activities are much lower. Eye sclera and meninges exhibit mainly MAO-A and MAO-B, respectively. Aorta is the only tissue where SSAO is the predominant amine oxidase. Relatively low, but significant amounts of SSAO were also detected in brain microvessels, meninges, retina and eye sclera. Methylamine was observed to be deaminated by SSAO from different tissues. The physiological and toxicological implications of amine oxidases in these tissues are discussed.

Deamination of aliphatic amines by type B monoamine oxidase and semicarbazide-sensitive amine oxidase; pharmacological implications

Straight and branched chain aliphatic monoamines, which are not normal tissue constituents, are deaminated selectively by type B monoamine oxidase (MAO-B). They exhibit a high affinity towards the active site of MAO-B and this made them very useful pharmacologically. An anticonvulsant prodrug, Milacemide [2-(N-pentyl)glycinamide] is deaminated by MAO-B and this facilitates a mechanism of delivering glycine into the CNS. We have found that 2-propyl-pentylamine (2-propyl-1-aminopentane) and N-(2-propylpentyl)glycinamide are also converted by MAO-B to valproic acid and glycine both in vitro and in vivo; these compounds, however, cause severe tremor. By attaching a propargylamine group the resultant series of aliphatic propargylamine derivatives have been shown to be very potent selective MAO-B inhibitors. They are chemically quite different from most other MAO-B inhibitors, since they do not possess any aromatic structures. The relatively short chain aliphatic propargylamines, i.e. N-2-pentyl-N-methylpropargylamine and N-2-hexyl-N-methylpropargylamine, are 4 to 5 times more potent and more selective than selegiline (1-deprenyl) with respect to the inhibition of MAO-B in brain following oral administration. Semicarbazide-sensitive amine oxidase (SSAO) catalyzes the deamination of not only longer chain aliphatic amines but also short chain aliphatic amines including methylamine. Formaldehyde is produced from methylamine by SSAO. Increased methylamine deamination may cause cellular damage in some pathological conditions, such as uraemia and diabetes. We have observed that cultured human endothelial cells are damaged by methylamine in the presence of SSAO. Inhibition of the SSAO activity completely protects these cells from the methylamine-SSAO induced damage.

Physiological and pathological influences on sheep blood plasma amine oxidase: effect of pregnancy and experimental alloxan-induced diabetes mellitus

A copper-containing amine oxidase is present in sheep blood plasma and has a high capacity to deaminate spermine and spermidine. The physiological function of this enzyme remains to be determined. Sheep blood plasma amine oxidase (SPAO) was measured by its ability to deaminate spermidine (700 microM) using a peroxidase-linked colorimetric assay developed for microtitre plates. SPAO activity has been studied in a group of Welsh Mountain sheep with experimental alloxan-induced diabetes. This resulted in an increase in SPAO activity which reached a peak of 70 days after alloxan treatment (60 per cent increase). This change could be seen in both pregnant and non-pregnant diabetic sheep. In normal pregnant ewes, SPAO activity remained stable for the first 100 days of pregnancy but declined by 50 per cent in the last month of pregnancy. Together, these findings suggest that SPAO activity is controlled by hormonal influences. This sensitive and convenient assay method could provide clues as to the physiological significance of SPAO and may be a useful clinical chemical indicator in the sheep.

A sensitive fluorometric assay for serum monoamine oxidase with kynuramine as substrate

The detailed procedure of a new sensitive fluorometric assay for human serum monoamine oxidase (MAO) with kynuramine as substrate is described. The data on its reproducibility, stability, correlation with another method, and serum MAO levels for 150 healthy subjects and 205 patients with various diseases, are presented to support the usefulness of this method. Since our method is much simpler and more sensitive than other methods conventionally used, we can recommend it for routine clinical investigation.

Pre- and post-partum plasma amine oxidase differences in the rhesus monkey (Macaca mulatta)

Plasma amine oxidase activity increased from 23.4 nmol/ml/h during pregnancy to 49.5 nmol/ml/h during an extended post partum period in 10 rhesus monkeys. Comparison with non-pregnant control monkeys sampled at similar times indicated that the significant differences were in the extended post partum period.

A clinical method for testing abnormal in vitro haemolysis from catecholamine metabolites in schizophrenia

Earlier work from this laboratory showed that rheomelanins (blood soluble melanins) form in normal and schizophrenic blood incubated with the catecholamines, with l-dopa, with adrenochrome and with adrenolutin-monohydrate (Hegedus and Altschule (1–4)). This work also showed that the erythrocytes of many patients with a diagnosis of chronic schizophrenia are abnormally susceptible to the haemolytic effects of the rheomelanins or their precursors, formed in their bloods during incubation with most of these chemicals (Hegedus and Altschule (2, 4)). Erythrocytes of the chronic patients showed especially marked haemolysis during incubation of their oxalated blood with adrenochrome, with adrenolutin-monohydrate and with dopamine. A test for routine use was developed on the basis of this phenomenon, and is reported in this paper. The conditions of this routine test were determined as regards quantity of chemicals, time and temperature of incubation, and rate of shaking during incubation so as to give maximum differences between normal and schizophrenic bloods.