Mechanism of mammalian soluble epoxide hydrolase inhibition by chalcone oxide derivatives

The Multifaceted Role of Epoxide Hydrolases in Human Health and Disease

Epoxide hydrolases (EHs) are key enzymes involved in the detoxification of xenobiotics and biotransformation of endogenous epoxides. They catalyze the hydrolysis of highly reactive epoxides to less reactive diols. EHs thereby orchestrate crucial signaling pathways for cell homeostasis. The EH family comprises 5 proteins and 2 candidate members, for which the corresponding genes are not yet identified. Although the first EHs were identified more than 30 years ago, the full spectrum of their substrates and associated biological functions remain partly unknown. The two best-known EHs are EPHX1 and EPHX2. Their wide expression pattern and multiple functions led to the development of specific inhibitors. This review summarizes the most important points regarding the current knowledge on this protein family and highlights the particularities of each EH. These different enzymes can be distinguished by their expression pattern, spectrum of associated substrates, sub-cellular localization, and enzymatic characteristics. We also reevaluated the pathogenicity of previously reported variants in genes that encode EHs and are involved in multiple disorders, in light of large datasets that were made available due to the broad development of next generation sequencing. Although association studies underline the pleiotropic and crucial role of EHs, no data on high-effect variants are confirmed to date.

Small Molecule Soluble Epoxide Hydrolase Inhibitors in Multitarget and Combination Therapies for Inflammation and Cancer

The enzyme soluble epoxide hydrolase (sEH) plays a central role in metabolism of bioactive lipid signaling molecules. The substrate-specific hydrolase activity of sEH converts epoxyeicosatrienoic acids (EETs) to less bioactive dihydroxyeicosatrienoic acids. EETs exhibit anti-inflammatory, analgesic, antihypertensive, cardio-protective and organ-protective properties. Accordingly, sEH inhibition is a promising therapeutic strategy for addressing a variety of diseases. In this review, we describe small molecule architectures that have been commonly deployed as sEH inhibitors with respect to angiogenesis, inflammation and cancer. We juxtapose commonly used synthetic scaffolds and natural products within the paradigm of a multitarget approach for addressing inflammation and inflammation induced carcinogenesis. Structural insights from the inhibitor complexes and novel strategies for development of sEH-based multitarget inhibitors are also presented. While sEH inhibition is likely to suppress inflammation-induced carcinogenesis, it can also lead to enhanced angiogenesis via increased EET concentrations. In this regard, sEH inhibitors in combination chemotherapy are described. Urea and amide-based architectures feature prominently across multitarget inhibition and combination chemotherapy applications of sEH inhibitors.

Epoxy-Oxylipins and Soluble Epoxide Hydrolase Metabolic Pathway as Targets for NSAID-Induced Gastroenteropathy and Inflammation-Associated Carcinogenesis

Polyunsaturated fatty acids (PUFAs) including epoxide-modified ω-3 and ω-6 fatty acids are made via oxidation to create highly polarized carbon-oxygen bonds crucial to their function as signaling molecules. A critical PUFA, arachidonic acid (ARA), is metabolized to a diverse set of lipids signaling molecules through cyclooxygenase (COX), lipoxygenase (LOX), cytochrome P450 epoxygenase, or cytochrome P450 hydroxylase; however, the majority of ARA is metabolized into anti-inflammatory epoxides via cytochrome P450 enzymes. These short-lived epoxide lipids are rapidly metabolized or inactivated by the soluble epoxide hydrolase (sEH) into diol-containing products. sEH inhibition or knockout has been a practical approach to study the biology of the epoxide lipids, and has been shown to effectively treat inflammatory conditions in the preclinical models including gastrointestinal ulcers and colitis by shifting oxylipins to epoxide profiles, inhibiting inflammatory cell infiltration and activation, and enhancing epithelial cell defense via increased mucin production, thus providing further evidence for the role of sEH as a pro-inflammatory protein. Non-steroidal anti-inflammatory drugs (NSAIDs) with COX-inhibitor activity are among the most commonly used analgesics and have demonstrated applications in the management of cardiovascular disease and intriguingly cancer. Major side effects of NSAIDs however are gastrointestinal ulcers which frequently precludes their long-term application. In this review, we hope to bridge the gap between NSAID toxicity and sEH-mediated metabolic pathways to focus on the role of epoxy fatty acid metabolic pathway of PUFAs in NSAIDS-ulcer formation and healing as well as inflammation-related carcinogenesis. Specifically we address the potential application of sEH inhibition to enhance ulcer healing at the site of inflammation via their activity on altered lipid signaling, mitochondrial function, and diminished reactive oxygen species, and further discuss the significance of dual COX and sEH inhibitor in anti-inflammation and carcinogenesis.

Discovery of Novel Soluble Epoxide Hydrolase Inhibitors as Potent Vasodilators

In view of the role of sEH (soluble epoxide hydrolase) in hypertension, we have developed a rigorously validated pharmacophore model containing one HBA (Hydrogen Bond Acceptor), two HY (Hydrophobic) and one RA (Ring Aromatic) features. The model was used as a query to search the NCI (National Cancer Institute) and Maybridge database leading to retrieval of many compounds which were sorted on the basis of predicted activity, fit value and Lipinski’s violation. The selected compounds were docked into the active site of enzyme soluble epoxide hydrolase. Potential interactions were observed between the features of the identified hits and the amino acids present in the docking site. The three selected compounds were subjected to in vitro evaluation using enzyme- based assay and the isolated rat aortic model followed by cytotoxicity studies. The results demonstrate that the identified compounds are potent, safe and novel soluble epoxide hydrolase inhibitors.

