Generation and inhalation toxicity of dichloroacetylene

Abiotic degradation of trichloroethylene under thermal remediation conditions

The degradation of trichloroethylene (TCE) to carbon dioxide (CO2) and chloride (Cl-) has been reported to occur during thermal remediation of subsurface environments. The effects of solid-phase composition and oxygen content on the chemical reactivity of TCE were evaluated in sealed ampules that were incubated at 22 and 120 degrees C for periods ranging from 4 to 40 days. For all treatments, no more than 15% of the initial amount of TCE was degraded, resulting in the formation of several non-chlorinated products including Cl-, CO2, carbon monoxide, glycolate, and formate. First-order rate coefficients for TCE disappearance ranged from 1.2 to 6.2 x 10(-3) day(-1) at 120 degrees C and were not dependent upon oxygen content orthe presence of Ottawa sand. However, the rate of TCE disappearance at 120 degrees C increased by more than 1 order-of-magnitude (1.6 to 5.3 x 10(-2) day(-1)), corresponding to a half-life of 13-44 days in ampules containing 1% (wt) goethite and Ottawa sand. These results indicate that the rate of TCE degradation in heated, three-phase systems is relatively insensitive to oxygen content, but may increase substantially in the presence of iron bearing minerals.

Formation and toxicity of anesthetic degradation products

Toxic degradation products are formed from a range of old and modern anesthetic agents. The common element in the formation of degradation products is the reaction of the anesthetic agent with the bases in the carbon dioxide absorbents in the anesthesia circuit. This reaction results in the conversion of trichloroethylene to dichloroacetylene, halothane to 2-bromo-2-chloro-1,1-difluoroethylene, sevoflurane to 2-(fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (Compound A), and desflurane, isoflurane, and enflurane to carbon monoxide. Dichloroacetylene, 2-bromo-2-chloro-1,1-difluoroethylene, and Compound A form glutathione S-conjugates that undergo hydrolysis to cysteine S-conjugates and bioactivation of the cysteine S-conjugates by renal cysteine conjugate beta-lyase to give nephrotoxic metabolites. The elucidation of the mechanisms of formation and bioactivation of degradation products has allowed for the safe use of anesthetics that may undergo degradation in the anesthesia circuit.

Chemical-induced nephrotoxicity mediated by glutathione S-conjugate formation

Glutathione conjugation has been identified as an important detoxication reaction. However, several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions is discussed in this article. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl) glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed and interact with nucleic acids. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cystein conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is likely to be responsible for the observed nephrotoxicity of the parent compounds. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they cause alkylation or redox cycling reactions, or both, that cause organ-selective damage.

Sudden infant death syndrome: a proposed discovery

Sudden cardiac death is a leading cause of fatality in the industrially developed world. Sudden infant death syndrome, has not hitherto been regarded as the same disease. However, the 55% reduction in the recorded rate of death from sudden infant death syndrome following the removal of stress-related problems caused by babies sleeping on their stomachs and overheating from tight and heavy clothing has, I propose, revealed that the babies may be dying from a similar stress-related cause, which can be prevented.

Dichloroacetylene: effects on the rat trigeminal nerve somatosensory evoked potential

Humans overexposed to trichloroethylene (TCE), under specific conditions, were reported to develop trigeminal nerve dysfunction. A degradation byproduct dichloroacetylene (DCA), however, has been suggested as the probable neurotoxicant rather than TCE. Studies in mice, rats, and rabbits support the hypothesis of DCA-induced trigeminal neurotoxicity. This study, therefore, was conducted to characterize DCA-induced trigeminal nerve dysfunction in rats using the electrodiagnostic procedure trigeminal nerve-stimulated somatosensory evoked potential (TSEP). A group of six rats was exposed once to DCA (approximately 300 ppm) or room air for 2.25 h and a separate group of six rats was not exposed and served as controls. Trigeminal nerve somatosensory evoked potentials (TSEPs) were collected before exposure and 2, 4, and 7 days postexposure. Because DCA was manufactured from TCE with acetylene added as a stabilizer, another group of rats was exposed to TCE and acetylene without generation of DCA. TSEPs from DCA-exposed rats were smaller and slower compared to their baseline recordings and to the concurrent negative controls. TSEPs from the controls and the TCE/acetylene-exposed rats were unchanged. Neuropathology did not reveal treatment-related lesions. It was concluded that the rat is mildly to markedly susceptible to DCA-induced trigeminal nerve dysfunction as assessed by TSEP, but that the kidney was the likely target organ based on gross observations and the DCA literature.

