Purification and characterization of rat hepatic microsomal low molecular weight fatty acid ethyl ester synthase and its relationship to carboxylesterases

Nonoxidative ethanol metabolism in humans-from biomarkers to bioactive lipids

Ethanol is a widely used psychoactive drug whose chronic abuse is associated with organ dysfunction and disease. Although the prevalent metabolic fate of ethanol in the human body is oxidation a smaller fraction undergoes nonoxidative metabolism yielding ethyl glucuronide, ethyl sulfate, phosphatidylethanol and fatty acid ethyl esters. Nonoxidative ethanol metabolites persist in tissues and body fluids for much longer than ethanol itself and represent biomarkers for the assessment of ethanol intake in clinical and forensic settings. Of note, the nonoxidative reaction of ethanol with phospholipids and fatty acids yields bioactive compounds that affect cellular signaling pathways and organelle function and may contribute to ethanol toxicity. Thus, despite low quantitative contributions of nonoxidative pathways to overall ethanol metabolism the resultant ethanol metabolites have important biological implications. In this review we summarize the current knowledge about the enzymatic formation of nonoxidative ethanol metabolites in humans and discuss the implications of nonoxidative ethanol metabolites as biomarkers of ethanol intake and mediators of ethanol toxicity. © 2016 IUBMB Life, 68(12):916-923, 2016.

Synthesis of fatty acid ethyl esters in mammalian tissues after ethanol exposure: a systematic review of the literature

The ability to undergo non-oxidative metabolism from ethanol to fatty acid ethyl esters (FAEEs) varies greatly among tissues and organs. To gain a greater understanding of non-oxidative ethanol metabolism to FAEE, we aimed to collect all published data on FAEE synthesis in mammalian organs and tissues to identify all tissues, organs, and enzymes that are known to, or likely possess FAEE-synthetic activity. A systematic search for relevant papers was performed and two independent reviewers examined potentially relevant abstracts (articles on FAEEs that pertain to ethanol exposure) to determine whether they met the inclusion criteria. Information on FAEE synthesis was retrieved from papers meeting the inclusion/exclusion criteria and summarized by organ/tissue/matrix examined. The systematic search through four databases yielded 78 articles that investigated FAEE synthesis by tissues, tissue fractions and cell lines, and 29 articles that attempted to purify and/or characterize the enzymes involved in FAEE synthesis. Two enzyme activities have been studied: FAEE synthase (FAEES, which conjugates ethanol and free fatty acid) and acyl-CoA: ethanol O-acyltransferase (AEAT, which conjugates ethanol and fatty acyl-CoA). Both activities are expressed by a variety of different enzymes. FAEES activity is the most widely studied and has been purified from several tissues and shown to be associated with several well-known enzymes, while the identity of enzymes possessing AEAT activity remains unknown. The organs and tissues that have been shown to synthesize FAEEs are discussed, with special emphasis on the studies that attempted to elucidate the enzymology of FAEE synthesis in those tissues.

Distinctive genes determine different intramuscular fat and muscle fiber ratios of the longissimus dorsi muscles in Jinhua and landrace pigs

Meat quality is determined by properties such as carcass color, tenderness and drip loss. These properties are closely associated with meat composition, which includes the types of muscle fiber and content of intramuscular fat (IMF). Muscle fibers are the main contributors to meat mass, while IMF not only contributes to the sensory properties but also to the plethora of physical, chemical and technological properties of meat. However, little is known about the molecular mechanisms that determine meat composition in different pig breeds. In this report we show that Jinhua pigs, a Chinese breed, contains much higher levels of IMF than do Landrace pigs, a Danish breed. We analyzed global gene expression profiles in the longissimus dorsi muscles in Jinhua and Landrace breeds at the ages of 30, 90 and 150 days. Cross-comparison analysis revealed that genes that regulate fatty acid biosynthesis (e.g., fatty acid synthase and stearoyl-CoA desaturase) are expressed at higher levels in Jinhua pigs whereas those that regulate myogenesis (e.g., myogenic factor 6 and forkhead box O1) are expressed at higher levels in Landrace pigs. Among those genes which are highly expressed in Jinhua pigs at 90 days (d90), we identified a novel gene porcine FLJ36031 (pFLJ), which functions as a positive regulator of fat deposition in cultured intramuscular adipocytes. In summary, our data showed that the up-regulation of fatty acid biosynthesis regulatory genes such as pFLJ and myogenesis inhibitory genes such as myostatin in the longissimus dorsi muscles of Jinhua pigs could explain why this local breed produces meat with high levels of IMF.