Visualizing the Mechanism of Epoxide Hydrolysis by the Bacterial Virulence Enzyme Cif

The CFTR inhibitory factor (Cif) is an epoxide hydrolase (EH) virulence factor secreted by the bacterium Pseudomonas aeruginosa. Sequence alignments reveal a pattern of Cif-like substitutions that proved to be characteristic of a new subfamily of bacterial EHs. At the same time, crystallographic and mutagenetic data suggest that EH activity is required for virulence and that Cif's active site remains generally compatible with a canonical two-step EH mechanism. A hallmark of this mechanism is the formation of a covalent hydroxyalkyl-enzyme intermediate by nucleophilic attack. In several well-studied EHs, this intermediate has been captured at near stoichiometric levels, presumably reflecting rate-limiting hydrolysis. Here we show by mass spectrometry that only minimal levels of the expected intermediate can be trapped with WT Cif. In contrast, substantial amounts of intermediate are recovered from an active-site mutant (Cif-E153Q) that selectively targets the second, hydrolytic release step. Utilizing Cif-E153Q and a previously reported nucleophile mutant (Cif-D129S), we then captured Cif in the substrate-bound, hydroxyalkyl-intermediate, and product-bound states for 1,2-epoxyhexane, yielding the first crystallographic snapshots of an EH at these key stages along the reaction coordinate. Taken together, our data illuminate the proposed two-step hydrolytic mechanism of a new class of bacterial virulence factor. They also suggest that the failure of WT Cif to accumulate a covalent hydroxyalkyl-enzyme intermediate reflects an active-site chemistry in which hydrolysis is no longer the rate-limiting step, a noncanonical kinetic regime that may explain similar observations with a number of other EHs.

Open Source Bayesian Models. 3. Composite Models for Prediction of Binned Responses

Bayesian models constructed from structure-derived fingerprints have been a popular and useful method for drug discovery research when applied to bioactivity measurements that can be effectively classified as active or inactive. The results can be used to rank candidate structures according to their probability of activity, and this ranking benefits from the high degree of interpretability when structure-based fingerprints are used, making the results chemically intuitive. Besides selecting an activity threshold, building a Bayesian model is fast and requires few or no parameters or user intervention. The method also does not suffer from such acute overtraining problems as quantitative structure–activity relationships or quantitative structure–property relationships (QSAR/QSPR). This makes it an approach highly suitable for automated workflows that are independent of user expertise or prior knowledge of the training data. We now describe a new method for creating a composite group of Bayesian models to extend the method to work with multiple states, rather than just binary. Incoming activities are divided into bins, each covering a mutually exclusive range of activities. For each of these bins, a Bayesian model is created to model whether or not the compound belongs in the bin. Analyzing putative molecules using the composite model involves making a prediction for each bin and examining the relative likelihood for each assignment, for example, highest value wins. The method has been evaluated on a collection of hundreds of data sets extracted from ChEMBL v20 and validated data sets for ADME/Tox and bioactivity.

Is Promiscuous CALB a Good Scaffold for Designing New Epoxidases?

Candida Antarctica lipase B (CALB) is a well-known enzyme, especially because of its promiscuous activity. Due to its properties, CALB was widely used as a benchmark for designing new catalysts for important organic reactions. The active site of CALB is very similar to that of soluble epoxide hydrolase (sEH) formed by a nucleophile-histidine-acid catalytic triad and an oxyanion hole typical for molecular structures derived from processes of α/β hydrolases. In this work we are exploring these similarities and proposing a Ser105Asp variant of CALB as a new catalyst for epoxide hydrolysis. In particular, the hydrolysis of the trans-diphenylpropene oxide (t-DPPO) is studied by means of quantum cluster models mimicking the active site of both enzymes. Our results, based on semi-empirical and DFT calculations, suggest that mutant Ser105Asp CALB is a good protein scaffold to be used for the bio-synthesis of chiral compounds.

Soluble epoxide hydrolase inhibitors and heart failure

Cardiovascular disease remains one of the leading causes of death in the Western societies. Heart failure (HF) is due primarily to progressive myocardial dysfunction accompanied by myocardial remodeling. Once HF develops, the condition is, in most cases, irreversible and is associated with a very high mortality rate. Soluble epoxide hydrolase (sEH) is an enzyme that catalyzes the hydrolysis of epoxyeicosatrienoic acids (EETs), which are lipid mediators derived from arachidonic acid through the cytochrome P450 epoxygenase pathway. EETs have been shown to have vasodilatory, antiinflammatory, and cardioprotective effects. When EETs are hydrolyzed by sEH to corresponding dihydroxyeicosatrienoic acids, their cardioprotective activities become less pronounced. In line with the recent genetic study that has identified sEH as a susceptibility gene for HF, the sEH enzyme has received considerable attention as an attractive therapeutic target for cardiovascular diseases. Indeed, sEH inhibition has been demonstrated to have antihypertensive and antiinflammatory actions, presumably due to the increased bioavailability of endogenous EETs and other epoxylipids, and several potent sEH inhibitors have been developed and tested in animal models of cardiovascular disease including hypertension, cardiac hypertrophy, and ischemia/reperfusion injury. sEH inhibitor treatment has been shown to effectively prevent pressure overload- and angiotensin II-induced cardiac hypertrophy and reverse the pre-established cardiac hypertrophy caused by chronic pressure overload. Application of sEH inhibitors in several cardiac ischemia/reperfusion injury models reduced infarct size and prevented the progressive cardiac remodeling. Moreover, the use of sEH inhibitors prevented the development of electrical remodeling and ventricular arrhythmias associated with cardiac hypertrophy and ischemia/reperfusion injury. The data published to date support the notion that sEH inhibitors may represent a promising therapeutic approach for combating detrimental cardiac remodeling and HF.

The role of epoxide hydrolases in health and disease

Epoxide hydrolases (EH) are ubiquitously expressed in all living organisms and in almost all organs and tissues. They are mainly subdivided into microsomal and soluble EH and catalyze the hydration of epoxides, three-membered-cyclic ethers, to their corresponding dihydrodiols. Owning to the high chemical reactivity of xenobiotic epoxides, microsomal EH is considered protective enzyme against mutagenic and carcinogenic initiation. Nevertheless, several endogenously produced epoxides of fatty acids function as important regulatory mediators. By mediating the formation of cytotoxic dihydrodiol fatty acids on the expense of cytoprotective epoxides of fatty acids, soluble EH is considered to have cytotoxic activity. Indeed, the attenuation of microsomal EH, achieved by chemical inhibitors or preexists due to specific genetic polymorphisms, is linked to the aggravation of the toxicity of xenobiotics, as well as the risk of cancer and inflammatory diseases, whereas soluble EH inhibition has been emerged as a promising intervention against several diseases, most importantly cardiovascular, lung and metabolic diseases. However, there is reportedly a significant overlap in substrate selectivity between microsomal and soluble EH. In addition, microsomal and soluble EH were found to have the same catalytic triad and identical molecular mechanism. Consequently, the physiological functions of microsomal and soluble EH are also overlapped. Thus, studying the biological effects of microsomal or soluble EH alterations needs to include the effects on both the metabolism of reactive metabolites, as well as epoxides of fatty acids. This review focuses on the multifaceted role of EH in the metabolism of xenobiotic and endogenous epoxides and the impact of EH modulations.

