Formation of fatty acid ethyl esters in rat liver microsomes. Evidence for a key role for acyl-CoA: ethanol O-acyltransferase

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.

Monoacylglycerol Lipases Act as Evolutionarily Conserved Regulators of Non-oxidative Ethanol Metabolism

Fatty acid ethyl esters (FAEEs) are non-oxidative metabolites of ethanol that accumulate in human tissues upon ethanol intake. Although FAEEs are considered as toxic metabolites causing cellular dysfunction and tissue damage, the enzymology of FAEE metabolism remains poorly understood. In this study, we used a biochemical screen in Saccharomyces cerevisiae to identify and characterize putative hydrolases involved in FAEE catabolism. We found that Yju3p, the functional orthologue of mammalian monoacylglycerol lipase (MGL), contributes >90% of cellular FAEE hydrolase activity, and its loss leads to the accumulation of FAEE. Heterologous expression of mammalian MGL in yju3Δ mutants restored cellular FAEE hydrolase activity and FAEE catabolism. Moreover, overexpression or pharmacological inhibition of MGL in mouse AML-12 hepatocytes decreased or increased FAEE levels, respectively. FAEEs were transiently incorporated into lipid droplets (LDs) and both Yju3p and MGL co-localized with these organelles. We conclude that the storage of FAEE in inert LDs and their mobilization by LD-resident FAEE hydrolases facilitate a controlled metabolism of these potentially toxic lipid metabolites.

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.

Induction of apoptosis by fatty acid ethyl esters in HepG2 cells

Fatty acid ethyl esters (FAEEs) are esterification products of ethanol and fatty acids which have been found particularly in the organ damaged by ethanol abuse. To evaluate any effect of FAEEs on HepG2 cells, we added FAEEs to cell culture medium. Electrophoresis of DNA from HepG2 cells exposed to 18.5 microM ethyl palmitate (EP) and 10.6 microM ethyl stearate (ES) for 24 h revealed a smear which is typical of non-specific degradation by DNA ladder assay. Apoptosis was characterized by electron microscopy, flow cytometry revealed that the cell cycle of HepG2 cells was perturbed by exposure to FAEEs. In the present study we demonstrate that treatment of HepG2 cells with EP and ES induces apoptosis, as well as perturbing the cell cycle as the number of cells in the G(2)/M and S phases decreased.

On Sensitivity, Specificity, and the Influence of Various Parameters on Ethyl Glucuronide Levels in Urine???Results From the WHO/ISBRA Study

BACKGROUND

Ethyl glucuronide (EtG), a direct ethanol metabolite, seems to meet the need for a sensitive and specific marker for monitoring recent alcohol consumption in different settings. Our aim was to study sensitivity, specificity, and the influence of various parameters on EtG levels in urine.

PATIENTS AND METHODS

Urine samples for a total of 453 patients (373 male, 80 female) were statistically analyzed. The mean age was 37.1 years (median 36, SD 12.59), body mass index was 24.7, total ethanol consumed last month was 1817.66 g (each median), and 80 patients reported cannabis use within the last 30 days. Determination of EtG was performed with a liquid chromatography-tandem mass spectrometry method with deuterium-labeled EtG as internal standard.

RESULTS

For EtG in urine, a good correlation was found with other state markers and days of sobriety. In a regression analysis, age, gender, marijuana use, kidney disease, and total grams of ethanol consumed last month were the variables that significantly influenced EtG levels in contrast to race, smoking, body mass index, cirrhosis of liver, age began drinking regularly, packs of cigarettes smoked last month, and total body water. Furthermore, in a receiver operating characteristic curve analysis to distinguish between nondrinkers and individuals sober > 4 days versus individuals drinking in the recent 4 days, area under the curve was 0.834. At a cutoff of 0.145 mg/liter, sensitivity was 83.5% and specificity 68.3%. A receiver operating characteristic curve was calculated for lifetime alcohol abuse or dependence against those who had never been abusers or dependent. In this case, subjects were either never dependent or lifetime dependent, but those currently dependent were excluded. The resulting area under the curve was 0.694. At a cutoff of 0.145 mg/liter, sensitivity was 73.8% and specificity 60.3%. For those with a self-reported sobriety of less than 24 hr, the area under the curve was 0.899, sensitivity was 90.8%, and specificity was 76.5% at a cutoff of 0.435 mg/liter when we calculated nondrinkers and light drinkers against heavy drinkers and drinkers needing treatment. Cannabis-using patients showed significant differences with regard to almost all state markers when compared with nonconsuming subjects.

