A structure-activity study to identify novel and efficient substrates of the human semicarbazide-sensitive amine oxidase/VAP-1 enzyme

Effects of Chemical Structures Interacting with Amine Oxidases on Glucose, Lipid and Hydrogen Peroxide Handling by Human Adipocytes

Benzylamine is a natural molecule present in food and edible plants, capable of activating hexose uptake and inhibiting lipolysis in human fat cells. These effects are dependent on its oxidation by amine oxidases present in adipocytes, and on the subsequent hydrogen peroxide production, known to exhibit insulin-like actions. Virtually, other substrates interacting with such hydrogen peroxide-releasing enzymes potentially can modulate lipid accumulation in adipose tissue. Inhibition of such enzymes has also been reported to influence lipid deposition. We have therefore studied in human adipocytes the lipolytic and lipogenic activities of pharmacological entities designed to interact with amine oxidases highly expressed in this cell type: the semicarbazide-sensitive amine oxidase (SSAO also known as PrAO or VAP-1) and the monoamine oxidases (MAO). The results showed that SZV-2016 and SZV-2017 behaved as better substrates than benzylamine, releasing hydrogen peroxide once oxidized, and reproduced or even exceeded its insulin-like metabolic effects in fat cells. Additionally, several novel SSAO inhibitors, such as SZV-2007 and SZV-1398, have been evidenced and shown to inhibit benzylamine metabolic actions. Taken as a whole, our findings reinforce the list of molecules that influence the regulation of triacylglycerol assembly/breakdown, at least in vitro in human adipocytes. The novel compounds deserve deeper investigation of their mechanisms of interaction with SSAO or MAO, and constitute potential candidates for therapeutic use in obesity and diabetes.

Soluble Vascular Adhesion Protein-1 Level Correlates With Adropin and Inflammatory Biomarkers in Patients With Obstructive Sleep Apnea

PURPOSE

Soluble vascular adhesion protein-1 (sVAP-1) and adropin are 2 biomarkers of endothelial dysfunction. The main purpose of this study was to evaluate the levels of sVAP-1 in patients with moderate and severe obstructive sleep apnea (OSA) compared to healthy controls, and to further determine the relationship between sVAP-1 and adropin levels, as well as inflammatory biomarkers and sleep parameters.

METHODS

In this study, we included 50 male patients with OSA (25 moderate and 25 severe) and 20 age- and sex-matched control subjects. Patients with OSA underwent polysomnography. All subjects underwent fasting peripheral blood sampling for laboratory analysis.

RESULTS

Serum sVAP-1 and inflammatory biomarkers (IL-6, TNF-α, and hsCRP) levels were significantly higher in patients with severe OSA compared to those with moderate OSA and control groups, while plasma levels of adropin showed the opposite trend. Furthermore, sVAP-1 levels had a significant positive correlation with AHI, ODI, TNF-α, IL-6, and hsCRP levels and a significant negative correlation with adropin levels. The receiver operating characteristic analysis showed an Area Under Curve (AUC) of .876 (P < .001) for sVAP-1 levels predicting OSA. Serum sVAP-1 threshold of > 445.5 ng/mL had an 88% sensitivity and 80% specificity for detecting OSA status. Multivariate regression analysis demonstrated that sVAP-1 remained a significant positive predictor of OSA severity.

CONCLUSIONS

Increased sVAP-1 levels in OSA patients are significantly correlated with indices of OSA severity, adropin levels, and inflammatory biomarkers, suggesting that sVAP-1 plays a vital role in the pathophysiology of OSA and may become a potential screening tool in the evaluation of OSA severity.

Amine oxidase, copper containing 3 exerts anti‑mesenchymal transformation and enhances CD4+ T‑cell recruitment to prolong survival in lung cancer