Effect of soluble epoxide hydrolase polymorphism on substrate and inhibitor selectivity and dimer formation

Epoxy FAs (EpFAs) are important lipid mediators that are mainly metabolized by soluble epoxide hydrolase (sEH). Thus, sEH inhibition is a promising therapeutic target to treat numerous ailments. Several sEH polymorphisms result in amino acid substitutions and alter enzyme activity. K55R and R287Q are associated with inflammatory, cardiovascular, and metabolic diseases. R287Q seems to affect sEH activity through reducing formation of a catalytically active dimer. Thus, understanding how these SNPs affect the selectivity of sEH for substrates and inhibitors is of potential clinical importance. We investigated the selectivity of four sEH SNPs toward a series of EpFAs and inhibitors. We found that the SNPs alter the catalytic activity of the enzyme but do not alter the relative substrate and inhibitor selectivity. We also determined their dimer/monomer constants (KD/M). The WT sEH formed a very tight dimer, with a KD/M in the low picomolar range. Only R287Q resulted in a large change of the KD/M However, human tissue concentrations of sEH suggest that it is always in its dimer form independently of the SNP. These results suggest that the different biologies associated with K55R and R287Q are not explained by alteration in dimer formation or substrate selectivity.

Design and discovery of soluble epoxide hydrolase inhibitors for the treatment of cardiovascular diseases

INTRODUCTION

Cardiovascular diseases are a leading cause of death in developed countries. Increasing evidence shows that the alteration in the normal functions of the vascular endothelium plays a major role in the development of cardiovascular diseases. However, specific agents designed to prevent endothelial dysfunction and related cardiovascular complications are still lacking. One emerging strategy is to increase the bioavailability of epoxyeicosatrienoic acids (EETs), synthesized by cytochrome P450 epoxygenases from arachidonic acid. EETs are endothelium-derived hyperpolarising and relaxing factors and display attractive anti-inflammatory and metabolic properties. Genetic polymorphism studies in humans, and experiments in animal models of diseases, have identified soluble epoxide hydrolase (sEH), the major enzyme involved in EET degradation, as a potential pharmacological target.

AREAS COVERED

This review presents EET pathway and its functions and summarises the data supporting the development of sEH inhibitors for the treatment of cardiovascular and metabolic diseases. Furthermore, the authors present the different chemical families of sEH inhibitors developed and their effects in animal models of cardiovascular and metabolic diseases.

EXPERT OPINION

Several generations of sEH inhibitors have now been designed to treat endothelial dysfunction and cardiovascular complications for a variety of diseases. The safety of these drugs remains to be carefully investigated, particularly in relation to carcinogenesis. The increasing knowledge of the biological role of each of the EET isomers and of their metabolites may improve their pharmacological profile. This, in turn, could potentially lead to the identification of new pharmacological agents that achieve the cellular effects needed without the deleterious side effects.

Development of an HTS assay for EPHX2 phosphatase activity and screening of nontargeted libraries

The EPXH2 gene encodes soluble epoxide hydrolase (sEH), which has two distinct enzyme activities: epoxide hydrolase (Cterm-EH) and phosphatase (Nterm-phos). The Cterm-EH is involved in the metabolism of arachidonic acid epoxides that play important roles in blood pressure, cell growth, inflammation, and pain. While recent findings suggested complementary biological roles for Nterm-phos, research is limited by the lack of potent bioavailable inhibitors of this phosphatase activity. Also, a potent bioavailable inhibitor of this activity could be important in the development of therapy for cardiovascular diseases. We report herein the development of an HTS enzyme-based assay for Nterm-phos (Z'>0.9) using AttoPhos as the substrate. This assay was used to screen a wide variety of chemical entities, including a library of known drugs that have reached through clinical evaluation (Pharmakon 1600), as well as a library of pesticides and environmental toxins. We discovered that ebselen inhibits sEH phosphatase activity. Ebselen binds to the N-terminal domain of sEH (K(I)=550 nM) and chemically reacts with the enzyme to quickly and irreversibly inhibit Nterm-phos, and subsequently Cterm-EH, and thus represents a new class of sEH inhibitor.

Investigation of human exposure to triclocarban after showering and preliminary evaluation of its biological effects

The antibacterial soap additive triclocarban (TCC) is widely used in personal care products. TCC has a high environmental persistence. We developed and validated a sensitive online solid-phase extraction-LC-MS/MS method to rapidly analyze TCC and its major metabolites in urine and other biological samples to assess human exposure. We measured human urine concentrations 0-72 h after showering with a commercial bar soap containing 0.6% TCC. The major route of renal elimination was excretion as N-glucuronides. The absorption was estimated at 0.6% of the 70±15 mg of TCC in the soap used. The TCC-N-glucuronide urine concentration varied widely among the subjects, and continuous daily use of the soap led to steady state levels of excretion. In order to assess potential biological effects arising from this exposure, we screened TCC for the inhibition of human enzymes in vitro. We demonstrate that TCC is a potent inhibitor of the enzyme soluble epoxide hydrolase (sEH), whereas TCC's major metabolites lack strong inhibitory activity. Topical administration of TCC at similar levels to rats in a preliminary in vivo study, however, failed to alter plasma biomarkers of sEH activity. Overall the analytical strategy described here revealed that use of TCC soap causes exposure levels that warrant further evaluation.

Inhibition of soluble epoxide hydrolase by trans-4- [4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid is protective against ischemia-reperfusion injury

Arachidonic acid, a polyunsaturated fatty acid, can be metabolized to cardioprotective epoxyeicosatrienoic acids (EETs) by cytochrome P450 epoxygenases, which are subsequently hydrolyzed to less bioactive dihydroxyeicosatrienoic acids by soluble epoxide hydrolase (sEH). To study the effects of pharmacological inhibitor of sEH (sEHi), C57BL6 mice hearts were perfused in Langendorff mode for 40 minutes of baseline and subjected to 30 minutes of global no-flow ischemia followed by 40 minutes of reperfusion. Hearts were perfused with the sEHi, trans-4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (t-AUCB; 0.05, 0.1, 0.5, and 1 microM). To study the mechanism(s), hearts were perfused with 0.1 microM t-AUCB in the presence or absence of putative EET receptor antagonist 14,15-epoxyeicosa-5(Z)-enoic acid (10 microM) or phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin (200 nM) or LY294002 (5 microM).Infarct size was determined at the end of 2-hour reperfusion by 2,3,5-triphenyltetrazolium chloride staining. Inhibition of sEH by t-AUCB significantly improved postischemic left ventricular developed pressure (LVDP) recovery and reduced the infarct size after ischemia and reperfusion, as compared with control hearts. Perfusion with 14,15-epoxyeicosa-5(Z)-enoic acid, wortmannin or LY294002 before ischemia abolished the cardioprotective phenotype; however, co-perfusion of both t-AUCB and 11,12-EET did not result in an additive effect on improved LVDP recovery. Together, our data suggest that pharmacological inhibition of sEH by t-AUCB is cardioprotective.