CONCLUSIONS

Age, gender, marijuana use, kidney disease, and total grams of ethanol consumed last month should be taken into consideration when interpreting results of EtG in urine. Sensitivity and specificity seem promising. Cannabis use can be regarded as an indicator for other serious mental problems in alcohol-using subjects.

Stearic acid stimulates FA ethyl ester synthesis in HepG2 cells exposed to ethanol

FA ethyl esters (FAEE) are nonoxidative metabolites of ethanol produced by the esterification of FA and ethanol. FAEE have been implicated as mediators of ethanol-induced organ damage in vivo and in vitro, and are markers of ethanol intake. Upon ethanol intake, FAEE are synthesized in the liver and pancreas in significant quantities. There is limited information on the stimulation of FAEE synthesis upon addition of exogenous FA in vitro. HepG2 cells were incubated with ethanol alone, ethanol with 25 microM linoleate, and ethanol with 25 microM stearate. The amount of FAEE in human hepatoblastoma (HepG2) cells was determined 1-3 h after ethanol and FA addition. Stearate increased the FAEE concentration in HepG2 cells when incubated with the cells for 1 h, whereas linoleate did not increase the cellular FAEE concentration at any time. Ethyl palmitate, ethyl stearate, and ethyl oleate were the predominant FAEE species identified in all cases, independent of the specific supplemental FA added to the medium.

Membrane transport of fatty acylcarnitine and free L-carnitine by rat liver microsomes

Recent studies have suggested that parts of the hepatic activities of diacylglycerol acyltransferase and acyl cholesterol acyltransferase are expressed in the lumen of the endoplasmic reticulum (ER). However the ER membrane is impermeable to the long-chain fatty acyl-CoA substrates of these enzymes. Liver microsomal vesicles that were shown to be at least 95% impermeable to palmitoyl-CoA were used to demonstrate the membrane transport of palmitoylcarnitine and free L-carnitine - processes that are necessary for an indirect route of provision of ER luminal fatty acyl-CoA through a luminal carnitine acyltransferase (CAT). Experimental conditions and precautions were established to permit measurement of the transport of [14C]palmitoylcarnitine into microsomes through the use of the luminal CAT and acyl-CoA:ethanol acyltransferase as a reporter system to detect formation of luminal [14C]palmitoyl-CoA. Rapid, unidirectional transport of free L-[3H]carnitine by microsomes was measured directly. This process, mediated either by a channel or a carrier, was inhibited by mersalyl but not by N-ethylmaleimide or sulfobetaine - properties that differentiate it from the mitochondrial inner membrane carnitine/acylcarnitine exchange carrier. These findings are relevant to the understanding of processes for the reassembly of triacylglycerols that lipidate very low density lipoprotein particles as part of a hepatic triacylglycerol lipolysis/re-esterification cycle.