Lung cancer remains notorious for its poor prognosis. Despite the advent of tyrosine kinase inhibitors and immune checkpoint inhibitors, the probability of curing the disease in lung cancer patients remains low. Novel mechanisms and treatment strategies are needed to provide hope to patients. Advanced strategies of next generation sequencing (NGS) and bioinformatics were used to analyze normal and lung cancer tissues from lung cancer patients. Amine oxidases have been linked to leukocyte migration and tumorigenesis. However, the roles of amine oxidases in lung cancer are not well-understood. Our results indicated that amine oxidase, copper containing 3 (AOC3) was significantly decreased in the tumor tissue compared with the normal tissue, at both the mRNA and protein level, in the included lung cancer patients and public databases. Lower expression of AOC3 conferred a poorer survival probability across the different cohorts. Epigenetic silencing of AOC3 via miR-3691-5p caused tumor promotion and progression by increasing migration and epithelial-mesenchymal transition (EMT). Furthermore, knockdown of AOC3 caused less CD4⁺ T-cell attachment onto lung cancer cells and reduced transendothelial migration in vitro, as well as reducing CD4⁺ T-cell trafficking to the lung in vivo. In conclusion, the present study revealed that downregulation of AOC3 mediated lung cancer promotion and progression, as well as decrease of immune cell recruitment. This novel finding could expand our understanding of the dysregulation of the tumor immune microenvironment and could help to develop a novel strategy for the treatment of lung cancer.

Inhibition of vascular adhesion protein-1 modifies hepatic steatosis in vitro and in vivo

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is associated with obesity, insulin resistance and dyslipidaemia and currently is estimated to affect up to a third of all individuals in developed countries. Current standard of care for patients varies according to disease stage, but includes lifestyle interventions common insulin sensitizers, antioxidants and lipid modifiers. However, to date specific therapies have shown little histological or fibrosis stage improvement in large clinical trials, and there is still no licensed therapy for NAFLD. Given the high prevalence, limited treatment options and significant screening costs for the general population, new treatments are urgently required.

AIM

To assess the potential for inhibition of the amine oxidase enzyme vascular adhesion protein-1 (VAP-1) to modify hepatic lipid accumulation in NAFLD.

METHODS

We have used immunochemical and qPCR analysis to document expression of VAP-1 and key functional proteins and transporters across the NAFLD spectrum. We then utilised hepatocytes in culture and human precision cut liver slices in concert with selective enzyme activity inhibitors to test the effects of activating the semicarbazide-sensitive amine oxidase activity of VAP-1 on hepatic lipid uptake and triglyceride export. A murine model of NAFLD was also used to determine the consequences of VAP-1 knockout and gene expression arrays were used to quantify the effects of VAP-1 activity on key lipid modifying and proinflammatory gene expression.

RESULTS

We confirmed that increasing severity of NAFLD and progression to cirrhosis was associated with a significant increase in hepatocellular VAP-1 expression. Hepatocytes in vitro exposed to recombinant VAP-1 and its substrate methylamine showed increased lipid accumulation as determined by quantification of Oil Red O uptake. This was recapitulated using hydrogen peroxide, and lipid accumulation was accompanied by changes in expression of the lipid transporter molecules FABP3, FATP6, insulin receptor subunits and PPARα. Human liver tissue exposed to recombinant VAP-1 or substrates for endo/exogenous VAP-1 produced less triglyceride than untreated tissue and demonstrated an increase in steatosis. This response could be inhibited by using bromoethylamine to inhibit the SSAO activity of VAP-1, and mice deficient in VAP-1/AOC3 also demonstrated reduced steatosis on high fat diet. Exposure of human liver tissue to methylamine to activate VAP-1 resulted in increased expression of FABP2 and 4, FATP3-5, caveolin-1, VLDLR, PPARGC1 and genes associated with the inflammatory response.

CONCLUSION

Our data confirm that the elevations in hepatic VAP-1 expression reported in nonalcoholic steatohepatitis can contribute to steatosis, metabolic disturbance and inflammation. This suggests that targeting the semicarbazide sensitive amine oxidase capacity of VAP-1 may represent a useful adjunct to other therapeutic strategies in NAFLD.

Exploring the activity of polyamine analogues on polyamine and spermine oxidase: methoctramine, a potent and selective inhibitor of polyamine oxidase

Fourteen polyamine analogues, asymmetric or symmetric substituted spermine (1-9) or methoctramine (10-14) analogues, were evaluated as potential inhibitors or substrates of two enzymes of the polyamine catabolic pathway, spermine oxidase (SMOX) and acetylpolyamine oxidase (PAOX). Compound 2 turned out to be the best substrate for PAOX, having the highest affinity and catalytic efficiency with respect to its physiological substrates. Methoctramine (10), a well-known muscarinic M2 receptor antagonist, emerged as the most potent competitive PAOX inhibitor known so far (Ki = 10 nM), endowed with very good selectivity compared with SMOX (Ki=1.2 μM vs SMOX). The efficacy of methoctramine in inhibiting PAOX activity was confirmed in the HT22 cell line. Methoctramine is a very promising tool in the design of drugs targeting the polyamine catabolism pathway, both to understand the physio-pathological role of PAOX vs SMOX and for pharmacological applications, being the polyamine pathway involved in various pathologies.