Mammalian epoxide hydrolases in xenobiotic metabolism and signalling

Epoxide hydrolases catalyse the hydrolysis of electrophilic--and therefore potentially genotoxic--epoxides to the corresponding less reactive vicinal diols, which explains the classification of epoxide hydrolases as typical detoxifying enzymes. The best example is mammalian microsomal epoxide hydrolase (mEH)-an enzyme prone to detoxification-due to a high expression level in the liver, a broad substrate selectivity, as well as inducibility by foreign compounds. The mEH is capable of inactivating a large number of structurally different, highly reactive epoxides and hence is an important part of the enzymatic defence of our organism against adverse effects of foreign compounds. Furthermore, evidence is accumulating that mammalian epoxide hydrolases play physiological roles other than detoxification, particularly through involvement in signalling processes. This certainly holds true for soluble epoxide hydrolase (sEH) whose main function seems to be the turnover of lipid derived epoxides, which are signalling lipids with diverse functions in regulatory processes, such as control of blood pressure, inflammatory processes, cell proliferation and nociception. In recent years, the sEH has attracted attention as a promising target for pharmacological inhibition to treat hypertension and possibly other diseases. Recently, new hitherto uncharacterised epoxide hydrolases could be identified in mammals by genome analysis. The expression pattern and substrate selectivity of these new epoxide hydrolases suggests their participation in signalling processes rather than a role in detoxification. Taken together, epoxide hydrolases (1) play a central role in the detoxification of genotoxic epoxides and (2) have an important function in the regulation of physiological processes by the control of signalling molecules with an epoxide structure.

Gerry Brooks and epoxide hydrolases: four decades to a pharmaceutical

The pioneering work of Gerry Brooks on cyclodiene insecticides led to the discovery of a class of enzymes known as epoxide hydrolases. The results from four decades of work confirm Brooks' first observations that the microsomal epoxide hydrolase is important in foreign compound metabolism. Brooks and associates went on to be the first to carry out a systematic study of the inhibition of this enzyme. A second role for this enzyme family was in the degradation of insect juvenile hormone (JH). JH epoxide hydrolases have now been cloned and expressed from several species, and there is interest in developing inhibitors for them. Interestingly, the distantly related mammalian soluble epoxide hydrolase has emerged as a promising pharmacological target for treating hypertension, inflammatory disease and pain. Tight-binding transition-state inhibitors were developed with good ADME (absorption, distribution, metabolism and excretion). These compounds stabilize endogenous epoxides of fatty acids, including arachidonic acid, which have profound therapeutic effects. Now EHs from microorganisms and plants are used in green chemistry. From his seminal work, Dr Brooks opened the field of epoxide hydrolase research in many directions including xenobiotic metabolism, insect physiology and human health, as well as asymmetric organic synthesis.

The soluble epoxide hydrolase as a pharmaceutical target for hypertension

The soluble epoxide hydrolase appears to be a promising target for the development of antihypertensive therapies based on a previously unexplored mechanism of action. Epoxide hydrolases are enzymes that add water to three membered cyclic ethers known as epoxides. The soluble epoxide hydrolase in mammalian systems (sEH) is a member of the alpha/beta-hydrolase fold family of enzymes and it shows a high degree of selectivity for epoxides of fatty acids. The regioisomeric epoxides of arachidonic acid or epoxyeicosanoids (EETs) are particularly good substrates. These EETs appear to be major components of the endothelium-derived hyperpolarizing factors (EDHFs). As such, EETs cause vasodilation and reduce blood pressure. The EETs also are strongly anti-inflammatory and analgesic. By inhibiting sEH, the increase in circulating EETs leads to a reduction in blood pressure in a number of animal models. Potent transition state mimic inhibitors have been developed for the sEH. Some of these sEH inhibitors (sEHIs) show nanomolar to picomolar potency and good pharmacokinetic properties. Because of their unique mode of action they show promise in treating hypertension while reducing problems with end organ failure, vascular inflammation and diabetes. Indeed, the anti-inflammatory properties of the sEHI may make them particularly suitable for treating hypertension in patients with other concomitant metabolic syndromes. They are more potent on a molar basis than most nonsteroidal anti-inflammatory drugs (NSAIDs) in reducing PGE2 in inflammation models, they strongly synergize with NSAIDs, and appear to ameliorate apparently unfavorable eicosanoid profiles associated with some cyclo-oxygenase-2 inhibitors.

Cardiovascular therapeutic aspects of soluble epoxide hydrolase inhibitors

Soluble epoxide hydrolase (sEH) is an enzyme responsible for the conversion of lipid epoxides to diols by the addition of water. Biological actions on the cardiovascular system that are attributed to epoxides include vasodilation, antiinflammatory actions and vascular smooth muscle cell antimigratory actions. Conversion of arachidonic acid epoxides to diols by sEH diminishes the beneficial cardiovascular properties of these epoxyeicosano-ids. Cardiovascular diseases in animal models and humans have been associated with decreased epoxygenase activity or increased sEH activity and these changes are responsible for the progression of the disease state. More recently, sEH gene polymorphisms in the human population have been associated with increased risk for cardiovascular diseases. Thus the biological actions of epoxyeicosanoids and the sEH enzyme are ideal therapeutic targets for cardiovascular diseases. The rapid development of 1,3-disubstituted urea based sEH inhibitors over the past five years has resulted in a number of studies demonstrating cardiovascular protection. sEH inhibitors have antihypertensive and antiinflammatory actions and have been demonstrated to decrease cerebral ischemic and renal injury in rat models of hypertension. These findings of beneficial actions in animal models of disease position the sEH enzyme as a promising therapeutic target for cardiovascular diseases.