Placental handling of fatty acid ethyl esters: perfusion and subcellular studies

The measurement of fatty acid ethyl esters (FAEE) in neonatal meconium is a novel test to confirm prenatal ethanol exposure. The origin of FAEE in the maternal-placental-fetal unit is not known. The objectives of this study were to investigate whether FAEE are transferred and metabolized by the human placenta. Isolated placental cotyledons were perfused with a mixture of four FAEE (palmitic, stearic, oleic, and linoleic acid ethyl esters) commonly detected in the meconium of neonates exposed to ethanol in utero, and the transfer of FAEE to the fetal unit was investigated in the absence and presence of albumin. The metabolic degradation of FAEE by human placental microsomes was subsequently determined. FAEE disappeared from the maternal circulation but remained undetectable in the fetal unit following perfusions. The addition of albumin had no effect on FAEE transfer. The unrecoverable fraction of individual FAEE in the perfusion system accounted for >50% of the initial amount used, suggesting significant metabolic degradation. Subcellular studies documented the enzymatic degradation of FAEE by placental microsomes (mean Km, 35-95 microM; Vmax, 0.6-1.8 nmol/min/mg for individual FAEE). FAEE at levels found in alcoholics that are originated from the mother are not transferred to the fetus because they are taken up and degraded extensively by the human placenta. Hence, FAEE detected in neonatal matrices are likely produced by the fetus from ethanol that has been transferred to and metabolized by the fetus, rendering FAEE a powerful direct biomarker reflective of true fetal exposure to ethanol in utero.

Ethyl glucuronide--the direct ethanol metabolite on the threshold from science to routine use

AIMS

Current biological state markers remain suboptimal with regard to sensitivity and specificity for monitoring recent alcohol consumption in various settings. Furthermore, these biomarkers can be influenced by age, gender and a variety of substances and non-alcohol-associated diseases and do not cover fully the time axis for alcohol intake. Ethyl glucuronide (EtG) is a non-volatile, water-soluble, stable, direct metabolite of ethanol that can be detected in various body fluids, tissues and hair. Shortly after the consumption even of small amounts of ethanol, EtG becomes positive. It can detect ethanol intake up to 80 hours after the complete elimination of alcohol from the body, covering a unique and important time spectrum for recent alcohol use. EtG seems to meet the need for a sensitive and specific marker to elucidate alcohol use not detected by standard testing.

DESIGN, SETTING, PARTICIPANTS, METHODS AND FINDINGS

The literature was reviewed with a focus on possible diagnostic and therapeutic applications, currently available methods and future perspectives. To date, more than 4000 samples of body fluids, tissues and hair from approximately 1500 individuals have been assessed.

CONCLUSIONS

The data suggest that EtG is a useful tool in numerous settings, including alcohol and drug treatment (to detect lapse/relapse and for motivational feedback), in safety sensitive work settings where use is dangerous or in other settings where alcohol use may be risky (e.g. such as driving, work-place, pregnancy or monitoring physicians or other professionals who are in recovery and working) or for resolving forensic questions. If the question of recent alcohol consumption has to be answered in a binary way (yes/no), such as for determining lapses. the use of EtG in urine is among the preferred tests. The use of this marker alone and complementary with other biological state markers and self-reports is expected to lead to significant improvement in treatment outcome, therapy efficacy and cost reduction.

Ethyl glucuronide discloses recent covert alcohol use not detected by standard testing in forensic psychiatric inpatients

BACKGROUND

Considerable lives and money could be saved if one could detect early stages of lapsing/relapsing behavior in addicted persons (e.g., in safety-sensitive workplaces) and could disclose harmful drinking in social drinkers. Due to the serious public health problem of alcohol use and abuse worldwide, markers of alcohol use have been sought. Both ethyl glucuronide (EtG) and phosphatidyl ethanol (PEth) appear to have high sensitivity and specificity and a time frame of detection that may elucidate alcohol use not detected by standard testing. Our aim was to assess their potential for detecting recent covert alcohol use under controlled conditions.

METHODS

Thirty-five forensic psychiatric inpatients in a closed ward who had committed a substance-related offense ( section sign 64 StGB), were followed for 12 months. The complete time spectrum of possible alcohol consumption was covered by the complementary use of breath and urinary ethanol (hours), urinary EtG (days), %carbohydrate-deficient transferrin (CDT)/PEth (weeks), and gamma-glutamyltranspeptidase (GGT)/mean corpuscular volume (MCV) (weeks-months).

