Fatty acid conjugates of xenobiotics

Comprehensive characterization of European house dust contaminants: Concentrations and profiles, geographical variability, and implications for chemical regulation and health risk

This study investigated the concentration profiles and geographical variability of contaminants in house dust across Europe. A collaborative trial (CT) was organized by the NORMAN network using pooled dust and advanced chromatographic and mass spectrometric techniques combined with suspect screening and non-target screening (NTS). Over 1200 anthropogenic compounds were tentatively identified. Additionally, seventy-five individual samples were subjected to target analysis and NTS. The median concentrations of most contaminants varied <3-fold across Europe, and the contaminant profile of European dust was similar to that of North American dust, which was investigated in a previous CT. This similarity may be attributed to the use of similar consumer articles and building materials throughout the developed world. Multivariate data analysis revealed geographical trends in contaminant distribution, with north-south gradients across Europe. Geographical trends were more frequently found for compounds with rapid release (pharmaceuticals, personal care products, fragrances, pesticides, biocides) and smoke-related compounds. The concentrations of chlorinated paraffins, polycyclic aromatic hydrocarbons (PAHs), perfluorinated alkyl substances and stimulants generally increased from north to south, whereas the biocides levels decreased from north to south. Despite widespread presence of in-use contaminants in dusts, some of the highest risks come from compounds that have been restricted for decades or more. These include di(2-ethylhexyl) phthalate (DEHP), polychlorinated biphenyl (PCB) 118 and polybrominated diphenyl ethers 47, 99, and 153. DEHP remains the most abundant contaminant in European house dust, while the other compounds are classified as persistent organic pollutants (POPs). Moreover, there is a striking lack of reliable toxicity data, particularly for emerging compounds. For instance, although acceptable daily intakes (ADIs) were examined for 202 compounds, only 46 had consensus-based ADI values. The results highlight the need for proactive measures to prevent hazardous chemicals from entering the market and for careful selection of substitute chemicals, when such are needed, to avoid regrettable substitutions.

On the Fate of Butyl Methoxydibenzoylmethane (Avobenzone) in Coral Tissue and Its Effect on Coral Metabolome

The intensive use of sunscreen products has raised concerns regarding their environmental toxicity and the adverse impacts of ultraviolet (UV) filters on ecologically important coral communities. Prior metabolomic analyses on symbiotic coral Pocillopora damicornis exposed to the UV filter butyl methoxydibenzoylmethane (BM, avobenzone) revealed unidentified ions in the holobiont metabolome. In the present study, follow-up differential metabolomic analyses in BM-exposed P. damicornis detected 57 ions with significantly different relative concentrations in exposed corals. The results showed an accumulation of 17 BM derivatives produced through BM reduction and esterification. The major derivative identified C16:0-dihydroBM, which was synthesized and used as a standard to quantify BM derivatives in coral extracts. The results indicated that relative amounts of BM derivatives made up to 95% of the total BM (w/w) absorbed in coral tissue after 7 days of exposure. Among the remaining metabolites annotated, seven compounds significantly affected by BM exposure could be attributed to the coral dinoflagellate symbiont, indicating that BM exposure might impair the photosynthetic capacity of the holobiont. The present results suggest that the potential role of BM in coral bleaching in anthropogenic areas should be investigated and that BM derivatives should be considered in future assessments on the fate and effects of BM in the environment.

The Various Roles of Fatty Acids

Lipids comprise a large group of chemically heterogeneous compounds. The majority have fatty acids (FA) as part of their structure, making these compounds suitable tools to examine processes raging from cellular to macroscopic levels of organization. Among the multiple roles of FA, they have structural functions as constituents of phospholipids which are the "building blocks" of cell membranes; as part of neutral lipids FA serve as storage materials in cells; and FA derivatives are involved in cell signalling. Studies on FA and their metabolism are important in numerous research fields, including biology, bacteriology, ecology, human nutrition and health. Specific FA and their ratios in cellular membranes may be used as biomarkers to enable the identification of organisms, to study adaptation of bacterial cells to toxic compounds and environmental conditions and to disclose food web connections. In this review, we discuss the various roles of FA in prokaryotes and eukaryotes and highlight the application of FA analysis to elucidate ecological mechanisms. We briefly describe FA synthesis; analyse the role of FA as modulators of cell membrane properties and FA ability to store and supply energy to cells; and inspect the role of polyunsaturated FA (PUFA) and the suitability of using FA as biomarkers of organisms.