Evolution and functional classification of mammalian copper amine oxidases

Mammalian copper-containing amine oxidases (CAOs), encoded by four genes (AOC1-4) and catalyzing the oxidation of primary amines to aldehydes, regulate many biological processes and are linked to various diseases including inflammatory conditions and histamine intolerance. Despite the known differences in their substrate preferences, CAOs are currently classified based on their preference for either primary monoamines (EC 1.4.3.21) or diamines (EC 1.4.3.22). Here, we present the first extensive phylogenetic study of CAOs that, combined with structural analyses of the CAO active sites, provides in-depth knowledge of their relationships and guidelines for classification of mammalian CAOs into AOC1-4 sub-families. The phylogenetic results show that CAOs can be classified based on two residues, X1 and X2, from the active site motif: T/S-X1-X2-N-Y-D. Residue X2 discriminates among the AOC1 (Tyr), AOC2 (Gly), and AOC3/AOC4 (Leu) proteins, while residue X1 further classifies the AOC3 (Leu) and AOC4 (Met) proteins that so far have been poorly identified and annotated. Residues X1 and X2 conserved within each sub-family and located in the catalytic site seem to be the key determinants for the unique substrate preference of each CAO sub-family. Furthermore, one residue located at 10 Å distance from the catalytic site is different between the sub-families but highly conserved within each sub-family (Asp in AOC1, His in AOC2, Thr in AOC3 and Asn in AOC4) and likely contributes to substrate selectivity. Altogether, our results will benefit the design of new sub-family specific inhibitors and the design of in vitro tests to detect individual CAO levels for diagnostic purposes.

Evidence for existence of thyroid hormone inducible semicarbazide-sensitive amine oxidase (SSAO) in rat heart cytosol

BACKGROUND

Semicarbazide-sensitive amine oxidase (SSAO; EC; 1.4.3.6.) has widespread tissue distribution, and the physiological role of SSAO is quite well known through its involvement in several pathological states.

AIMS

The present study examined modulators of SSAO which might be present in the rat heart cytosol and looked for changes in SSAO modulatory activity.

METHODS

An endogenous inhibitor of SSAO was separated by gel filtration from 105,000g supernate of T4-treated rat heart cytosol. SSAO inhibition fraction was referred to as "endogenous SSAO inhibitor".

RESULTS

The inhibition by this inhibitor was concentration-dependent. Inhibition of SSAO was not enhanced by varying the time of preincubation of the enzyme, indicating reversible inhibition of SSAO. The molecular weight of this inhibitor was estimated to be 1000-1100 by gel filtration. The isoelectric point (pI) value was determined to be 4.8 isoelectric focusing. This inhibitor was found to be heat-stable and resistant to protease treatment. SSAO inhibition activity was much lower in the cytosol of thyroidectomized, non-T4-treated rats than T4-treated rats, suggesting that this inhibitor was induced by thyroid hormone T4. SSAO activity in rat heart might be regulated by the level of this inhibitor.

CONCLUSION

These results suggest the presence of SSAO inhibitor in T4-treated rat cytosol and that the level of this inhibitor is regulated by thyroid hormone.

A novel enzyme with spermine oxidase properties in bovine liver mitochondria: identification and kinetic characterization

The uptake of spermine into mammalian mitochondria indicated the need to identify its catabolic pathway in these organelles. Bovine liver mitochondria were therefore purified and their capacity for natural polyamine uptake was verified. A kinetic approach was then used to determine the presence of an MDL 72527-sensitive enzyme with spermine oxidase activity in the matrix of bovine liver mitochondria. Western blot analysis of mitochondrial fractions and immunogold electron microscopy observations of purified mitochondria unequivocally confirmed the presence of a protein recognized by anti-spermine oxidase antibodies in the mitochondrial matrix. Preliminary kinetic characterization showed that spermine is the preferred substrate of this enzyme; lower activity was detected with spermidine and acetylated polyamines. Catalytic efficiency comparable to that of spermine was also found for 1-aminododecane. The considerable effect of ionic strength on the Vmax/KM ratio suggested the presence of more than one negatively charged zone inside the active site cavity of this mitochondrial enzyme, which is probably involved in the docking of positively charged substrates. These findings indicate that the bovine liver mitochondrial matrix contains an enzyme belonging to the spermine oxidase class. Because H2O2 is generated by spermine oxidase activity, the possible involvement of the latter as an important signaling transducer under both physiological and pathological conditions should be considered.