Biotransformation and membrane transport in nephrotoxicity

The kidney is a frequent target organ for toxic effects of xenobiotics. In recent years, the molecular mechanisms responsible for the selective renal toxicity of many nephrotoxic xenobiotics have been elucidated. Accumulation by renal transport mechanisms, and thus aspects of renal physiology, plays an important role in the renal toxicity of some antibiotics, metals, and agents binding to low molecular weight proteins such as alpha(2u)-globulin. The accumulation by active transport of metabolites formed in other organs is involved in the kidney-specific toxicity of certain polyhaloalkanes, polyhaloalkenes, hydroquinones, and aminophenols. Other xenobiotics are selectively metabolized to reactive electrophiles by enzymes expressed in the kidney. This review summarizes the present knowledge on the mechanistic basis of target organ selectivity of these compounds.

Biotransformation and renal processing of nephrotoxic agents

Nephrotoxicity is often observed as an endpoint in animal toxicity studies. In recent years, the mechanisms of biotransformation, which often provide the basis for renal toxicity, have been elucidated for a variety of compounds. These studies showed that nephrotoxicity of chemicals is either due to accumulation of certain metabolites in the kidney and further bioactivation or due to intrarenal bioactivation of the parent xenobiotic. Both types of mechanisms will be discussed using two relevant samples. The polychlorinated olefin hexachlorobutadiene and other haloolefins cause necrosis of the S-3 segment of the proximal tubules; their nephrotoxicity is dependent on bioactivation reactions. In the liver, hexachlorobutadiene is transformed by conjugation with glutathione to (S-pentachlorobutadienyl)glutathione. This S-conjugate is processed by the enzymes of mercapturic acid formation to give N-acetyl-(S-pentachlorobutadienyl)-L-cysteine, which is accumulated in the proximal tubule cells and deacetylated there to give (S-pentachlorobutadienyl)-L-cysteine. Further bioactivation is catalyzed by renal cysteine conjugate beta-lyase. Both the renal accumulation by the organic anion transporter and the topographical distribution of cysteine conjugate beta-lyase along the nephron are major determinants of organ and cell selectivity. Vinylidene chloride (VDC) is nephrotoxic in mice after inhalation, but not after oral or intraperitoneal administration. The nephrotoxicity of VDC is due to the selective expression of an androgen-dependent cytochrome P450 in the proximal tubules of male mice. This enzyme oxidizes VDC to an electrophile and is not present in female mice, but can be induced be androgen treatment. The observation of nephrotoxicity of VDC after inhalation only is due to the high blood flow to the kidney and thus high concentrations of VDC delivered to the kidney after inhalation. After oral or intraperitoneal application, hepatic first-pass metabolism efficiently reduces the amount of VDC delivered to the kidney. The results demonstrated here demonstrate that prior to in vitro nephrotoxicity screening, toxicokinetics and biotransformation pathways for a chemical have to be elucidated and metabolites have to be included into the testing regimen.

Glutathione-dependent bioactivation of xenobiotics

Glutathione conjugation has been identified as an important detoxication reaction. However, in recent years several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions are discussed. 1. Dichloromethane is metabolized by glutathione conjugation to formaldehyde via S-(chloromethyl)glutathione. Both compounds are reactive intermediates and may be responsible for the dichloromethane-induced tumorigenesis in sensitive species. 2. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl)glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed. 3. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cysteine conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is responsible for the observed toxicity of the parent compounds. 4. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they undergo alkylation or redox cycling reactions, or both, that cause organ-selective damage.

