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

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.

Vitamin E delta-tocotrienol levels in tumor and pancreatic tissue of mice after oral administration

Tocotrienols are natural vitamin E compounds that are known to have a neuroprotective effect at nanomolar concentration and anti-carcinogenic effect at micromolar concentration. In this report, we investigated the pharmacokinetics, tumor and pancreatic tissue levels, and toxicity of delta-tocotrienol in mice because of its anti-tumor activity against pancreatic cancer. Following a single oral administration of delta-tocotrienol at 100 mg/kg, the peak plasma concentration (C(max)) was 57 +/- 5 micromol/l, the time required to reach peak plasma concentration (T(max)) was 2 h and plasma half-life (t(1/2)) was 3.5 h. The delta-tocotrienol was cleared from plasma and liver within 24 h, but delayed from the pancreas. When mice were fed delta-tocotrienol for 6 weeks, the concentration in tumor tissue was 41 +/- 3.5 nmol/g. This concentration was observed with the oral dose (100 mg/kg) of delta-tocotrienol which inhibited tumor growth by 80% in our previous studies. Interestingly, delta-tocotrienol was 10-fold more concentrated in the pancreas than in the tumor. We observed no toxicity due to delta-tocotrienol as mice gained normal weight with no histopathological changes in tissues. Our data suggest that bioactive levels of delta-tocotrienol can be achieved in the pancreas following oral administration and supports its clinical investigation in pancreatic cancer.

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.