Obesogenic endocrine disruptors and obesity: myths and truths

Obesogenic endocrine disruptors, also known as obesogens, are chemicals potentially involved in weight gain by altering lipid homeostasis and promoting adipogenesis and lipid accumulation. They included compounds to which human population is exposed over daily life such as pesticides/herbicides, industrial and household products, plastics, detergents and personal care products. The window of life during which the exposure happens could lead to different effects. A critical window is during utero and/or neonatal period in which the obesogens could cause subtle changes in gene expression and tissue organization or blunt other levels of biological organization leading to increased susceptibility to diseases in the adulthood. Some of the reasons for this increased sensitivity include the lack of the protective mechanisms that are available in adult such as DNA repair mechanisms, a competent immune system, detoxifying enzymes, liver metabolism and the blood/brain barrier still not fully functional in the fetus or newborn. The mechanisms of action of obesogens lay on their ability to increase the number and/or the size of the adipocytes and to alter appetite, satiety and food preferences. The ability of obesogens to increase fat deposition results in an increased capacity for their own retention due to their lipophilic properties; thus prolonging the exposure and increasing the detrimental metabolic consequences.

Endocrine Disruptors and Obesity

PURPOSE OF REVIEW

The purpose of this review was to summarise current evidence that some environmental chemicals may be able to interfere in the endocrine regulation of energy metabolism and adipose tissue structure.

RECENT FINDINGS

Recent findings demonstrate that such endocrine-disrupting chemicals, termed "obesogens", can promote adipogenesis and cause weight gain. This includes compounds to which the human population is exposed in daily life through their use in pesticides/herbicides, industrial and household products, plastics, detergents, flame retardants and as ingredients in personal care products. Animal models and epidemiological studies have shown that an especially sensitive time for exposure is in utero or the neonatal period. In summarising the actions of obesogens, it is noteworthy that as their structures are mainly lipophilic, their ability to increase fat deposition has the added consequence of increasing the capacity for their own retention. This has the potential for a vicious spiral not only of increasing obesity but also increasing the retention of other lipophilic pollutant chemicals with an even broader range of adverse actions. This might offer an explanation as to why obesity is an underlying risk factor for so many diseases including cancer.

Lipid-soluble conjugates of hydroxylated polybrominated diphenyl ethers in blue mussels from the Baltic Sea

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) of proposed natural origin have been detected throughout the food web of the Baltic Sea. Some OH-PBDEs have been shown to disrupt oxidative phosphorylation and the thyroid hormone system in exposed organisms. This paper describes an investigation into the fate of OH-PBDEs in the Baltic Sea's predominant specie, the blue mussel. The main focus was on the conjugation of OH-PBDEs with lipophilic moieties (e.g., fatty acids) and the potential role this transformation mechanism may have in heavily exposed mussels in nature. Analytical methods were developed to accurately determine the concentrations of these conjugates in blue mussels collected on different occasions during the summer in a coastal area of the Baltic proper. The measured concentrations of conjugated OH-PBDEs were compared to those of the unconjugated parent compounds, and it was found that in some cases, the levels of the conjugated derivatives can be equal or even higher than the levels of the unconjugated OH-PBDEs. This is, to our knowledge, the first study on lipid-soluble OH-PBDE conjugates, and the first study to investigate the occurrence of such conjugates of halogenated phenolic compounds in environmentally exposed mussels. The mussels were also found to contain hydrolysable water-soluble derivatives of OH-PBDEs (such as e.g., glucuronic acid and/or sulfate conjugates etc.). These were tentatively determined to be of lower concentration (by up to an order of magnitude) than that of the OH-PBDEs which were conjugated with lipophilic moieties.

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.