Altered hepatic lipid metabolism in C57BL/6 mice fed alcohol: a targeted lipidomic and gene expression study

Chronic alcohol consumption is associated with fatty liver disease in mammals. The object of this study was to gain an understanding of dysregulated lipid metabolism in alcohol-fed C57BL/6 mice using a targeted lipidomic approach. Liquid chromatography tandem mass spectrometry was used to analyze several lipid classes, including free fatty acids, fatty acyl-CoAs, fatty acid ethyl esters, sphingolipids, ceramides, and endocannabinoids, in plasma and liver samples from control and alcohol-fed mice. The interpretation of lipidomic data was augmented by gene expression analyses for important metabolic enzymes in the lipid pathways studied. Alcohol feeding was associated with i) increased hepatic free fatty acid levels and decreased fatty acyl-CoA levels associated with decreased mitochondrial fatty acid oxidation and decreased fatty acyl-CoA synthesis, respectively; ii) increased hepatic ceramide levels associated with higher levels of the precursor molecules sphingosine and sphinganine; and iii) increased hepatic levels of the endocannabinoid anandamide associated with decreased expression of its catabolic enzyme fatty acid amide hydrolase. The unique combination of lipidomic and gene expression analyses allows for a better mechanistic understanding of dysregulated lipid metabolism in the development of alcoholic fatty liver disease.

Multisite promiscuity in the processing of endogenous substrates by human carboxylesterase 1

Human carboxylesterase 1 (hCE1) is a drug and endobiotic-processing serine hydrolase that exhibits relatively broad substrate specificity. It has been implicated in a variety of endogenous cholesterol metabolism pathways including the following apparently disparate reactions: cholesterol ester hydrolysis (CEH), fatty acyl Coenzyme A hydrolysis (FACoAH), acyl-Coenzyme A:cholesterol acyltransfer (ACAT), and fatty acyl ethyl ester synthesis (FAEES). The structural basis for the ability of hCE1 to perform these catalytic actions involving large substrates and products has remained unclear. Here we present four crystal structures of the hCE1 glycoprotein in complexes with the following endogenous substrates or substrate analogues: Coenzyme A, the fatty acid palmitate, and the bile acids cholate and taurocholate. While the active site of hCE1 was known to be promiscuous and capable of interacting with a variety of chemically distinct ligands, these structures reveal that the enzyme contains two additional ligand-binding sites and that each site also exhibits relatively non-specific ligand-binding properties. Using this multisite promiscuity, hCE1 appears structurally capable of assembling several catalytic events depending, apparently, on the physiological state of the cellular environment. These results expand our understanding of enzyme promiscuity and indicate that, in the case of hCE1, multiple non-specific sites are employed to perform distinct catalytic actions.

Metabolic basis of ethanol-induced cytotoxicity in recombinant HepG2 cells: role of nonoxidative metabolism

Chronic alcohol abuse, a major health problem, causes liver and pancreatic diseases and is known to impair hepatic alcohol dehydrogenase (ADH). Hepatic ADH-catalyzed oxidation of ethanol is a major pathway for the ethanol disposition in the body. Hepatic microsomal cytochrome P450 (CYP2E1), induced in chronic alcohol abuse, is also reported to oxidize ethanol. However, impaired hepatic ADH activity in a rat model is known to facilitate a nonoxidative metabolism resulting in formation of nonoxidative metabolites of ethanol such as fatty acid ethyl esters (FAEEs) via a nonoxidative pathway catalyzed by FAEE synthase. Therefore, the metabolic basis of ethanol-induced cytotoxicity was determined in HepG2 cells and recombinant HepG2 cells transfected with ADH (VA-13), CYP2E1 (E47) or ADH + CYP2E1 (VL-17A). Western blot analysis shows ADH deficiency in HepG2 and E47 cells, compared to ADH-overexpressed VA-13 and VL-17A cells. Attached HepG2 cells and the recombinant cells were incubated with ethanol, and nonoxidative metabolism of ethanol was determined by measuring the formation of FAEEs. Significantly higher levels of FAEEs were synthesized in HepG2 and E47 cells than in VA-13 and VL-17A cells at all concentrations of ethanol (100-800 mg%) incubated for 6 h (optimal time for the synthesis of FAEEs) in cell culture. These results suggest that ADH-catalyzed oxidative metabolism of ethanol is the major mechanism of its disposition, regardless of CYP2E1 overexpression. On the other hand, diminished ADH activity facilitates nonoxidative metabolism of ethanol to FAEEs as found in E47 cells, regardless of CYP2E1 overexpression. Therefore, CYP2E1-mediated oxidation of ethanol could be a minor mechanism of ethanol disposition. Further studies conducted only in HepG2 and VA-13 cells showed lower ethanol disposition and ATP concentration and higher accumulation of neutral lipids and cytotoxicity (apoptosis) in HepG2 cells than in VA-13 cells. The apoptosis observed in HepG2 vs. VA-13 cells incubated with ethanol appears to be mediated by release of mitochondrial cytochrome c via activation of caspase-9 and caspase-3. These results strongly support our hypothesis that diminished hepatic ADH activity facilitates nonoxidative metabolism of ethanol and the products of ethanol nonoxidative metabolism cause apoptosis in HepG2 cells via intrinsic pathway.

