Alteration of the enzymology of chloral hydrate reduction in the presence of ethanol

Trichloroethylene biotransformation and its role in mutagenicity, carcinogenicity and target organ toxicity

Metabolism is critical for the mutagenicity, carcinogenicity, and other adverse health effects of trichloroethylene (TCE). Despite the relatively small size and simple chemical structure of TCE, its metabolism is quite complex, yielding multiple intermediates and end-products. Experimental animal and human data indicate that TCE metabolism occurs through two major pathways: cytochrome P450 (CYP)-dependent oxidation and glutathione (GSH) conjugation catalyzed by GSH S-transferases (GSTs). Herein we review recent data characterizing TCE processing and flux through these pathways. We describe the catalytic enzymes, their regulation and tissue localization, as well as the evidence for transport and inter-organ processing of metabolites. We address the chemical reactivity of TCE metabolites, highlighting data on mutagenicity of these end-products. Identification in urine of key metabolites, particularly trichloroacetate (TCA), dichloroacetate (DCA), trichloroethanol and its glucuronide (TCOH and TCOG), and N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (NAcDCVC), in exposed humans and other species (mostly rats and mice) demonstrates function of the two metabolic pathways in vivo. The CYP pathway primarily yields chemically stable end-products. However, the GST pathway conjugate S-(1,2-dichlorovinyl)glutathione (DCVG) is further processed to multiple highly reactive species that are known to be mutagenic, especially in kidney where in situ metabolism occurs. TCE metabolism is highly variable across sexes, species, tissues and individuals. Genetic polymorphisms in several of the key enzymes metabolizing TCE and its intermediates contribute to variability in metabolic profiles and rates. In all, the evidence characterizing the complex metabolism of TCE can inform predictions of adverse responses including mutagenesis, carcinogenesis, and acute and chronic organ-specific toxicity.

Dose-dependent liver regeneration in chloroform, trichloroethylene and allyl alcohol ternary mixture hepatotoxicity in rats

The present study was designed to examine the hypothesis that liver tissue repair induced after exposure to chloroform (CF) + trichloroethylene (TCE) + allyl alcohol (AA) ternary mixture (TM) is dose-dependent similar to that elicited by exposure to these compounds individually. Male Sprague Dawley (S-D) rats (250-300 g) were administered with fivefold dose range of CF (74-370 mg/kg, ip), and TCE (250-1250 mg/kg, ip) in corn oil and sevenfold dose range of AA (5-35 mg/kg, ip) in distilled water. Liver injury was assessed by plasma alanine amino transferase (ALT) activity and liver tissue repair was measured by (3) H-thymidine incorporation into hepatonuclear DNA. Blood and liver levels of parent compounds and two major metabolites of TCE [trichloroacetic acid (TCA) and trichloroethanol (TCOH)] were quantified by gas chromatography. Blood and liver CF and AA levels after TM were similar to CF alone or AA alone, respectively. However, the TCE levels in blood and liver were substantially decreased after TM in a dose-dependent fashion compared to TCE alone. Decreased plasma and liver TCE levels were consistent with decreased production of metabolites and elevated urinary excretion of TCE. The antagonistic interaction resulted in lower liver injury than the summation of injury caused by the individual components at all three-dose levels. On the other hand, tissue repair showed a dose-response leading to regression of injury. Although the liver injury was lower and progression was contained by timely tissue repair, 50% mortality occurred only with the high dose combination, which is several fold higher than environmental levels. The mortality could be due to the central nervous system toxicity. These findings suggest that exposure to TM results in lower initial liver injury owing to higher elimination of TCE, and the compensatory liver tissue repair stimulated in a dose-dependent manner mitigates progression of injury after exposure to TM.