Metabolism of 14C-dichloroethyne in rats

1. The metabolism of 14C-dichloroethyne was studied in rats by inhalation in a dynamic nose-only exposure system. 14C-Dichloroethyne was generated in 95-99% yield from 14C-trichloroethene by alkaline dehydrochlorination. 2. After inhalation of 20 ppm and 40 ppm dichloroethyne for 1 h, the retention rates were 17.6% and 15.6% of the radioactivity introduced into the exposure system, respectively. During the period of observation (96 h), almost quantitative elimination of the dose was observed. Elimination with urine accounted for 60.0% (40 ppm) and 67.8% (20 ppm) of absorbed radioactivity and elimination with faeces for 27% (40 ppm) and 27.7% (20 ppm), 3.4-3.5% remained in the carcasses. 3. Metabolites of dichloroethyne identified are: N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine, dichloroethanol, dichloroacetic acid, oxalic acid and chloroacetic acid in urine; N-acetyl-S-(1,2-dichlorovinyl-L-cysteine in faeces. 4. In bile of rats exposed to 40 ppm of dichloroethyne, S-(1,2-dichlorovinyl)glutathione was the only metabolite identified. Biliary cannulation did not influence the renal excretion of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine, indicating that glutathione conjugate formation occurs in the kidney. 5. The results suggest that two metabolic pathways are operative in dichloroethyne metabolism in vivo. Cytochrome P450-dependent oxidation represents a minor pathway accounting for the formation of 1,1-dichloro compounds after chlorine migration. The major pathway is the biosynthesis of toxic glutathione conjugates. Organ-specific toxicity and carcinogenicity of dichloroethyne is due most likely to the topographical distribution of gamma-glutamyl transpeptidase which is concentrated mainly in the kidney in rats.

Haemolytic activity and nephrotoxicity of 2-hydroxy-1,4-naphthoquinone in rats

The short-term toxicity of 2-hydroxy-1,4-naphthoquinone (lawsone) and 2-methyl-1,4-naphthoquinone (menadione) has been compared in rats. 2-Methyl-1,4-naphthoquinone has been shown previously to cause haemolytic anaemia in animals, and this was confirmed in the present experiment. 2-Hydroxyl-1,4-naphthoquinone was found also to cause haemolysis, in a dose-dependent manner, as reflected by decreased blood packed cell volumes and haemoglobin levels and by histopathological changes in spleen, liver and kidney. With both naphthoquinones, the haemolysis was of the oxidative type, characterized by the presence of Heinz bodies within erythrocytes. Haemolysis was the only toxic change identified in rats dosed with 2-methyl-1,4-naphthoquinone. In contrast, 2-hydroxyl-1,4-naphthoquinone was not only a haemolytic agent but also a nephrotoxin, causing renal enlargement, elevated plasma levels of urea and creatinine and histologically-identified tubular necrosis, largely confined to the distal segment of the proximal convoluted tubules. The relationship between the in vivo toxic effects of these naphthoquinones and previously-reported data on their in vitro cytotoxic action is discussed.

Bioactivation of xenobiotics by formation of toxic glutathione conjugates

Evidence has been accumulating that several classes of compounds are converted by glutathione conjugate formation to toxic metabolites. The aim of this review is to summarize the current knowledge on the biosynthesis and toxicity of glutathione S-conjugates derived from halogenated alkanes, halogenated alkenes, and hydroquinones and quinones. Different types of toxic glutathione conjugates have been identified and will be discussed in detail: (i) conjugates which are transformed to electrophilic sulfur mustards, (ii) conjugates which are converted to toxic metabolites in an enzyme-catalyzed multistep mechanism, (iii) conjugates which serve as a transport form for toxic quinones and (iv) reversible glutathione conjugate formation and release of the toxic agent in cell types with lower glutathione concentrations. The kidney is the main, with some compounds the exclusive, target organ for compounds metabolized by pathways (i) to (iii). Selective toxicity to the kidney is easily explained due to the capability of the kidney to accumulate intermediates formed by processing of S-conjugates and to bioactivate these intermediates to toxic metabolites. The influences of other factors participating in the renal susceptibility are discussed.

Influence of aging on chemically induced hepatotoxicity: role of age-related changes in metabolism

The effects on hepatotoxicity of age-associated changes in drug metabolism are not always straightforward. In the case of allyl alcohol hepatotoxicity in male rats, there is a good relationship between increased metabolic activation by liver alcohol dehydrogenase and enhanced hepatotoxicity in old age. With regard to two other hepatotoxicants, some tentative conclusions about the role of metabolism can be drawn, but they must be tempered with caution due to gaps in the available information. Acetaminophen-induced hepatotoxicity is reduced in old age, and decreased formation of the toxic intermediate may be the reason. There is a prominent effect of aging on acetaminophen conjugation, a shift from sulfation to glucuronidation, but this change does not affect total clearance. The situation with carbon tetrachloride is difficult to interpret because the final outcome is unaltered hepatotoxicity in old age. Nevertheless, the available data suggest that an age-associated decrease in activation of carbon tetrachloride is counterbalanced by a loss in resistance to lipid peroxidation. These conclusions are summarized in Table 5. Again, it must be emphasized that all of these age-dependent changes in toxicity could be related to effects on other systems that are not necessarily involved in the metabolism of hepatotoxicants. Future research is needed to identify pathways of metabolic activation and detoxification in which age-dependent changes occur that result in significant changes in hepatotoxicity. The entire sequence of events from changes at the molecular level to their sequelae at the level of the cell, tissue and intact animal should be investigated, and the results should be confirmed in more than one mammalian model of aging. The aim would be to identify basic mechanisms that result in increased hazard for the aged liver from exposure to toxic compounds.