RESULTS

Fourteen of the 146 urine samples examined were positive for EtG. In all EtG-positive cases, patients reported alcohol consumption of between 40 and 200 g of ethanol 12-60 hr prior to testing. Urinary and breath ethanol were positive in only one case. In the blood samples, PEth was not positive in any case and %CDT did not exceed the reference value. Isoelectric focusing showed no abnormal Tf subtypes.

CONCLUSIONS

The findings emphasize the diagnostic and therapeutic usefulness, specificity, and sensitivity of EtG as a marker of recent alcohol use. Such a test is needed in numerous settings, including alcohol and drug treatment (to detect lapse/relapse), in safety-sensitive work settings where use is dangerous or in other settings where use may be inappropriate (e.g., such as driving, workplace, pregnancy, or monitoring physicians or other professionals who are in recovery and working), or for testing other groups (such as children or those with medical problems) where alcohol use would be unhealthy or unsafe. The health, social and socioeconomic benefits arising from the future use of these markers is hard to overestimate.

Ethyl glucuronide: on the time course of excretion in urine during detoxification

Ethyl glucuronide (EtG) is a promising new biological state marker of recent alcohol consumption that detects alcohol use reliably over a definite time period. Other currently available markers lack acceptable sensitivity and specificity. Our aim is to elucidate under naturalistic conditions the time course of EtG excretion in urine following alcohol consumption and to show how this can be utilized for monitoring and prognosis in patients. There are no other existing data on this issue to date. One hundred and thirty-eight urine samples from 28 male alcohol withdrawal patients were drawn every 3-24 hours for up to 94 hours after hospitalization. Breath ethanol concentration (mean) at hospitalization was 900 mg/L. Patient age in years was 40.3 (mean). Determination of urine EtG was performed by gas chromatography/mass spectrometry (GC/MS) with deuterium-labelled EtG as an internal standard. The strongest correlations (p<0.01) were found between EtG determinations in the different patient when breath ethanol concentrations (BEC) were 0 and 48 hours after BEC=0 (r=0.747), EtG 24 and 48 hours after BEC=0 (r=0.872), and in the time frame of detection (hours) of EtG and EtG 48 hours after BEC=0 (r=0.762). No significant correlation was found (Mann-Whitney test) between EtG concentrations in urine at different time points between the groups of patients with (a) 1 or less-2, (b) 3-4 or more previous hospitalizations, (c) a history of seizures (yes/no) or (d) an age above or below the median (40.5). EtG excretion in urine is not random, but seems rather to follow a kinetic profile. Furthermore our preliminary data indicate, that there is no significant difference for EtG concentration in urine when correlated to group variables such as age, seizures and hospitalizations.

The ethanol conjugate ethyl glucuronide is a useful marker of recent alcohol consumption

BACKGROUND

Current biological state markers remain suboptimal with regard to sensitivity and specificity for monitoring alcohol consumption. The currently used state markers can be influenced by age, sex, and a variety of substances and non-alcohol-associated diseases and do not fully cover the time axis for alcohol intake. Ethyl glucuronide (EtG) is a promising, nonvolatile, water-soluble marker of recent alcohol consumption that is stable during storage and can be detected for an extended time period (up to 80 hr) after alcohol is completely eliminated from the body.

PATIENTS AND METHODS

In the WHO/ISBRA Study of State and Trait Markers of Alcohol Use and Dependence, EtG was determined in urine samples from 304 patients with an liquid chromotography, electrospray ionization double mass spectrometry (ESI-LC/MS-MS) method. Deuterium labeled EtG was used as internal standard. Determination limit was 0.1 mg/liter. All measurements were performed in duplicate. A calibration solution was measured after each 10 samples.