Dysregulated hepatic expression of glucose transporters in chronic disease: contribution of semicarbazide-sensitive amine oxidase to hepatic glucose uptake

Insulin resistance is common in patients with chronic liver disease (CLD). Serum levels of soluble vascular adhesion protein-1 (VAP-1) are also increased in these patients. The amine oxidase activity of VAP-1 stimulates glucose uptake via translocation of transporters to the cell membrane in adipocytes and smooth muscle cells. We aimed to document human hepatocellular expression of glucose transporters (GLUTs) and to determine if VAP-1 activity influences receptor expression and hepatic glucose uptake. Quantitative PCR and immunocytochemistry were used to study human liver tissue and cultured cells. We also used tissue slices from humans and VAP-1-deficient mice to assay glucose uptake and measure hepatocellular responses to stimulation. We report upregulation of GLUT1, -3, -5, -6, -7, -8, -9, -10, -11, -12, and -13 in CLD. VAP-1 expression and enzyme activity increased in disease, and provision of substrate to hepatic VAP-1 drives hepatic glucose uptake. This effect was sensitive to inhibition of VAP-1 and could be recapitulated by H2O2. VAP-1 activity also altered expression and subcellular localization of GLUT2, -4, -9, -10, and -13. Therefore, we show, for the first time, alterations in hepatocellular expression of glucose and fructose transporters in CLD and provide evidence that the semicarbazide-sensitive amine oxidase activity of VAP-1 modifies hepatic glucose homeostasis and may contribute to patterns of GLUT expression in chronic disease.

Exploiting the repertoire of CK2 inhibitors to target DYRK and PIM kinases

Advantage has been taken of the relative promiscuity of commonly used inhibitors of protein kinase CK2 to develop compounds that can be exploited for the selective inhibition of druggable kinases other than CK2 itself. Here we summarize data obtained by altering the scaffold of CK2 inhibitors to give rise to novel selective inhibitors of DYRK1A and to a powerful cell permeable dual inhibitor of PIM1 and CK2. In the former case one of the new compounds, C624 (naphto [1,2-b]benzofuran-5,9-diol) displays a potency comparable to that of the first-in-class DYRK1A inhibitor, harmine, lacking however the drawback of drastically inhibiting monoamine oxidase-A (MAO-A) as harmine does. On the other hand the promiscuous CK2 inhibitor 4,5,6,7-tetrabromo-1H-benzimidazole (TBI,TBBz) has been derivatized with a sugar moiety to generate a 1-(β-D-2'-deoxyribofuranosyl)-4,5,6,7-tetrabromo-1H-benzimidazole (TDB) compound which inhibits PIM1 and CK2 with comparably high efficacy (IC50 values<100nM) and remarkable selectivity. TDB, unlike other dual PIM1/CK2 inhibitors described in the literature is readily cell permeable and displays a cytotoxic effect on cancer cells consistent with concomitant inhibition of both its onco-kinase targets. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

VAP-1, a novel molecule linked to endothelial damage and kidney function in kidney allograft recipients

BACKGROUND/AIMS

VAP-1 (vascular adhesion protein-1) is a copper-containing SSAO (semicarbazide sensitive amine oxidase) secreted by vascular smooth muscle cells, adipocytes, endothelial cells with functional monoamine oxidase activity. The oxidation process generates harmful products that may be involved in atherosclerosis and vascular damage. Elevation of SSAO activity is observed in atherosclerosis, diabetes mellitus and obesity. On the other hand, renalase, with possible monoamine oxidase activity, which breaks down catecholamines like SSAO, is also expressed in the endothelium as well as in the kidney. The aim of the study was to assess VAP-1 levels and its correlations with endothelial injury markers and renalase in 50 kidney allograft recipients.

METHODS

Hemoglobin, urea, creatinine, rate were studied by standard laboratory method in the hospital central laboratory. We assessed markers of endothelial function/injury: vWF, thrombomodulin, ICAM, VCAM, CD40L, CD44, CD146, inflammation: hsCRP, and IL-6 and adipocytokines: leptin, adiponectin, visfatin, apelin with commercially available assays.