Theoretical Study of the Full Reaction Mechanism of Human Soluble Epoxide Hydrolase

The complete reaction mechanism of soluble epoxide hydrolase (sEH) has been investigated by using the B3LYP density functional theory method. Epoxide hydrolases catalyze the conversion of epoxides to their corresponding vicinal diols. In our theoretical study, the sEH active site is represented by quantum-chemical models that are based on the X-ray crystal structure of human soluble epoxide hydrolase. The trans-substituted epoxide (1S,2S)-beta-methylstyrene oxide has been used as a substrate in the theoretical investigation of the sEH reaction mechanism. Both the alkylation and the hydrolytic half-reactions have been studied in detail. We present the energetics of the reaction mechanism as well as the optimized intermediates and transition-state structures. Full potential energy curves for the reactions involving nucleophilic attack at either the benzylic or the homo-benzylic carbon atom of (1S,2S)-beta-methylstyrene oxide have been computed. The regioselectivity of epoxide opening has been addressed for the two substrates (1S,2S)-beta-methylstyrene oxide and (S)-styrene oxide.

Solid-phase combinatorial approach for the optimization of soluble epoxide hydrolase inhibitors

A 192-member library of N,N'-disubstituted urea inhibitors was synthesized by a solid-phase method. The ureas were tested for their inhibitory activities against recombinant human soluble epoxide hydrolase. Simple carbocyclic or para/meta-substituted phenyl groups showed inhibition potencies that were equal to or greater than adamantane-based sEH inhibitors, while the presence of bulky or ionizable groups close to the urea group dramatically decreased their activities.

Development of a high-throughput screen for soluble epoxide hydrolase inhibition

Mammalian soluble epoxide hydrolase (sEH) represents a highly promising new target for drug development. Chemical inhibition of this enzyme in animal models was shown to treat hypertension and vascular inflammation as well as related syndromes. Existing sEH inhibitors are relatively potent and specific. However, the low solubility and relatively fast metabolism of described sEH inhibitors make them less than therapeutically efficient, stating the need for novel inhibitor structures. Therefore, a series of alpha-cyanoester and alpha-cyanocarbonate epoxides were evaluated as potential human sEH (HsEH) substrates for the high-throughput screen (HTS) of compound libraries. (3-Phenyl-oxiranyl)-acetic acid cyano-(6-methoxy-naphthalen-2-yl)-methyl ester (PHOME), which displayed the highest aqueous stability and solubility, was selected for the development of an HTS assay with long incubation times at room temperature. Concentrations of HsEH and PHOME were optimized to ensure assay sensitivity, reliability, and reproducibility. Assay validation, which employed these optimized concentrations, resulted in good accuracy (60-100%) and high precision (<7% relative standard deviation). In addition, an overall Z' value of 0.7 proved the system's robustness and potential for HTS. The developed assay system will be a valuable tool to discover new structures for the therapeutic inhibition of sEH to treat various cardiovascular diseases.

Lipid sulfates and sulfonates are allosteric competitive inhibitors of the N-terminal phosphatase activity of the mammalian soluble epoxide hydrolase

The EPXH2 gene encodes for the soluble epoxide hydrolase (sEH), a homodimeric enzyme with each monomer containing two domains with distinct activities. The C-terminal domain, containing the epoxide hydrolase activity (Cterm-EH), is involved in the metabolism of arachidonic acid epoxides, endogenous chemical mediators that play important roles in blood pressure regulation, cell growth, and inflammation. We recently demonstrated that the N-terminal domain contains a Mg2+-dependent lipid phosphate phosphatase activity (Nterm-phos). However, the biological role of this activity is unknown. The inability of known phosphatase inhibitors to inhibit the Nterm-phos constitutes a significant barrier to the elucidation of its function. We describe herein sulfate, sulfonate, and phosphonate lipids as novel potent inhibitors of Nterm-phos. These compounds are allosteric competitive inhibitors with K(I) in the hundred nanomolar range. These inhibitors may provide a valuable tool to investigate the biological role of the Nterm-phos. We found that polyisoprenyl phosphates are substrates of Nterm-phos, suggesting a possible role in sterol synthesis or inflammation. Furthermore, some of these compounds inhibit the C-terminal sEH activity through a noncompetitive inhibition mechanism involving a new binding site on the C-terminal domain. This novel site may play a role in the natural in vivo regulation of epoxide hydrolysis by sEH.

Human soluble epoxide hydrolase: structural basis of inhibition by 4-(3-cyclohexylureido)-carboxylic acids

X-ray crystal structures of human soluble epoxide hydrolase (sEH) complexed with four different dialkylurea inhibitors bearing pendant carboxylate "tails" of varying length have been determined at 2.3-3.0 A resolution. Similarities among inhibitor binding modes reinforce the proposed roles of Y381 and/or Y465 as general acids that protonate the epoxide ring of the substrate in concert with nucleophilic attack of D333 at the electrophilic epoxide carbon. Additionally, the binding of these inhibitors allows us to model the binding mode of the endogenous substrate 14,15-epoxyeicosatrienoic acid. Contrasts among inhibitor binding modes include opposite orientations of inhibitor binding in the active-site hydrophobic tunnel. Alternative binding orientations observed for this series of inhibitors to human sEH, as well as the binding of certain dialkylurea inhibitors to human sEH and murine sEH, complicate the structure-based design of human sEH inhibitors with potential pharmaceutical applications in the treatment of hypertension. Thus, with regard to the optimization of inhibitor designs targeting human sEH, it is critical that human sEH and not murine sEH be utilized for inhibitor screening, and it is critical that structures of human sEH-inhibitor complexes be determined to verify inhibitor binding orientations that correlate with measured affinities.

Fluorescent substrates for soluble epoxide hydrolase and application to inhibition studies

Inhibition of the mammalian soluble epoxide hydrolase (sEH) is a promising new therapy in the treatment of disorders resulting from hypertension and vascular inflammation. A spectrophotometric assay (4-nitrophenyl-trans-2,3-epoxy-3-phenylpropyl carbonate, NEPC) is currently used to screen libraries of chemicals; however this assay lacks the required sensitivity to differentiate the most potent inhibitors. A series of fluorescent alpha-cyanoester and alpha-cyanocarbonate epoxides that produce a strong fluorescent signal on epoxide hydrolysis by both human and murine sEH were designed as potential substrates for an in vitro inhibition assay. The murine enzyme showed a broad range of specificities, whereas the human enzyme showed the highest specificity for cyano(6-methoxy-naphthalen-2-yl)methyl trans-[(3-phenyloxiran-2-yl)methyl] carbonate. An in vitro inhibition assay was developed using this substrate and recombinant enzyme. The utility of the fluorescent assay was confirmed by determining the IC(50) values for a series of known inhibitors. The new IC(50) values were compared with those determined by spectrophotometric NEPC and radioactive tDPPO assays. The fluorescent assay ranked these inhibitors on the basis of IC(50) values, whereas the NEPC assay did not. The ranking of inhibitor potency generally agreed with that determined using the tDPPO assay. These results show that the fluorescence-based assay is a valuable tool in the development of sEH inhibitors by revealing structure-activity relationships that previously were seen only by using the costly and labor-intensive radioactive tDPPO assay.