Tissue disposition, excretion and metabolism of 2,2′,4,4′,6-pentabromodiphenyl ether (BDE-100) in male Sprague–Dawley rats

The absorption, disposition, metabolism and excretion study of orally administered 2,2',4,4',6-pentabromodiphenyl ether (BDE-100) was studied in conventional and bile-duct cannulated male rats. In conventional rats, >70% of the radiolabelled oral dose was retained at 72 h, and lipophilic tissues were the preferred sites for disposition, i.e. adipose tissue, gastrointestinal tract, skin, liver and lungs. Urinary excretion of BDE-100 was very low (0.1% of the dose). Biliary excretion of BDE-100 was slightly greater than that observed in urine, i.e. 1.7% at 72 h, and glucuronidation of phenolic metabolites was suggested. Thiol metabolites were not observed in the bile as had been reported in other PBDE metabolism studies. Almost 20% of the dose in conventional male rats and over 26% in bile-duct cannulated rats was excreted in the faeces, mainly as the unmetabolized parent, although large amounts of non-extractable radiolabel were also observed. Extractable metabolites in faeces were characterized by mass spectrometry. Monohydroxylated pentabromodiphenyl ether metabolites were detected; mono- and di-hydroxylated metabolites with accompanying oxidative debromination were also observed as faecal metabolites. Tissue residues of [(14)C]BDE-100 in liver, gastrointestinal tract and adipose tissue contained only parent material. The majority of the 0-72-h biliary radioactivity was associated with an unidentified 79-kDa protein or to albumin.

Comparative absorption and bioaccumulation of polybrominated diphenyl ethers following ingestion via dust and oil in male rats

Household dust has been implicated as a major source of polybrominated diphenyl ether (PBDE) exposure in humans. This finding has important implications for young children, who tend to ingest more dust than adults and may be more susceptible to some of the putative developmental effects of PBDEs. Absorption parameters of PBDEs from ingested dust are unknown; therefore, the objectives of this study were to determine and to compare the uptake of PBDEs from either household dust (NIST Standard Reference Material 2585) or a corn oil solution. Male rats were administered dust or corn oil doses at 1 or 6 microg of PBDEs kg(-1) body wt in the diet for 21 days (n = 4 rats per group). The concentrations of 15 PBDEs were measured in adipose tissue and liver from each treatment group and showed that bioconcentration was congener dependent, but for the majority of congeners, the concentrations did not differ with either dose level or dose vehicle. Hepatic Cyp2b1 and 2b2 mRNA expression increased in rats receiving the higher PBDE doses, suggesting potential effects on metabolic activity. Retention of PBDEs in tissues ranged from <5% of the dose for BDE-209 to 70% for BDEs-47, 100, and 153 but generally did not differ between the high dust and high oil treatment groups. Excretion via the feces was significantly lower in the high oil dosed rats suggesting differences in absorption, excretion, and/or metabolism. The present study shows that PBDEs in dust are readily bioavailable and are biologically active, as indicated by increased transcription of hepatic enzymes.

The Saccharomyces cerevisiae EHT1 and EEB1 genes encode novel enzymes with medium-chain fatty acid ethyl ester synthesis and hydrolysis capacity

Fatty acid ethyl esters are secondary metabolites produced by Saccharomyces cerevisiae and many other fungi. Their natural physiological role is not known but in fermentations of alcoholic beverages and other food products they play a key role as flavor compounds. Information about the metabolic pathways and enzymology of fatty acid ethyl ester biosynthesis, however, is very limited. In this work, we have investigated the role of a three-member S. cerevisiae gene family with moderately divergent sequences (YBR177c/EHT1, YPL095c/EEB1, and YMR210w). We demonstrate that two family members encode an acyl-coenzymeA:ethanol O-acyltransferase, an enzyme required for the synthesis of medium-chain fatty acid ethyl esters. Deletion of either one or both of these genes resulted in severely reduced medium-chain fatty acid ethyl ester production. Purified glutathione S-transferase-tagged Eht1 and Eeb1 proteins both exhibited acyl-coenzymeA:ethanol O-acyltransferase activity in vitro, as well as esterase activity. Overexpression of Eht1 and Eeb1 did not enhance medium-chain fatty acid ethyl ester content, which is probably due to the bifunctional synthesis and hydrolysis activity. Molecular modeling of Eht1 and Eeb1 revealed the presence of a alpha/beta-hydrolase fold, which is generally present in the substrate-binding site of esterase enzymes. Hence, our results identify Eht1 and Eeb1 as novel acyl-coenzymeA:ethanol O-acyltransferases/esterases, whereas the third family member, Ymr210w, does not seem to play an important role in medium-chain fatty acid ethyl ester formation.