Evaluating human variability in chemical risk assessment: hazard identification and dose-response assessment for noncancer oral toxicity of trichloroethylene

Human variability can be addressed during each stage in the risk assessment of chemicals causing noncancer toxicities. Noncancer toxicities arising from oral exposure to trichloroethylene (TCE) are used in this paper as a case study for exploring strategies for identifying and incorporating information about human variability in the chemical specific hazard identification and dose-response assessment steps. Toxicity testing in laboratory rodents is the most commonly used method for hazard identification. By using animal models for sensitive populations, such as developing fetuses, testing can identify some potentially sensitive populations. A large variety of reproductive and developmental studies with TCE were reviewed. The results were mostly negative and the limited positive findings generally occurred at doses similar to those causing liver and kidney toxicity. Physiologically based pharmacokinetic modeling using Monte Carlo simulation is one method for evaluating human variability in the dose-response assessment. Three strategies for obtaining data describing this variability for TCE are discussed: (1) using in vivo human pharmacokinetic data for TCE and its metabolites, (2) studying metabolism in vitro, and (3) identifying the responsible enzymes and their variability. A review of important steps in the metabolic pathways for TCE describes known metabolic variabilities including genetic polymorphisms, enzyme induction, and disease states. A significant problem for incorporating data on pharmacokinetic variability is a lack of information on how it relates to alterations in toxicity. Response modeling is still largely limited to empirical methods due to the lack of knowledge about toxicodynamic processes. Empirical methods, such as reduction of the No-Observed-Adverse-Effect-Level or a Benchmark Dose by uncertainty factors, incorporate human variability only qualitatively by use of an uncertainty factor. As improved data and methods for biologically based dose-response assessment become available, use of quantitative information about variability will increase in the risk assessment of chemicals.

Chloroacetaldehyde-induced hepatocyte cytotoxicity. Mechanisms for cytoprotection

2-Chloroacetaldehyde (CAA)-induced cytotoxicity in isolated hepatocytes was enhanced markedly if hepatocyte alcohol or aldehyde dehydrogenase was inhibited prior to CAA addition. Hepatocyte GSH depletion, ATP depletion and lipid peroxidation by CAA were also enhanced markedly. Furthermore, CAA was about 10- and 70-fold more cytotoxic than its oxidative or reductive metabolite chloroacetate or chloroethanol, respectively. Nutrients such as lactate, xylitol, sorbitol or glycerol, which increase cytosolic NADH levels, prevented CAA cytotoxicity in normal hepatocytes but further enhanced cytotoxicity toward alcohol dehydrogenase inactivated hepatocytes, suggesting that increased cytosolic NADH reduces CAA via alcohol dehydrogenase in normal hepatocytes but prevents CAA oxidation in alcohol dehydrogenase inactivated hepatocytes. However, increasing cytosolic NADH levels with ethanol or NADH-generating nutrients after CAA had been metabolized also prevented cytotoxicity and caused a partial ATP recovery, whereas oxidation of cytosolic NADH with pyruvate markedly increased cytotoxicity. This indicates that cytotoxic CAA concentrations cause oxidative stress and that ATP levels can be restored if cellular redox homeostasis is normalized with reductants. Furthermore, except for fructose, nutrients that did not increase NADH did not affect CAA-induced cytotoxicity. Fructose also caused a partial ATP recovery, and its protection was prevented by the glycolytic inhibitor fluoride. Hepatocytes isolated from fasted animals were 4- to 6-fold more susceptible to CAA-induced ATP depletion and cytotoxicity. No lipid peroxidation occurred at these lower CAA concentrations. Furthermore, all nutrients, including alanine, glutamine and glucose, prevented cytotoxicity toward hepatocytes isolated from fasted animals. The susceptibility of hepatocytes to CAA cytotoxicity, therefore, depends on both cellular redox homeostasis and cellular energy supply.