Epoxide hydrolases: mechanisms, inhibitor designs, and biological roles

Organisms are exposed to epoxide-containing compounds from both exogenous and endogenous sources. In mammals, the hydration of these compounds by various epoxide hydrolases (EHs) can not only regulate their genotoxicity but also, for lipid-derived epoxides, their endogenous roles as chemical mediators. Recent findings suggest that the EHs as a family represent novel drug discovery targets for regulation of blood pressure, inflammation, cancer progression, and the onset of several other diseases. Knowledge of the EH mechanism provides a solid foundation for the rational design of inhibitors, and this review summarizes the current understanding of the catalytic mechanism of the EHs. Although the overall EH mechanism is now known, the molecular basis of substrate selectivity, possible allosteric regulation, and many fine details of the catalytic mechanism remain to be solved. Finally, recent development in the design of EH inhibitors and the EH biological role are discussed.

Soybean epoxide hydrolase: identification of the catalytic residues and probing of the reaction mechanism with secondary kinetic isotope effects

Soybean epoxide hydrolase catalyzes the oxirane ring opening of 9,10-epoxystearate via a two-step mechanism involving the formation of an alkylenzyme intermediate, which, in contrast to most epoxide hydrolases studied so far, was found to be the rate-limiting step. We have probed residues potentially involved in catalysis by site-directed mutagenesis. Mutation of His(320), a residue predicted from sequence analysis to belong to the catalytic triad of the enzyme, considerably slowed down the second half-reaction. This kinetic manipulation provoked an accumulation of the reaction intermediate, which could be trapped and characterized by electrospray ionization mass spectrometry. As expected, mutation of Asp(126) totally abolished the activity of the enzyme from its crucial function as nucleophile involved in the formation of the alkylenzyme. In line with its role as the partner of His(320) in the "charge relay system," mutation of Asp(285) dramatically reduced the rate of catalysis. However, the mutant D285L still exhibited a very low residual activity, which, by structural analysis and mutagenesis, has been tentatively attributed to Glu(195), another acidic residue of the active site. Our studies have also confirmed the fundamental role of the conserved Tyr(175) and Tyr(255) residues, which are believed to activate the oxirane ring. Finally, we have determined the secondary tritium kinetic isotope effects on the epoxide opening step of 9,10-epoxystearate. The large observed values, i.e. (T)(V/K(m)) approximately 1.30, can be interpreted by the occurrence of a very late transition state in which the epoxide bond is broken before the nucleophilic attack by Asp(126) takes place.

QSAR and classification of murine and human soluble epoxide hydrolase inhibition by urea-like compounds

A data set of 348 urea-like compounds that inhibit the soluble epoxide hydrolase enzyme in mice and humans is examined. Compounds having IC(50) values ranging from 0.06 to >500 microM (murine) and 0.10 to >500 microM (human) are categorized as active or inactive for classification, while quantitation is performed on smaller compound subsets ranging from 0.07 to 431 microM (murine) and 0.11 to 490 microM (human). Each compound is represented by calculated structural descriptors that encode topological, geometrical, electronic, and polar surface features. Multiple linear regression (MLR) and computational neural networks (CNNs) are employed for quantitative models. Three classification algorithms, k-nearest neighbor (kNN), linear discriminant analysis (LDA), and radial basis function neural networks (RBFNN), are used to categorize compounds as active or inactive based on selected data split points. Quantitative modeling of human enzyme inhibition results in a nonlinear, five-descriptor model with root-mean-square errors (log units of IC(50) [microM]) of 0.616 (r(2) = 0.66), 0.674 (r(2) = 0.61), and 0.914 (r(2) = 0.33) for training, cross-validation, and prediction sets, respectively. The best classification results for human and murine enzyme inhibition are found using kNN. Human classification rates using a seven-descriptor model for training and prediction sets are 89.1% and 91.4%, respectively. Murine classification rates using a five-descriptor model for training and prediction sets are 91.5% and 88.6%, respectively.

The soluble epoxide hydrolase encoded by EPXH2 is a bifunctional enzyme with novel lipid phosphate phosphatase activity

The gene EPXH2 encodes for the soluble epoxide hydrolase (sEH), an enzyme involved in the regulation of cardiovascular and renal physiology containing two distinct domains connected via a proline-rich linker. The C-terminal domain containing the EH catalytic activity has been well studied. In contrast, a function for the N-terminal domain, which has high homology to the haloacid dehalogenase family of phosphatases, has not been definitively reported. In this study we describe the N-terminal domain as a functional phosphatase unaffected by a number of classic phosphatase inhibitors. Assuming a functional association between these catalytic activities, dihydroxy lipid phosphates were rationalized as potential endogenous substrates. A series of phosphorylated hydroxy lipids were therefore synthesized and found to be excellent substrates for the human sEH. The best substrate tested was the monophosphate of dihydroxy stearic acid (threo-910-phosphonoxy-hydroxy-octadecanoic acid) with K(m) = 21 +/- 0.3 microM, V(Max) = 338 +/- 12 nmol x min(-1) x mg(-1), and k(cat) = 0.35 +/- 0.01 s(-1). Therefore dihydroxy lipid phosphates are possible candidates for the endogenous substrates of the sEH N-terminal domain, which would represent a novel branch of fatty acid metabolism with potential signaling functions.