RESULTS

The following significant correlations were found for the Spearman rank correlation for the total sample between EtG and other variables: sobriety in days (r = -0.6), 5-hydroxytryptophol to 5-hydroxyindole-3-acetic acid (HTOL/HIAA) ratio (r = 0.58), ethanol level (r = 0.433), methanol level (r = 0.198), carbohydrate-deficient transferrin (r = 0.458), gamma-glutamyltransferase (r = 0.428), aspartate aminotransferase (r = 0.260), age (r = 0.264), and total grams of ethanol consumed in the previous month (r = 0.467). In a subsample of 277 subjects in whom no ethanol was detectable in urine, the following correlations with EtG levels were found: sobriety (days; r = -0.597), HTOL/HIAA ratio (r = 0.478), gamma-glutamyltransferase (r = 0.422), total grams of ethanol consumed last month (r = 0.395), and carbohydrate-deficient transferrin (r = 0.366; all significant at p < 0.05). When we compared results between EtG levels and the HTOL/HIAA ratio, 68.8% (n = 119) of those positive for EtG did not have elevated values for the HTOL/HIAA ratio. Thirty-one percent (31.2%) of these 119 subjects were positive for both parameters, but of those negative for EtG, only 4.4% had an elevated HTOL/HIAA ratio.

CONCLUSIONS

EtG is a good candidate for a sensitive, specific, and reliable marker of recent alcohol intake. The complementary use of this marker with other biological state markers should significantly improve treatment outcome and therapy effectiveness and reduce costs.

Involvement of liver carboxylesterases in the in vitro metabolism of lidocaine

Although lidocaine has been used clinically for more than half a century, the metabolism has still not been fully elucidated. In the present study we have addressed the involvement of hydroxylations, deethylations, and ester hydrolysis in the metabolism of lidocaine to 2,6-xylidine. Using microsomes isolated from male rat liver, we found that lidocaine is mainly metabolized by deethylation to N-(N-ethylglycyl)-2,6-xylidine, and N-(N-ethylglycyl)-2,6-xylidine is mainly metabolized to N-glycyl-2,6-xylidine, also by deethylation. However, 2,6-xylidine can be formed both from lidocaine and N-(N-ethylglycyl)-2,6-xylidine, but not from N-glycyl-2,6-xylidine, in an NADPH-independent reaction, suggesting that the amido bond in these compounds can be directly hydrolyzed by esterases. To test this hypothesis, we incubated lidocaine, N-(N-ethylglycyl)-2,6-xylidine, and N-glycyl-2,6-xylidine with purified liver carboxylesterases. Rat liver microsomal carboxylesterase ES-10, but not carboxylesterase ES-4, hydrolyzed lidocaine and N-(N-ethylglycyl)-2,6-xylidine to 2,6-xylidine, identifying this esterase as a candidate enzyme in the metabolism of lidocaine.

Fatty Acid Ethyl Esters in Liver and Adipose Tissues as Postmortem Markers for Ethanol Intake

Background: Fatty acid ethyl esters (FAEEs) are nonoxidative metabolites of ethanol. FAEEs are found in liver, pancreas, and adipose tissues up to 24 h after consumption of ethanol, and on that basis, they are potentially useful markers for ethanol intake. In this study with rats, we investigated the efficacy of using FAEEs in liver and in adipose tissue as postmortem markers for premortem ethanol ingestion.

Methods: An animal study was conducted in which test rats received injections of ethanol and control rats received injections of normal saline. The rats were killed 2 h after the injections. The bodies of the animals were stored at 4 °C up to 12 h, and samples of liver and adipose tissues were collected at different time intervals and processed for FAEE quantification. In another set of experiments, the rats received injections and were killed as described above, but bodies of animals from both groups were stored at 4, 25, or 37 °C for up to 72 h, and liver samples were collected and processed for FAEE quantification.

Results: FAEEs were detected up to 12 h after death in liver and adipose tissue samples from the bodies of ethanol-treated animals stored at 4 °C; negligible amounts were detected in the bodies of animals that received normal saline. Adipose tissues contained higher amounts of FAEEs than liver, as well as more species: eight FAEE species in adipose tissue and five in liver tissue. Higher concentrations of FAEEs were detected in livers of treated animals stored at 25 °C for up to 48 h than in livers of controls stored under the same conditions.