Aldehyde dehydrogenases and their role in carcinogenesis

Aldehydes are highly reactive molecules that may have a variety of effects on biological systems. They can be generated from a virtually limitless number of endogenous and exogenous sources. Although some aldehyde-mediated effects such as vision are beneficial, many effects are deleterious, including cytotoxicity, mutagenicity, and carcinogenicity. A variety of enzymes have evolved to metabolize aldehydes to less reactive forms. Among the most effective pathways for aldehyde metabolism is their oxidation to carboxylic acids by aldehyde dehydrogenases (ALDHs). ALDHs are a family of NADP-dependent enzymes with common structural and functional features that catalyze the oxidation of a broad spectrum of aliphatic and aromatic aldehydes. Based on primary sequence analysis, three major classes of mammalian ALDHs--1, 2, and 3--have been identified. Classes 1 and 3 contain both constitutively expressed and inducible cytosolic forms. Class 2 consists of constitutive mitochondrial enzymes. Each class appears to oxidize a variety of substrates that may be derived either from endogenous sources such as amino acid, biogenic amine, or lipid metabolism or from exogenous sources, including aldehydes derived from xenobiotic metabolism. Changes in ALDH activity have been observed during experimental liver and urinary bladder carcinogenesis and in a number of human tumors, including some liver, colon, and mammary cancers. Changes in ALDH define at least one population of preneoplastic cells having a high probability of progressing to overt neoplasms. The most common change is the appearance of class 3 ALDH dehydrogenase activity in tumors arising in tissues that normally do not express this form. The changes in enzyme activity occur early in tumorigenesis and are the result of permanent changes in ALDH gene expression. This review discusses several aspects of ALDH expression during carcinogenesis. A brief introduction examines the variety of sources of aldehydes. This is followed by a discussion of the mammalian ALDHs. Because the ALDHs are a relatively understudied family of enzymes, this section presents what is currently known about the general structural and functional properties of the enzymes and the interrelationships of the various forms. The remainder of the review discusses various aspects of the ALDHs in relation to tumorigenesis. The expression of ALDH during experimental carcinogenesis and what is known about the molecular mechanisms underlying those changes are discussed. This is followed by an extended discussion of the potential roles for ALDH in tumorigenesis. The role of ALDH in the metabolism of cyclophosphamidelike chemotherapeutic agents is described. This work suggests that modulation of ALDH activity may an important determinant of the effectiveness of certain chemotherapeutic agents.(ABSTRACT TRUNCATED AT 400 WORDS)

Aldehyde dismutation catalyzed by pulmonary carbonyl reductase: kinetic studies of chloral hydrate metabolism to trichloroacetic acid and trichloroethanol

The kinetics of the NAD(P)(+)-linked aldehyde dismutation by pulmonary carbonyl reductase of guinea pig were studied using a highly hydrated substrate, chloral hydrate (CH). The enzyme irreversibly converted the substrate into trichloroacetic acid (TCA) and trichloroethanol (TCE) in the presence of the reduced or oxidized cofactors, of which NAD(P)+ gave a higher reaction rate than did NAD(P)H, and the concentration ratios of the two products (TCA plus TCE) to CH utilized were 1:1. In the NAD(P)(+)-linked reaction TCA was the predominant product and its amount was compatible with that of TCE plus NAD(P)H produced, whereas in the NAD(P)H-linked reaction equal amounts of TCA and TCE were formed and the cofactor was little oxidized. These results suggest that the enzyme oxidized the hydrated aldehydes to TCA with NAD(P)+ as the cofactor and reduced the unhydrated aldehyde to TCE with NAD(P)H. The steady-state kinetic measurements in the NADP(+)-linked CH oxidation were consistent with an ordered Bi Bi mechanism which is the same as that for the secondary alcohol oxidation by the enzyme. The dehydrogenase activity was inhibited competitively with respect to CH by a secondary alcohol substrate, propan-2-ol. The CH and propan-2-ol dehydrogenase activities were similarly inactivated by 2,4,6-trinitrobenzene-sulfonate, and NADP(H), several cofactor analogs and a cofactor-competitive inhibitor, Cibacron blue dye, protected against the inactivation, which suggest that lysine residues are essential for catalysis.