Urea and amide-based inhibitors of the juvenile hormone epoxide hydrolase of the tobacco hornworm (Manduca sexta: Sphingidae)

A new class of inhibitors of juvenile hormone epoxide hydrolase (JHEH) of Manduca sexta and further in vitro characterization of the enzyme are reported. The compounds are based on urea and amide pharmacophores that were previously demonstrated as effective inhibitors of mammalian soluble and microsomal epoxide hydrolases. The best inhibitors against JHEH activity so far within this class are N-[(Z)-9-octadecenyl]-N'-propylurea and N-hexadecyl-N'-propylurea, which inhibited hydrolysis of a surrogate substrate (t-DPPO) with an IC(50) around 90 nM. The importance of substitution number and type was investigated and results indicated that N, N'-disubstitution with asymmetric alkyl groups was favored. Potencies of pharmacophores decreased as follows: amide>urea>carbamate>carbodiimide>thiourea and thiocarbamate for N, N'-disubstituted compounds with symmetric substituents, and urea>amide>carbamate for compounds with asymmetric N, N'-substituents. JHEH hydrolyzes t-DPPO with a K(m) of 65.6 microM and a V(max) of 59 nmol min(-1) mg(-1) and has a substantially lower K(m) of 3.6 microM and higher V(max) of 322 nmol min(-1) mg(-1) for JH III. Although none of these compounds were potent inhibitors of hydrolysis of JH III by JHEH, they are the first leads toward inhibitors of JHEH that are not potentially subject to metabolism through epoxide degradation.

Inhibitors of soluble epoxide hydrolase attenuate vascular smooth muscle cell proliferation

Atherosclerosis, in its myriad incarnations the foremost killer disease in the industrialized world, is characterized by aberrant proliferation of vascular smooth muscle (VSM) cells in part as a result of the recruitment of inflammatory cells to the blood vessel wall. The epoxyeicosatrienoic acids are synthesized from arachidonic acid in a reaction catalyzed by the cytochrome P450 system and are vasoactive substances. Metabolism of these compounds by epoxide hydrolases results in the formation of compounds that affect the vasculature in a pleiotropic manner. As an outgrowth of our observations that urea inhibitors of the soluble epoxide hydrolase (sEH) reduce blood pressure in spontaneously hypertensive rats as well as the findings of other investigators that these compounds possess antiinflammatory actions, we have examined the effect of sEH inhibitors on VSM cell proliferation. We now show that the sEH inhibitor 1-cyclohexyl-3-dodecyl urea (CDU) inhibits human VSM cell proliferation in a dose-dependent manner and is associated with a decrease in the level of cyclin D1. In addition, cis-epoxyeicosatrienoic acid mimics the growth-suppressive activity of CDU; there is no evidence of cellular toxicity or apoptosis in CDU-treated cells when incubated with 20 microM CDU for up to 48 h. These results, in light of the antiinflammatory and antihypertensive properties of these compounds that have been demonstrated already, suggest that the urea class of sEH inhibitors may be useful for therapy for diseases such as hypertension and atherosclerosis characterized by exuberant VSM cell proliferation and vascular inflammation.

Broussochalcone A, a potent antioxidant and effective suppressor of inducible nitric oxide synthase in lipopolysaccharide-activated macrophages

The antioxidant properties of broussochalcone A (BCA) and its effects on nitric oxide (NO) production in lipopolysaccharide (LPS)-activated macrophages were investigated in this study. BCA, isolated from Broussonetia papyrifera Vent., inhibited iron-induced lipid peroxidation in rat brain homogenate in a concentration-dependent manner with an IC(50) of 0.63 +/- 0.03 microM. It was as potent as butylated hydroxytoluene, a common antioxidant used for food preservation. In a diphenyl-2-picrylhydrazyl assay system, the radical-scavenging activity of BCA seemed to be more potent than that of alpha-tocopherol, its IC(0.200) being 7.6 +/- 0.8 microM. BCA could directly scavenge superoxide anion and hydroxyl radicals. These results indicated that BCA was a powerful antioxidant with versatile free radical-scavenging activity. On the other hand, we found that BCA suppressed NO production concentration-dependently, with an IC(50) of 11.3 microM in LPS-activated macrophages. This effect was not the consequence of a direct inhibitory action on the enzyme activity of inducible NO synthase (iNOS). Our results indicated that BCA exerts potent inhibitory effects on NO production, apparently mediated by its suppression of IkappaBalpha phosphorylation, IkappaBalpha degradation, nuclear factor-kappa B activation, and iNOS expression. Therefore, we conclude that the antioxidant activities of BCA and its inhibition of IkappaBalpha degradation and iNOS protein expression may have therapeutic potential, given that excessive free radicals and NO production have been associated with various inflammatory diseases.

3-D QSAR analysis of inhibition of murine soluble epoxide hydrolase (MsEH) by benzoylureas, arylureas, and their analogues

Two hundred and seventy-one compounds including benzoylureas, arylureas and related compounds were assayed using recombinant murine soluble epoxide hydrolase (MsEH) produced from a baculovirus expression system. Among all the insect growth regulators assayed, 18 benzoylphenylurea congeners showed weak activity against MsEH. Newly synthesized cyclohexylphenylurea, 1-benzyl-3-phenylurea, and 1,3-dibenzylurea analogues were rather potent. The introduction of a methyl group at the para-position of the phenyl ring of cyclohexylphenylurea enhanced the activity 6-fold, though similar substituent effects were not seen for any of the benzoylphenylureas. The activities of these compounds, including several previously reported compounds, such as dicyclohexylurea, diphenylurea, and their related analogues (Morisseau et al., Proc. Natl. Acad. Sci., 1999, 96, 8849), were quantitatively analyzed using comparative molecular field analysis (CoMFA), a three-dimensional quantitative structure-activity relationship (3-D QSAR) method. Both steric and electrostatic factors contributing to variations in the activity were visualized using CoMFA. CoMFA results showed that one side of the cyclohexylurea moiety having a trans-amide conformation (A-ring moiety) is surrounded by large sterically unfavorable fields, while the other side of A-ring moiety and the other cyclohexyl group (B-ring moiety) is encompassed by sterically favored fields. Electrostatically negative fields were scattered around the entire molecule, and a positive field surrounds the carbon of the carbonyl group. Hydrophobic fields were visualized using Kellogg's hydropathic interaction (HINT) in conjunction with CoMFA. Hydrophobically favorable fields appeared beside the 4- and 4'-carbon atoms of the cyclohexyl groups, and hydrophobically unfavorable fields surrounded the urea bridge. The addition of the molecular hydrophobicity, log P [corrected], to CoMFA did not improve the correlation significantly. The ligand-binding interactions shown by X-ray crystallographic data were rationalized using the results of the CoMFA and HINT analyses, and the essential physicochemical parameters for the design of new MsEH inhibitors were disclosed.