RESULTS

The mean serum VAP-1 in Tx was significantly higher comparing to the control group. In kidney transplant recipients VAP-1 correlated with BMI (r=0.39, p<0.01), CD44 (r=0.27, p<0.05), hsCRP (r=0.28, p<0.05), serum creatinine (r=0.29, p<0.05), eGFR (CKD-EPI formula r=-0.27, p<0.05, MDRD r=-0.27,p<0.05, Cockcroft-Gault r=-0.35,p<0.01), serum urea (r=0.27, p<0.05), CD146 (r=0.49, p<0.001), CD40L (r=0.26, p<0.06), and renalase (r=0.34, p<0.05). In multiple regression analysis VAP-1 was predicted 80% by serum creatinine (beta value 0.33, p=0.01), and CD146 (beta value 43, p=0.0005).

CONCLUSION

VAP-1, elevated in kidney transplant recipients, is predominantly dependent on endothelial damage and kidney function, which deteriorated with time after kidney transplantation.

Vascular adhesion protein-1 and syndecan-1 in septic shock

BACKGROUND

Constituents of vascular endothelial surface layer (glycocalyx), e.g. an anchor protein syndecan-1 (SDC-1), can be detected in plasma in many inflammatory conditions. In inflammation, vascular adhesion protein-1 (VAP-1) is rapidly translocated to the apical side of the endothelial cells and may be released to plasma in a soluble form. We hypothesized that glycocalyx injury coincides with VAP-1 activation on endothelial cells. To test the hypothesis, we measured SDC-1 and VAP-1 levels in 20 patients with septic shock.

METHODS

A prospective observational study was conducted in two multidisciplinary critical care units in two tertiary academic teaching hospitals with 20 mechanically ventilated adult patients with septic shock, on days 1 and 4 of treatment. Twenty healthy adults were enrolled as a control group. Plasma SDC-1 content, serum VAP-1 activity, platelets, and leukocyte count were measured in septic shock group at baseline and at 72 h and compared with those of healthy controls.

RESULTS

VAP-1 activity and SDC-1 content were significantly increased in septic patients' group (P < 0.01) in comparison with controls. VAP-1 activity and SDC-1 content correlated positively to each other, and negatively to platelet count. In the septic shock group SDC-1 correlated on day 1 to SOFA score.

CONCLUSIONS

We found increased VAP-1 activity and SDC-1 content in critically ill patients with septic shock. Based on our results, the role of VAP-1 in shock pathogenesis should be studied with semicarbazide-sensitive amine oxidase activity blocking agents and substrate affinity testing.

Implication for functions of the ectopic adipocyte copper amine oxidase (AOC3) from purified enzyme and cell-based kinetic studies

AOC3 is highly expressed in adipocytes and smooth muscle cells, but its function in these cells is currently unknown. The in vivo substrate(s) of AOC3 is/are also unknown, but could provide an invaluable clue to the enzyme's function. Expression of untagged, soluble human AOC3 in insect cells provides a relatively simple means of obtaining pure enzyme. Characterization of enzyme indicates a 6% titer for the active site 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor and corrected k(cat) values as high as 7 s(-1). Substrate kinetic profiling shows that the enzyme accepts a variety of primary amines with different chemical features, including nonphysiological branched-chain and aliphatic amines, with measured k(cat)/K(m) values between 10(2) and 10(4) M(-1) s(-1). K(m)(O(2)) approximates the partial pressure of oxygen found in the interstitial space. Comparison of the properties of purified murine to human enzyme indicates k(cat)/K(m) values that are within 3 to 4-fold, with the exception of methylamine and aminoacetone that are ca. 10-fold more active with human AOC3. With drug development efforts investigating AOC3 as an anti-inflammatory target, these studies suggest that caution is called for when screening the efficacy of inhibitors designed against human enzymes in non-transgenic mouse models. Differentiated murine 3T3-L1 adipocytes show a uniform distribution of AOC3 on the cell surface and whole cell K(m) values that are reasonably close to values measured using purified enzymes. The latter studies support a relevance of the kinetic parameters measured with isolated AOC3 variants to adipocyte function. From our studies, a number of possible substrates with relatively high k(cat)/K(m) have been discovered, including dopamine and cysteamine, which may implicate a role for adipocyte AOC3 in insulin-signaling and fatty acid metabolism, respectively. Finally, the demonstrated AOC3 turnover of primary amines that are non-native to human tissue suggests possible roles for the adipocyte enzyme in subcutaneous bacterial infiltration and obesity.