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.

Fatty acid ethyl esters: markers of alcohol abuse and alcoholism

Chronic alcoholism, which is associated with hepatic, pancreatic, and myocardial diseases, is one of the major health problems in the United States with high morbidity and mortality. Many individuals who abuse alcohol chronically die even before reaching the clinical stage of the disease. Reliable biomarkers of the diseases induced by chronic alcohol abuse, as well as for alcoholism, currently are not available. In the current study, we measured plasma concentrations of fatty acid ethyl esters [(FAEEs), nonoxidative metabolites of ethanol] in 39 patients with a detectable concentration of alcohol in their blood samples. In turn, we determined the relation of FAEE concentrations with blood alcohol concentration (BAC). Of 39 patients in whom we evaluated this relation, only five had a history of chronic alcohol abuse, and six had a history of acute alcohol abuse. Patients' age ranged from 25 to 71 years. Within this age range, greater concentrations of FAEEs were found in the plasma samples obtained from patients in the 41- to 50-year age group. There were no sex-related differences in BAC, nor in FAEE concentrations. Thirteen patients had a BAC greater than 300 mg%. For 11 patients, the BAC ranged between 200 and 299 mg%, and, for 12 patients, the BAC ranged between 100 and 199 mg%. In comparison with findings for patients with a BAC that ranged between 100 and 299 mg%, the FAEE concentrations were approximately twofold higher in patients with a BAC greater than 300 mg%. Ethyl palmitate and ethyl oleate were the main FAEEs detected in most patients. In general, FAEE concentrations increased with increasing BAC. However, in comparison with patients with a history of acute alcohol abuse, a greater increase in total FAEE concentrations was observed in patients with a history of chronic alcohol abuse (4,250 ng/ml and 15,086 ng/ml, respectively). Fatty acid ethyl esters were either detected in trace amounts or not detectable in the plasma of control subjects with no known alcohol ingestion. These results support our hypothesis that nonoxidative metabolism of ethanol to FAEEs is an important pathway of ethanol disposition during chronic alcohol abuse, and that FAEE concentrations can be a more reliable biomarker of chronic alcohol abuse than a history of acute alcohol abuse.

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.

Metabolism, tissue disposition, and excretion of 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) in male Sprague-Dawley rats

A single oral dose of [14C] 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) was administered to conventional and bile-duct cannulated male Sprague-Dawley rats. Tissue disposition, excretion and metabolism was determined. BTBPE is a low-volume brominated flame retardant used in resins or plastics, and toxicity data in peer-reviewed journals is extremely limited. BTBPE was fairly insoluble in lipophilic solutions, which made dose preparation difficult. The great majority of 14C (>94%) was excreted in the feces of both groups of rats at 72 h, and tissue retention was minimal. Lipophilic tissues contained the highest concentrations of BTBPE, e.g. thymus, adipose tissue, adrenals, lung, and skin. Metabolites were excreted in the urine, bile and feces, but at a very low level. Fecal metabolites were characterized as monohydroxylated, monohydroxylated with debromination, dihydroxylated/debrominated on a single aromatic ring, monohydroxylated on each aromatic ring with accompanying debromination, and cleavage on either side of the ether linkage to yield tribromophenol and tribromophenoxyethanol. Despite a limited quantity of stable metabolites extractable in the feces, non-extractable 14C levels were relatively high (39% of the 0-24 h fecal 14C), which suggested that BTBPE could be metabolically activated in the rat and covalently bound to fecal proteins and/or lipids. It was concluded that limited absorption and metabolism of BTBPE would occur by ingestion in mammals.