Cloning of ABCA17, a novel rodent sperm-specific ABC (ATP-binding cassette) transporter that regulates intracellular lipid metabolism

The A subclass of the ABC (ATP-binding cassette) transporter superfamily has a structural feature that distinguishes it from other ABC transporters, and is proposed to be involved in the transmembrane transport of endogenous lipids. Here we have cloned mouse and rat full-length cDNAs of ABCA17, a novel ABC transporter belonging to the A subclass. Mouse and rat ABCA17 proteins comprise 1733 and 1773 amino acid residues respectively, having 87.3% amino acid identity; mouse ABCA17 has amino acid identities of 55.3% and 36.7% with mouse ABCA3 and sea urchin ABCA respectively. RNA blot and quantitative real-time PCR analyses showed that ABCA17 mRNA is expressed exclusively in the testis. Examination of testis by in situ hybridization showed that ABCA17 mRNA is expressed in germ cells, mainly spermatocytes, in the seminiferous tubule. Immunoblot analysis using a specific antibody showed that ABCA17 is a protein of 200 kDa, and immunohistochemical analysis demonstrated that the protein is detected in the anterior head of sperm and elongated spermatids. ABCA17 was localized in the endoplasmic reticulum in transiently transfected HEK293 cells. Metabolic labelling analysis showed that intracellular esterified lipids, including cholesteryl esters, fatty acid esters and triacylglycerols, were significantly decreased in HEK293 cells stably expressing ABCA17 compared with untransfected cells. These results suggest that ABCA17 may play a role in regulating lipid composition in sperm.

Structural insights into drug processing by human carboxylesterase 1: tamoxifen, mevastatin, and inhibition by benzil

Human carboxylesterase 1 (hCE1) exhibits broad substrate specificity and is involved in xenobiotic processing and endobiotic metabolism. We present and analyze crystal structures of hCE1 in complexes with the cholesterol-lowering drug mevastatin, the breast cancer drug tamoxifen, the fatty acyl ethyl ester (FAEE) analogue ethyl acetate, and the novel hCE1 inhibitor benzil. We find that mevastatin does not appear to be a substrate for hCE1, and instead acts as a partially non-competitive inhibitor of the enzyme. Similarly, we show that tamoxifen is a low micromolar, partially non-competitive inhibitor of hCE1. Further, we describe the structural basis for the inhibition of hCE1 by the nanomolar-affinity dione benzil, which acts by forming both covalent and non-covalent complexes with the enzyme. Our results provide detailed insights into the catalytic and non-catalytic processing of small molecules by hCE1, and suggest that the efficacy of clinical drugs may be modulated by targeted hCE1 inhibitors.

In vitro conjugation of ethanolamine with fatty acids by rat liver subcellular fractions