Synthetic polyamines as potential amine oxidase inhibitors: a preliminary study

In the last few decades, medicinal chemists have carried out extensive research on synthetic polyamines for use as anticancer drugs and multitarget-directed ligands in neurodegenerative diseases. The aim of this study was to evaluate the effect of some synthetic polyamines as inhibitors of two new potential targets, human semicarbazide-sensitive amine oxidase/vascular adhesion protein-1 (SSAO/VAP-1) and monoamine oxidases B (MAO B), enzymes involved in various multi-factorial diseases such as Alzheimer's disease. N,N'-Dibenzyl-dodecane-1,12-diamine (Bis-Bza-Diado), a newly synthesised compound, and ELP 12, a muscarinic cholinergic M(2) receptor antagonist, were found to behave as reversible and mixed non-competitive inhibitors of both amine oxidases (dissociation constants of about 100 μM). ELP 12 was found to be more selective for SSAO/VAP-1. Combining kinetic and structural approaches, the binding mode of ELP 12 to SSAO/VAP-1 was investigated. ELP 12 may bind at the entrance of the active site channel by ionic interactions with ASP446 and/or ASP180; one end of the polyamine may be accommodated inside the channel, reaching the TPQ cofactor area. The binding of ELP 12 induces rearrangement of the secondary structure of the enzyme and impedes substrate entry and/or product release and catalysis. These structural data reveal that the entrance and the first part of the SSAO/VAP-1 channel may be considered as a new target area, or a "secondary binding site", for modulators of human SSAO/VAP-1 activity. These results indicate ELP 12 and Bis-Bza-Diado as new "skeletons" for the development of novel SSAO/VAP-1 and MAO B inhibitors.

SSAO substrates exhibiting insulin-like effects in adipocytes as a promising treatment option for metabolic disorders

BACKGROUND

Benzylamine exerts insulin-like effects in adipocytes (e.g., glucose uptake and antilipolysis) and improves glucose handling in rodents.

RESULTS

In murine adipocytes, benzylamine mimics another insulin action: it enhances apelin expression in a manner that is blocked by the semicarbazide-sensitive amine oxidase/vascular adhesion protein-1 (SSAO/VAP-1) inhibitor semicarbazide. It is shown that in human adipocytes, benzylamine activates glucose transport, but its effects are not additive to maximal insulin stimulation. Benzylamine effects are hydrogen peroxide dependent. They can be reproduced by novel substrates, but not by benzaldehyde.

CONCLUSION

Owing to the parallelism between the in vitro insulin mimicry and the in vivo improvement of glucose handling elicited by benzylamine in rodents, the SSAO/VAP-1 substrates, with stronger effects on human adipocytes than benzylamine, show promising applications for the treatment of insulin resistance.

Reaction of vascular adhesion protein-1 (VAP-1) with primary amines: mechanistic insights from isotope effects and quantitative structure-activity relationships

Human vascular adhesion protein-1 (VAP-1) is an endothelial copper-dependent amine oxidase involved in the recruitment and extravasation of leukocytes at sites of inflammation. VAP-1 is an important therapeutic target for several pathological conditions. We expressed soluble VAP-1 in HEK293 EBNA1 cells at levels suitable for detailed mechanistic studies with model substrates. Using the model substrate benzylamine, we analyzed the steady-state kinetic parameters of VAP-1 as a function of solution pH. We found two macroscopic pK(a) values that defined a bell-shaped plot of turnover number k(cat,app) as a function of pH, representing ionizable groups in the enzyme-substrate complex. The dependence of (k(cat)/K(m))(app) on pH revealed a single pK(a) value (∼9) that we assigned to ionization of the amine group in free benzylamine substrate. A kinetic isotope effect (KIE) of 6 to 7.6 on (k(cat)/K(m))(app) over the pH range of 6 to 10 was observed with d(2)-benzylamine. Over the same pH range, the KIE on k(cat) was found to be close to unity. The unusual KIE values on (k(cat)/K(m))(app) were rationalized using a mechanistic scheme that includes the possibility of multiple isotopically sensitive steps. We also report the analysis of quantitative structure-activity relationships (QSAR) using para-substituted protiated and deuterated phenylethylamines. With phenylethylamines we observed a large KIE on k(cat,app) (8.01 ± 0.28 with phenylethylamine), indicating that C-H bond breakage is limiting for 2,4,5-trihydroxyphenylalanine quinone reduction. Poor correlations were observed between steady-state rate constants and QSAR parameters. We show the importance of combining KIE, QSAR, and structural studies to gain insight into the complexity of the VAP-1 steady-state mechanism.

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.