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.

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

We reported purification of a high molecular weight (HMW) (ca. 180 kD) and a low molecular weight (LMW) (ca. 60 kD) protein fractions from digitonized rat liver microsomes using ammonium sulfate precipitation followed by ion exchange and gel filtration column chromatography. Both fractions expressed fatty acid ethyl ester (FAEE) synthase as well as p-nitrophenyl acetate (PNPA)-hydrolyzing (esterase) activities. The HMW fraction was found to be a trimer with subunit molecular weight ca. 60 kD and structurally and functionally similar to rat hepatic microsomal carboxylesterase (CE, pI 6.1) and adipose tissue FAEE synthase. In this article, we report further purification and characterization of the LMW (minor) fraction expressing FAEE synthase activity and its structural and functional relationship to hepatic microsomal CEs. Using isoelectric focusing (IEF) followed by gel filtration-high-performance liquid chromatography (GF-HPLC), five proteins were purified, which expressed FAEE synthase as well as PNPA-hydrolyzing activity. The isoelectric point values of 6.5, 5.8, 5.6, 5.3, and 5.0 were found for the purified LMW proteins by IEF and each showed a peak corresponding to ca. 60 kD molecular weight by GF-HPLC, which expressed FAEE synthase as well as PNPA-hydrolyzing activity. Sodium dodecyl sulfate-polyacrylamide gel elecrophoresis (SDS-PAGE) analysis of the GF-HPLC purified LMW proteins revealed that these proteins are monomers (ca. 60 kD). All the purified LMW proteins cross-reacted with antibodies to rat adipose tissue FAEE synthase. Coelution of PNPA-hydrolyzing and FAEE synthase activity at each step of purification and cross-reactivity with rat adipose tissue FAEE synthase antibodies suggest that the purified proteins are related to various hepatic microsomal CEs. This conclusion is further supported by the homology of N-terminal amino acid sequence of the purified LMW proteins to various hepatic microsomal CEs and protease precursors. Therefore, LMW FAEE synthase activity most probably is expressed by various isozymes of hepatic microsomal CEs, which are also involved in the biotransformation of xenobiotic alcohols and amines.

Fatty acid anilides: in vivo formation and relevance to toxic oil syndrome

Toxic oil syndrome (TOS), a multisystemic epidemic outbreak in 1981 in Spain, was caused by the ingestion of a cooking oil mixture containing rapeseed oil denatured with aniline. The mechanisms and causative agents responsible for the TOS are still not known. Although primary lesions observed in TOS patients could not be reproduced experimentally, the levels of fatty acid anilides (FAAs) and aniline in TOS-related cooking oil were considered proximate markers of TOS. Aniline, available from free aniline and FAAs ingested with TOS-related cooking oil, and its reconjugation with endogenous fatty acids could be an early event leading to TOS. Therefore, the present study was undertaken to determine the formation of FAAs following an oral dose of 2 mmol/kg aniline hydrochloride (AH) via gavage in rats. Here, 16:0, 18:0, 18:1, 18:2, 18:3, and 20:4 FAAs were analyzed in the whole blood, brown fat, liver, and pancreas at 0 (control), 0.25, 0.5, 1, 3, 6, 12, 24, and 48 hours. Generally, 16:0 and 18:1 FAAs were detected in the whole blood, brown fat, and liver of AH-treated rats with highest mean levels at 0.25 or 0.5 hour, except 3 hours for the whole blood. Only 16:0 FAA was detectable in the pancreas of AH-treated animals. The 18:0 FAA was also detected frequently in the liver while other FAAs were either in trace amounts or not detectable in the tissues analyzed in the present study. Overall, highest formation of the 16:0 FAA was found in the liver followed by pancreas and of 18:1 FAA in the whole blood and brown fat. These results indicate a rapid formation and further metabolism and disposition of FAAs in rat model and support our previous findings that 18:1 and 16:0 fatty acids are better substrates for the conjugation with aniline. Surprisingly, a small or trace amount of a few FAAs also detected in the tissues of control rats indicates their endogenous biosynthesis and/or presence. Results of 18:1 fatty acid incubation and aniline in the presence of fatty acid ethyl ester synthase, purified to homogeneity from rat liver microsome, suggest that formation of FAAs is catalyzed by an enzyme involved in the conjugation of fatty acids with xenobiotic alcohols. Because the FAAs are known to exert a wide range of toxicity in experimental animals and primary cell cultures, in vivo formation of FAAs could be an early event leading to TOS.