Conclusions: For at least 12 h after death, FAEEs in liver and adipose tissues are useful postmortem markers of premortem ethanol ingestion.

Effects of acetyl-L-carnitine on the formation of fatty acid ethyl esters in brain and peripheral organs after short-term ethanol administration in rat

Increasing evidence suggests that Fatty acid ethyl esters (FAEE) play a central role in ethanol induced organ damage. In the current study we measured FAEE formation in rats after short-term oral administration of ethanol, in the presence and absence of pre-treatment with acetyl-L-carnitine. Ethanol treatment caused a significant increase in the levels of FAEE, particularly in the brain and heart, but also in the kidney and liver. Increases in FAEE were associated with a significant increase in FAEE synthase activity, GSH transferase activity, and lipid hydroperoxide levels. Pretreatment with acetyl-L-carnitine resulted in a significant reduction of FAEE accumulation, decrease in FAEE synthase and GSH transferase activities, and lipid hydroperoxide levels. Administration of acetyl-L-carnitine greatly reduced the metabolic abnormalities due to non-oxidative ethanol metabolism, through an increment in lipid metabolism/turnover and by the modulation of the activities of enzymes associated with FAEE synthesis. These results suggest a potentially important pharmacological role for acetyl-L-carnitine in the prevention of alcohol-induced cellular damage.

The dose-response effects of ethanol on the human fetal osteoblastic cell line

Alcohol is a risk factor for the development of osteoporosis, especially in men. Chronic alcohol abuse decreases bone mass, which contributes to the increased incidence of fractures. To better understand the mechanism of action of ethanol on bone metabolism, we have studied the dose-response effects of ethanol on conditionally immortalized human fetal osteoblasts (hFOB) in culture. Ethanol treatment had no significant effects on osteoblast number after 1 day or 7 days. Ethanol treatment did not reduce type I collagen protein levels at either time point at any dose but slightly reduced alkaline phosphatase activity after 7 days. The messenger RNA (mRNA) levels for alkaline phosphatase, type I collagen, and osteonectin were unaltered by 24 h of ethanol treatment but a high dose (200 mM) reduced mRNA levels for the two bone matrix proteins after 7 days. Ethanol treatment led to dose-dependent increases in transforming growth factor beta1 (TGF-beta1) mRNA levels and decreases in TGF-beta2 mRNA levels. The concentration of ethanol in the medium decreased with time because of evaporation but there was little degradation caused by metabolism. These results, which show that cultured osteoblasts are less sensitive than osteoblasts in vivo, suggest that the pronounced inhibitory effects of ethanol on bone formation are not caused by direct cell toxicity.

Microsomal long-chain acyl-CoA thioesterase (carboxylesterase ES-4) is regulated by thyroxine

Long chain acyl-CoA thioesterase activity is mainly located in microsomes after subcellular fractionation of liver from untreated rats. The physiological function and regulation of expression of this activity is not known. In the present study we have investigated the effect of thyroxine on expression of carboxylesterase ES-4, the major acyl-CoA thioesterase of liver microsomes. Thyroidectomy of rats decreased the palmitoyl-CoA thioesterase activity to about 25% of normal activity. This decrease was accompanied by similar decreases at the protein and mRNA levels (31% and 57%, respectively, of controls). Treatment with thyroxine completely reversed the effect of thyroidectomy and resulted in elevated levels in both thyroidectomized and control rats. For reasons of comparison we also studied the possibility that ES-10 and ES-2, two other members of the same gene family, are affected by thyroxine. ES-10 was not changed at the protein or mRNA level by any of the treatments, while ES-2 expression in liver was decreased by thyroxine treatment. The data shows that changes in activity and expression of ES-4 correlate to thyroxine status in the rat suggesting a physiological regulatory role by this hormone. Since thyroxine regulates the expression of lipogenic enzymes, these results are consistent with a function for this microsomal acyl-CoA thioesterase in fatty acid synthesis and/or secretion, rather than in oxidative degradation of fatty acids.