Non-specific prolongation of the effects of general depressants by pyrazole and 4-methylpyrazole

The liver alcohol dehydrogenase inhibitors, pyrazole and 4-methylpyrazole, have been tested for their ability to prolong drug-induced sleep times in mice. Both drugs (at 1 mmol kg-1 i.p.) prolonged the duration of loss of righting reflex following chloral hydrate, pentobarbitone, barbitone, temazepam and halothane, but not diethyl ether. This suggests that the effects of these pyrazoles are not specific to the inhibition of liver alcohol dehydrogenase.

Alteration of chloral hydrate metabolism in rats with carbon tetrachloride-induced liver damage

The metabolism of chloral hydrate (CH) was investigated in the isolated perfused rat liver system. The experiments were performed on rats that were administered carbon tetrachloride (CCl4) subcutaneously for 15 weeks to induce chronic liver damage and on untreated rats. Clearance of CH from the perfusion system was lower in damaged liver than in control liver. In both groups, 50-70% of the added CH was excreted into perfusate as trichloroethanol (TCE) and trichloroacetic acid (TCA) within 120 min. The TCE/TCA ratio was 1:1.3 in the control group compared to 2:1 in the damaged liver group. The findings suggest that CH metabolism in the liver is affected by chronic damage.

The metabolite ratio as a function of chloral hydrate dose and intracellular redox state in the perfused rat liver

Chloral hydrate (CH), an intermediate metabolite of trichloroethylene, is reduced to trichloroethanol (TCE) by alcohol dehydrogenase and aldehyde reductase, and is also oxidized to trichloroacetic acid (TCA) by the nicotinamide adenine dinucleotide (NAD)-dependent enzyme, CH dehydrogenase. Alcohol dehydrogenase requires reduced NAD (NADH), aldehyde reductase requires reduced nicotinamide adenine dinucleotide phosphate (NADPH) and CH dehydrogenase requires NAD to complete the reaction. It is unclear which reaction is predominant at the physiological redox level in intact liver cells. To study this question, we perfused the livers of well-fed rats with Krebs-Ringer buffer solution containing 0.1 mM pyruvate/1.0 mM lactate. The levels of TCE and TCA in the effluent were measured by gas chromatography, and the fluorescence of reduced pyridine nucleotides was measured with a surface fluorometer. When a low concentration (below 0.25 mM) of CH was administered, more TCA than TCE was produced. When a high concentration of CH was administered (over 0.5 mM), TCE production was greater. Reduced pyridine nucleotides decreased inversely with the CH concentration. Even at low CH concentrations, pyridine nucleotides were not reduced. When 10 mM lactate was added to the perfusate in order to reduce the pyridine nucleotides in the liver cells, the TCE/TCA ratio increased. On the other hand, the TCE/TCA ratio tended to fall following the addition of 5.0 mM pyruvate. In conclusion, the TCE/TCA ratio was altered according to the concentration of CH, and to the redox level of pyridine nucleotides in the liver.

Effects of disulfiram on the oxidation of benzaldehyde and acetaldehyde in rat liver

The in vitro oxidation of benzaldehyde and acetaldehyde was studied in liver samples from disulfiram-treated and control rats. With 25 microM substrate, both cytosol and mitochondria appeared to make a nearly equal contribution to the oxidation of benzaldehyde, whereas ca. 90% of acetaldehyde oxidation occurred in mitochondria. When the Km values for benzaldehyde with aldehyde dehydrogenase (ALDH) were determined, two Km values (3 and 120 microM) were obtained with mitochondria, but only a single Km value (25 microM) was obtained with the cytosolic fraction. The relatively high Km (2.9 mM) found with microsomes makes it unlikely that microsomes are important in the oxidation of benzaldehyde. In intact mitochondria, with 200 microM acetaldehyde or benzaldehyde the matrix space enzyme accounted for 77 and 62%, respectively, of the total ALDH activity. When the activity was determined in a mixture containing both substrates, the activity was found not to be additive, indicating that both substrates are oxidized by the same matrix space enzyme. With subcellular fractions, from livers of disulfiram-treated and control rats, a greater degree of inhibition of ALDH was obtained when acetaldehyde was a substrate compared to that with benzaldehyde in cytosol and mitochondria. Microsomal ALDH was not inhibited by disulfiram. In liver slices from rats given disulfiram, a statistically significant inhibition was found when either 25 or 250 microM acetaldehyde was used (46 and 33%). With benzaldehyde, a significant inhibition (24%) was observed only with the lower substrate concentration. Finding that both mitochondrial fractions and slices were less inhibited at the higher substrate concentration implies that the high Km enzyme is not inhibited. It can be concluded that, in rat, disulfiram inhibiting liver ALDH not only affects oxidation of acetaldehyde, but also that of benzaldehyde.