Previous studies from our laboratory have shown the enzymic formation of fatty acid (FA) conjugates of xenobiotic alcohols and amines. In the present study, the formation of FA conjugates of a bifunctional compound, ethanolamine was investigated by incubating [1-14C]oleic acid (1 mM) with ethanolamine (25 mM) at 37 degrees C in the presence of various rat liver subcellular fractions. The resultant product (or products) was separated by thin-layer chromatography (TLC) and the radioactivity corresponding to the relative flow of fatty acid amide was determined. Under similar conditions, formation of ethanolamides of palmitic, stearic, linoleic, linolenic, and arachidonic acids were also examined. The formation of ethanolamine conjugate with oleic acid was found to be 16.3 nmol/h/mg protein as compared to 6.7, 6.2, 8.1, 8.3, and 7.6 nmol/h/mg protein for palmitic, stearic, linoleic, linolenic, and arachidonic acids, respectively. The formation of oleoyl ethanolamide was found to be 18.9, 40.1, 65.9, and 0.3 nmol/h/mg protein in postnuclear, mitochondrial, microsomal, and cytosolic fractions, respectively. Mass spectrometric and nuclear magnetic resonance spectroscopic data of the TLC-purified product confirm the formation of oleoyl ethanolamide, and amidation appeared to be a preferred reaction over esterification. The results of this study suggest that the enzyme responsible for the amidation of fatty acids resides mainly in the microsomal fraction of the liver, and that oleic acid is a better substrate than other fatty acids used in the present study.

Mechanism of differential inhibition of hepatic and pancreatic fatty acid ethyl ester synthase by inhibitors of serine-esterases: in vitro and cell culture studies

Earlier, we have shown that rat hepatic and pancreatic fatty acid ethyl ester (FAEE) synthases are structurally and functionally similar to rat liver carboxylesterase (CE) and pancreatic cholesterol esterase (ChE), respectively. We have also reported that only hepatic FAEE synthase is inhibited by tri-o-tolylphosphate (TOTP) in vivo and in human hepatocellular carcinoma (HepG2) cells. The metabolism of TOTP is a prerequisite for the inhibition of hepatic FAEE synthase as well as esterase activity. To further elucidate the mechanism of such differential inhibition by inhibitors of serine esterases, we synthesized two metabolites of TOTP, 2-(o-cresyl)-4H-1:3:2-benzodioxaphosphoran-2-one (CBDP; cyclic saligenin phosphate) and di-o-tolyl-o-( proportional, variant -hydroxy)tolylphosphate (HO-TOTP), and one ChE inhibitor, 3-benzyl-6-chloro-2-pyrone (3-BCP). The inhibitory effect of CBDP, HO-TOTP, and 3-BCP on FAEE synthase and esterase activity was studied using rat hepatic and pancreatic postnuclear (PN) fractions, commercial porcine hepatic CE and pancreatic ChE, and in HepG2 and rat pancreatic tumor (AR42J) cell lines. Only HO-TOTP and CBDP inhibited FAEE synthase as well as esterase activity of hepatic PN fraction and commercial CE and ChE in a concentration-dependent manner, and the inhibition was found to be irreversible. However, no inhibition was found in pancreatic PN fraction by both TOTP metabolites and 3-BCP. Although 3-BCP inhibited only the esterase activity of commercial ChE in a concentration-dependent manner, the activity was reversible within 30 min of incubation. Studies with HepG2 cells also showed a significant inhibition of FAEE synthase-esterase activity by CBDP and HO-TOTP within 15 min of incubation, while no inhibition was observed in AR42J cells. 3-BCP did not inhibit FAEE synthase-esterase activity either in HepG2 or AR42J cells. Such differential inhibitory effect of the TOTP metabolites on hepatic and pancreatic FAEE synthase-esterase is supported by our earlier in vivo and in vitro studies. Further investigations are needed to understand the biochemical mechanism(s) of inactivation of TOTP metabolites and 3-BCP in the pancreas and AR42J cells towards FAEE synthase-esterase activities.

Modulation of fatty acid methyl esters in rats pretreated with tri-o-tolyl phosphate

Formation and toxicity of fatty acid methyl esters (FAMEs) have been reported both in vitro and in vivo. In previous studies, it was shown that fatty acid ethyl ester synthase (FAEES), which catalyzes the formation of FAMEs, also expresses esterase activity. Therefore, it was hypothesized that inhibitors of esterases such as tri-o-tolyl phosphate (TOTP) can modulate the formation of FAMEs. To test this, four groups of rats were used. Group 1 served as control (vehicle only). Group 2 was treated with methanol only (3 g/kg via gavage), group 3 was given TOTP only (100 mg/kg i.p. in corn oil), and group 4 was administered TOTP as in group 3, followed by methanol after 18 h. Three hours after exposure, animals were sacrificed and FAEES activity and FAME levels were measured in blood, liver, pancreas, and brown fat. About 95% of FAEES activity was inhibited in the liver and whole blood of TOTP-treated rats (group 3) but no inhibition was observed in the pancreas or brown fat. Total hepatic FAMEs were found to be lowest for the TOTP-treated group (3) and highest in the methanol-treated animals (group 2). Total pancreatic FAMEs in different groups were not statistically different, while significant increases were observed in the brown fat in both methanol-treated groups. To verify that the oxidative metabolism of methanol was unaffected by TOTP, alcohol dehydrogenase activity was also measured and found to be unchanged in any group as compared to control. These results demonstrate that the formation of FAMEs can be modulated in the liver and probably in blood, but not in the pancreas or brown fat by the inhibitors of FAEES.