Fatty acid ethyl and methyl ester synthases, and fatty acid anilide synthase in HepG2 and AR42J cells: interrelationships and inhibition by tri-o-tolyl phosphate

Synthesis of fatty acid ethyl esters (FAEEs), fatty acid methyl esters (FAMEs), and fatty acid anilides (FAAs) in humans and/or experimental animals and in vitro have been reported by us and other investigators. In previous studies, we have demonstrated that fatty acid ethyl ester synthase (FAEES), purified from rat liver microsomes, is structurally and functionally identical to the rat liver microsomal carboxylesterase (pI 6.1) and suggested a role in the conjugation of a variety of xenobiotic alcohols with endogenous fatty acids (B. S. Kaphalia, R. R. Fritz, and G. A. S. Ansari, Chem. Res. Toxicol. 11, 211-218, 1997). However, hepatic FAEES was found to be structurally and functionally different from that of pancreas. Therefore, the present study was undertaken to determine structural and functional interrelationships among the enzyme(s) involved in the synthesis of FAEEs, FAMEs, and FAAs, in HepG2 and AR42J cells using tri-o-tolyl phosphate (TOTP), a specific inhibitor for beta-esterases. Synthesis of FAEEs, FAMEs, and FAAs, studied in the HepG2 cells, was found to be dose- and time-dependent following incubation with methanol, ethanol, or aniline, respectively. Approximately 86-90% inhibition of FAEE, FAME, and FAA synthesizing activities was found in HepG2 cells following exposure to 2.5 microM TOTP. Identical profiles of dose- and time-dependent inhibition of FAEE, FAME, and FAA synthesizing activities by TOTP in HepG2 cells suggest that synthesis of FAEEs, FAMEs, and FAAs is catalyzed by the same enzyme(s). However, FAEE, FAME, and FAA synthesizing activities in AR42J cells could not be inhibited by TOTP under similar experimental conditions. A differential pattern of p-nitrophenyl acetate hydrolyzing activity (a measure of esterase activity) similar to that of fatty acid ester/anilide synthesizing activities was observed in the two cell lines. These results are further substantiated by the presence of approximately 60 kDa (subunit molecular weight) protein in the postnuclear fraction of HepG2 but not in AR42J cells by Western blot analysis using antibodies raised against FAEES, purified from rat liver microsomes or adipose tissue. Therefore, the enzyme responsible for the FAEE, FAME, or FAA synthesizing activities is most probably carboxylesterase in HepG2 cells and is structurally and functionally different than that present in AR42J cells. These studies also indicate the utility of HepG2 and AR42J cell cultures as an alternative to the animal model regarding studies on nonoxidative metabolism of alcohols and amines, in general.

Cytotoxicity of short-chain alcohols

Ethanol and other short-chain alcohols elicit a number of cellular responses that are potentially cytotoxic and, to some extent, independent of cell type. Aberrations in phospholipid and fatty acid metabolism, changes in the cellular redox state, disruptions of the energy state, and increased production of reactive oxygen metabolites have been implicated in cellular damage resulting from acute or chronic exposure to short-chain alcohols. Resulting disruptions of intracellular signaling cascades through interference with the synthesis of phosphatidic acid, decreases in phosphorylation potential and lipid peroxidation are mechanisms by which solvent alcohols can affect the rate of cell proliferation and, consequently, cell number. Nonoxidative metabolism of short-chain alcohols, including phospholipase D-mediated synthesis of alcohol phospholipids, and the synthesis of fatty acid alcohol esters are additional mechanisms by which alcohols can affect membrane structure and compromise cell function.