Inhibition of insect juvenile hormone epoxide hydrolase: asymmetric synthesis and assay of glycidol-ester and epoxy-ester inhibitors of trichoplusia ni epoxide hydrolase

Juvenile hormone (JH) undergoes metabolic degradation by two major pathways involving JH esterase and JH epoxide hydrolase (EH). While considerable effort has been focussed on the study of JH esterase and the development of inhibitors for this enzyme, much less has been reported on the study of JH-EH. In this work, the asymmetric synthesis of two classes of inhibitors of recombinant JH-EH from Trichoplusia ni, a glycidol-ester series and an epoxy-ester series is reported. The most effective glycidol-ester inhibitor, compound 1, exhibited an I(50) of 1.2x10(-8) M, and the most effective epoxy-ester inhibitor, compound 11, exhibited an I(50) of 9.4x10(-8) M. The potency of the inhibitors was found to be dependent on the absolute configuration of the epoxide. In both series of inhibitors, the C-10 R-configuration was found to be significantly more potent that the corresponding C-10 S-configuration. A mechanism for epoxide hydration catalyzed by insect EH is also presented.

Biochemical evidence for the involvement of tyrosine in epoxide activation during the catalytic cycle of epoxide hydrolase

Epoxide hydrolases (EH) catalyze the hydrolysis of epoxides and arene oxides to their corresponding diols. The crystal structure of murine soluble EH suggests that Tyr(465) and Tyr(381) act as acid catalysts, activating the epoxide ring and facilitating the formation of a covalent intermediate between the epoxide and the enzyme. To explore the role of these two residues, mutant enzymes were produced and the mechanism of action was analyzed. Enzyme assays on a series of substrates confirm that both Tyr(465) and Tyr(381) are required for full catalytic activity. The kinetics of chalcone oxide hydrolysis show that mutation of Tyr(465) and Tyr(381) decreases the rate of binding and the formation of an intermediate, suggesting that both tyrosines polarize the epoxide moiety to facilitate ring opening. These two tyrosines are, however, not implicated in the hydrolysis of the covalent intermediate. Sequence comparisons showed that Tyr(465) is conserved in microsomal EHs. The substitution of analogous Tyr(374) with phenylalanine in the human microsomal EH dramatically decreases the rate of hydrolysis of cis-stilbene oxide. These results suggest that these tyrosines perform a significant mechanistic role in the substrate activation by EHs.

Cress and potato soluble epoxide hydrolases: purification, biochemical characterization, and comparison to mammalian enzymes

Affinity chromatographic methods were developed for the one-step purification to homogeneity of recombinant soluble epoxide hydrolases (sEHs) from cress and potato. The enzymes are monomeric, with masses of 36 and 39 kDa and pI values of 4.5 and 5.0, respectively. In spite of a large difference in sequence, the two plant enzymes have properties of inhibition and substrate selectivity which differ only slightly from mammalian sEHs. Whereas mammalian sEHs are highly selective for trans- versus cis-substituted stilbene oxide and 1,3-diphenylpropene oxide (DPPO), plant sEHs exhibit far greater selectivity for trans- versus cis-stilbene oxide, but little to no selectivity for DPPO isomers. The isolation of a covalently linked plant sEH-substrate complex indicated that the plant and mammalian sEHs have a similar mechanism of action. We hypothesize an in vivo role for plant sEH in cutin biosynthesis, based on relatively high plant sEH activity on epoxystearate to form a cutin precursor, 9,10-dihydroxystearate. Plant sEHs display a high thermal stability relative to mammalian sEHs. This stability and their high enantioselectivity for a single substrate suggest that their potential as biocatalysts for the preparation of enantiopure epoxides should be evaluated.

Detoxification of environmental mutagens and carcinogens: structure, mechanism, and evolution of liver epoxide hydrolase

The crystal structure of recombinant murine liver cytosolic epoxide hydrolase (EC 3.3.2.3) has been determined at 2.8-A resolution. The binding of a nanomolar affinity inhibitor confirms the active site location in the C-terminal domain; this domain is similar to that of haloalkane dehalogenase and shares the alpha/beta hydrolase fold. A structure-based mechanism is proposed that illuminates the unique chemical strategy for the activation of endogenous and man-made epoxide substrates for hydrolysis and detoxification. Surprisingly, a vestigial active site is found in the N-terminal domain similar to that of another enzyme of halocarbon metabolism, haloacid dehalogenase. Although the vestigial active site does not participate in epoxide hydrolysis, the vestigial domain plays a critical structural role by stabilizing the dimer in a distinctive domain-swapped architecture. Given the genetic and structural relationships among these enzymes of xenobiotic metabolism, a structure-based evolutionary sequence is postulated.

Potent urea and carbamate inhibitors of soluble epoxide hydrolases

The soluble epoxide hydrolase (sEH) plays a significant role in the biosynthesis of inflammation mediators as well as xenobiotic transformations. Herein, we report the discovery of substituted ureas and carbamates as potent inhibitors of sEH. Some of these selective, competitive tight-binding inhibitors with nanomolar K(i) values interacted stoichiometrically with the homogenous recombinant murine and human sEHs. These inhibitors enhance cytotoxicity of trans-stilbene oxide, which is active as the epoxide, but reduce cytotoxicity of leukotoxin, which is activated by epoxide hydrolase to its toxic diol. They also reduce toxicity of leukotoxin in vivo in mice and prevent symptoms suggestive of acute respiratory distress syndrome. These potent inhibitors may be valuable tools for testing hypotheses of involvement of diol and epoxide lipids in chemical mediation in vitro or in vivo systems.

Multiple epoxide hydrolases in Alternaria alternata f. sp. lycopersici and their relationship to medium composition and host-specific toxin production

The production of Alternaria alternata f. sp. lycopersici host-specific toxins (AAL toxins) and epoxide hydrolase (EH) activity were studied during the growth of this plant-pathogenic fungus in stationary liquid cultures. Media containing pectin as the primary carbon source displayed peaks of EH activity at day 4 and at day 12. When pectin was replaced by glucose, there was a single peak of EH activity at day 6. Partial characterization of the EH activities suggests the presence of three biochemically distinguishable EH activities. Two of them have a molecular mass of 25 kDa and a pI of 4.9, while the other has a molecular mass of 20 kDa and a pI of 4.7. Each of the EH activities can be distinguished by substrate preference and sensitivity to inhibitors. The EH activities present at day 6 (glucose) or day 12 (pectin) are concomitant with AAL toxin production.