Reductases for carbonyl compounds in human liver

Two aldehyde reductases with mol. wt 78,000 and 32,000 and one carbonyl reductase with mol. wt 31,000 were purified to homogeneity from human liver cytosol. The high molecular weight aldehyde reductase exhibited properties similar to alcohol dehydrogenase; it had a single subunit of mol. wt 41,000 and a pI value of 10 to 10.5, and showed preference for NADH over NADPH as cofactor and sensitivity to SH-reagents, pyrazole, o-phenanthroline and isobutyramide. The enzyme reduced aliphatic and aromatic aldehydes, alicyclic ketones and alpha-diketones and an optimal pH of 6.0, and oxidized various alcohols with NAD as a cofactor at an optimal pH of 8.8. The identity of the enzyme with alcohol dehydrogenase was established by starch gel electrophoresis and co-purification of the two enzymes. The other enzymes were NADPH-dependent and monomeric reductases; the aldehyde reductase reduced aldehydes, hexonates and alpha-diketones and was sensitive to barbiturates, diphenylhydantoin and valproate, while the carbonyl reductase showed a broad substrate specificity for aldehydes, ketones and quinones and was inhibited by SH-reagent, quercitrin and benzoic acid. The latter enzyme appeared in three multiforms with different charges which occurred in differing ratios in liver specimens. Comparison of kinetic constants for aldehydes among the enzymes indicated that alcohol dehydrogenase is the best reductase with the highest affinity and Kcat values. The enzyme also catalyzed oxidation and reduction of aromatic aldehydes in the presence of NAD at physiological pH of 7.2. Tissue distribution of the three enzymes and variation of their specific activities in human livers were examined.

Application of metabolic profiling to study the effects of ethanol on metabolism in rats

Rat urine was analyzed by both gas chromatography and a combination of gas chromatography/mass spectroscopy in an attempt to apply the technique of metabolic profiling to determine if ethanol consumption produced an alteration in acid excretion products. Rats were fed a liquid diet for seven days then fed ethanol in the same diet. The 24 hr urine for the last day of control and the first day of ethanol differed greatly with respect to four compounds. These were an increase in threonic, glucuronic and an undetermined acid and a decrease in pyroglutamic acid. The biological basis for the alterations was not investigated. Glucuronic acid forms conjugates with many compounds. Possibly an acute dose of ethanol may alter the removal of some compounds from the liver.

Metabolic interaction between m-xylene and ethanol

Ingestion of a moderate dose of ethanol (0.8 g/kg) by volunteers prior to 4-h inhalation exposure to m-xylene (6.0 or 11.5 mmol/m3) caused marked alterations in xylene kinetics. After ethanol intake the blood xylene level rose about 1.5-2.0-fold and urinary methylhippuric acid excretion declined by about 50% suggesting that ethanol decreased the metabolic clearance of xylene by about one half during xylene inhalation. This effect was noticeable up until a few hours after completed xylene exposure. Urinary excretion of 2,4-xylenol, the minor m-xylene metabolite, was generally not decreased by ethanol and sometimes the reverse seemed to be the case. The disturbance of xylene kinetics can be hypothesized to be caused mainly by ethanol-mediated inhibition of microsomal metabolism. When four volunteers who ingested ethanol prior to m-xylene inhalation at the higher concentration were monitored for blood acetaldehyde, transiently raised levels were found without notable effects on ethanol elimination. This observation may explain why some individuals experienced dizziness and nausea during the combined ethanol-xylene exposure.