Purification and characterization of rat pancreatic fatty acid ethyl ester synthase and its structural and functional relationship to pancreatic cholesterol esterase

Formation of fatty acid ethyl esters (FAEEs, catalyzed by FAEE synthase) has been implicated in the pathogenesis of chronic pancreatitis. In previous studies, we demonstrated that FAEE synthase, purified from rat liver microsomes, is identical to rat liver carboxylesterase (pI 6.1), and structurally and functionally different than that from pancreas. In this study, we purified and characterized rat pancreatic microsomal FAEE synthase, and determined its relationship with rat pancreatic cholesterol esterase (ChE). Since most of the serine esterases express p-nitrophenyl acetate (PNPA)-hydrolyzing activity as well as synthetic activity to form fatty acid esters or amides with a wide spectrum of alcohols and amines, respectively, we used PNPA-hydrolyzing activity to monitor the purification of FAEE synthase during various chromatographic purification steps. Synthesizing activity towards FAEEs, fatty acid methyl esters, and fatty acid anilides was measured only in the pooled fractions. At each step of purification (ammonium sulfate saturation, Q Sepharose XL, and heparin-agarose column chromatographies, and high performance liquid chromatography (anion exchange and gel filtration)) synthetic as well as hydrolytic activities copurified. Using ethanol, methanol, or aniline as substrates, the ester or anilide synthesizing activity of the purified protein was found to be 8709, 13000, and 2201 nmol/h/mg protein, respectively. The purified protein displayed a single band with an estimated molecular mass of approximately 68 kD upon SDS-PAGE under reduced denaturing conditions, cross-reacted with antisera against rat pancreatic ChE and showed 100% N-terminal sequence homology of the first 15 amino acids to that of rat pancreatic ChE. These results suggest that the purified protein has broad substrate specificity towards the conjugation of endogenous long chain fatty acids with substrates having hydroxyl and amino groups and is identical to ChE.

Differential inhibition of hepatic, pancreatic, and plasma fatty acid ethyl ester synthase by tri-o-tolylphosphate in rats

Fatty acid conjugation of alcohols, catalyzed by fatty acid ethyl ester synthase (FAEES), results in the formation of lipophilic esters. Although the activity of FAEES is reported in almost all organs, including plasma, the interrelationship among various proteins expressing FAEES activity in different organs/tissues is not well understood. Earlier, we have reported an inhibition of FAEES activity in human hepatoma cells by tri-o-tolylphosphate (TOTP; serine esterase inhibitor). The present study was undertaken to further characterize the hepatic, plasma, and pancreatic FAEES in rats after ip injection of 10, 25, 50, or 100 mg/kg TOTP in corn oil or vehicle alone. After 18 h, animals were euthanized and FAEES activity in the plasma and postnuclear fractions of hepatic and pancreatic homogenates were assayed by measuring the ester formation following incubation with [1-(14)C]oleic acid and ethanol or methanol as substrates. Significant inhibition of FAEES activity was observed in hepatic postnuclear fraction. The esterase activity also showed a pattern similar to fatty acid ethyl and methyl ester synthesizing activity. A trend similar to hepatic synthesizing and hydrolyzing activities was also found in the plasma of TOTP-treated rats. However, no inhibition of synthetic activity toward formation of fatty acid ethyl or methyl esters or p-nitrophenyl acetate hydrolyzing activity was observed in the pancreas of rats after TOTP exposure. Our results also show that the protein expressing FAEES activity in the pancreas does not cross-react with antibodies to rat adipose tissue FAEES using Western blot analysis, which recognizes approximately 60- and approximately 84-kDa proteins in the liver and plasma, respectively. Furthermore, the inhibition in liver is at the functional level of enzyme as no change was observed between control and treated animals by immunohistochemistry. We conclude that fatty acid ethyl or methyl ester synthesizing enzyme(s) in the liver and plasma, which are inhibited by TOTP, are different from that present in the pancreas.