Formation of dichloroacetylene from trichloroethylene in the presence of alkaline material--possible cause of intoxication after abundant use of chloroethylene-containing solvents

Inhabitants of a private home suffered from symptoms possibly due to dichloroacetylene intoxication. Subsequent anamnesis revealed that abundant amounts of trichloroethylene had been used to remove a wax coating from a concrete-lined stone floor. This prompted us to examine whether dichloroacetylene could have been formed. Incubation of two commercial samples of trichloroethylene with aqueous alkaline solutions between pH 11 and 13, with mortar and tile filling material resulted in the formation of dichloroacetylene. This finding suggests formation of dichloroacetylene, when trichloroethylene comes into contact with moderately alkaline material, such as moist concrete.

Mutagenicity of dichloroacetylene and its degradation products trichloroacetyl chloride, trichloroacryloyl chloride and hexachlorobutadiene

Dichloroacetylene (DCA) is a highly reactive compound that decomposes rapidly in contact with air into a series of chlorinated aliphatic hydrocarbons (e.g., phosgene, trichloroacetyl chloride, trichloroacryloyl chloride and hexachlorobutadiene). Experiments were performed to compare the mutagenic properties of DCA and its degradation products on the histidine-dependent tester strains TA98 and TA100 of Salmonella typhimurium. In these experiments, DCA vapour was streamed under analytical control through the bacterial suspensions. DCA is soluble in aqueous solution and was stable under the experimental steady-state conditions of the bacterial exposure. There is a linear correlation between the supply of DCA vapour and solubilized DCA in the range of 1000 and 16 000 ppm. Mutagenic response was observed with strain TA100 if the bacteria were suspended in Oxoid medium. No mutagenicity could be detected with strain TA98. DCA mixtures with acetylene, as used as stabilizer for animal experiments, were not mutagenic in either bacterial strain, irrespective of the presence or absence of S9 mix in the cell suspension. One of the degradation products of DCA, trichloroacryloyl chloride, showed pronounced mutagenic properties with and without drug-metabolizing enzymes. Other degradation products of DCA, such as trichloroacetyl chloride and hexachlorobutadiene, were not mutagenic, either in the presence or absence of liver homogenate.

Application and results of whole-body autoradiography in distribution studies of organic solvents

With the growing concern for the health hazards of occupational exposure to toxic substances attention has been focused on the organic solvents, which are associated with both deleterious nervous system effects and specific tissue injuries. Relatively little is known about the distribution of organic solvents and their metabolites in the living organism. Knowledge of the specific tissue localizations and retention of solvents and solvent metabolites is of great value in revealing and understanding the sites and mechanisms of organic solvent toxicity. Whole-body autoradiography has been modified and applied to distribution studies of benzene, toluene, m-xylene, styrene, methylene chloride, chloroform, carbon tetrachloride, trichloroethylene and carbon disulfide. The high volatility of these substances has led to the development of cryo-techniques. Whole-body autoradiographic techniques applicable to the study of volatile substances are reviewed. The localizations of nonvolatile solvent metabolites and firmly bound metabolites have also been examined. The obtained results are discussed in relation to toxic effects and evaluated by comparison with other techniques used in distribution studies of organic solvents and their metabolites.

Neurotoxic effects of dichloroacetylene

In inhalation tests involving lethal as well as sublethal doses of dias examined in rabbits by means ofhistologic and neurofunctional methods. Histologic examination revealed chromatolysis, disintegration of Nissl bodies, and cell shrinkage in proportion to the doses employed in the nuclei of the unpaired brain stem, in the sensory cortical regions and especially in the sensory cranial nerve nuclei. The sensory trigeminal nucleus was involved most severely, followed in decreasing intensity 0y the facial and oculomotor nerves and the motor trigeminal nucleus; the least changes were observed in the acoustic nerve. By testing the protopathic sensitivity of the facial skin it was possible for the first time to produce evidence of a sensory loss in the distribution of the trigeminal nerve by animal experimentation. The neuropathological and functional deficits observed may explain the cranial nerve lesions in human DCA intoxication; however, these lesions seem to be less distinct in experimental animals.