Oxidation of thiodiglycol (2,2'-thiobis-ethanol) by alcohol dehydrogenase: comparison of human isoenzymes

Xenobiotica-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models

Studies on the metabolic fate of medical drugs, skin care products, cosmetics and other chemicals intentionally or accidently applied to the human skin have become increasingly important in order to ascertain pharmacological effectiveness and to avoid toxicities. The use of freshly excised human skin for experimental investigations meets with ethical and practical limitations. Hence information on xenobiotic-metabolizing enzymes (XME) in the experimental systems available for pertinent studies compared with native human skin has become crucial. This review collects available information of which—taken with great caution because of the still very limited data—the most salient points are: in the skin of all animal species and skin-derived in vitro systems considered in this review cytochrome P450 (CYP)-dependent monooxygenase activities (largely responsible for initiating xenobiotica metabolism in the organ which provides most of the xenobiotica metabolism of the mammalian organism, the liver) are very low to undetectable. Quite likely other oxidative enzymes [e.g. flavin monooxygenase, COX (cooxidation by prostaglandin synthase)] will turn out to be much more important for the oxidative xenobiotic metabolism in the skin. Moreover, conjugating enzyme activities such as glutathione transferases and glucuronosyltransferases are much higher than the oxidative CYP activities. Since these conjugating enzymes are predominantly detoxifying, the skin appears to be predominantly protected against CYP-generated reactive metabolites. The following recommendations for the use of experimental animal species or human skin in vitro models may tentatively be derived from the information available to date: for dermal absorption and for skin irritation esterase activity is of special importance which in pig skin, some human cell lines and reconstructed skin models appears reasonably close to native human skin. With respect to genotoxicity and sensitization reactive-metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the Conclusions section in the end of this review.

Toxicity Assessment of Thiodiglycol

Sulfur mustard (HD) undergoes hydrolysis to form various products such as thiodiglycol (TG) in biological and environmental systems. TG is a precursor in the production of HD and it is also considered as a “Schedule 2” compound (dual-use chemicals with low to moderate commercial use and high-risk precursors). Several toxicological studies on TG were conducted to assess environmental and health effects. The oral LD50 values were >5000 mg/kg in rats. It was a mild skin and moderate ocular irritant and was not a skin sensitizer in animals. It was not mutagenic in Ames Salmonella, Escherichia coli, mouse lymphoma, and in vivo mouse micronucleus assays, but it induced chromosomal aberrations in Chinese hamster ovarian (CHO) cells. A 90-day oral subchronic toxicity study with neat TG at doses of 0, 50, 500, and 5000 mg/kg/day (5 days/week) in Sprague-Dawley rats results show that there are no treatment-related changes in food consumption, hematology, and clinical chemistry in rats of either sex. The body weights of both sexes were significantly lower than controls at 5000 mg/kg/day. Significant changes were also noted in both sexes in absolute weights of kidneys, kidney to body weight ratios, and kidney to brain weight ratios, in the high-dose group. The no-observed-adverse-effect level (NOAEL) for oral toxicity was 500 mg/kg/day. The developmental toxicity conducted at 0, 430, 1290, and 3870 mg/kg by oral gavage showed maternal toxicity in dams receiving 3870 mg/kg. TG was not a developmental toxicant. The NOAEL for the developmental toxicity in rats was 1290 mg/kg. The provisional oral reference dose (RfD) of 0.4 mg/kg/day was calculated for health risk assessments. The fate of TG in the environment and soil showed biological formation of thiodiglycalic acid with formation of an intermediate ((2-hydroxyethyl)thio)acetic acid. It was slowly biodegraded under anaerobic conditions. It was not toxic to bluegill sunfish at 1000 mg/L and its metabolism and environmental and biochemical effects are summarized.

Xenobiotic-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models

The exposure of the skin to medical drugs, skin care products, cosmetics, and other chemicals renders information on xenobiotic-metabolizing enzymes (XME) in the skin highly interesting. Since the use of freshly excised human skin for experimental investigations meets with ethical and practical limitations, information on XME in models comes in the focus including non-human mammalian species and in vitro skin models. This review attempts to summarize the information available in the open scientific literature on XME in the skin of human, rat, mouse, guinea pig, and pig as well as human primary skin cells, human cell lines, and reconstructed human skin models. The most salient outcome is that much more research on cutaneous XME is needed for solid metabolism-dependent efficacy and safety predictions, and the cutaneous metabolism comparisons have to be viewed with caution. Keeping this fully in mind at least with respect to some cutaneous XME, some models may tentatively be considered to approximate reasonable closeness to human skin. For dermal absorption and for skin irritation among many contributing XME, esterase activity is of special importance, which in pig skin, some human cell lines, and reconstructed skin models appears reasonably close to human skin. With respect to genotoxicity and sensitization, activating XME are not yet judgeable, but reactive metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the “Overview and Conclusions” section in the end of this review.

Human Metabolism and Metabolic Interactions of Deployment-Related Chemicals

It has been suggested that chemicals and, more specifically, chemical interactions, are involved as causative agents in deployment-related illnesses. Unfortunately, this hypothesis has proven difficult to test, because toxicological investigations of deployment-related chemicals are usually carried out on surrogate animals and are difficult to extrapolate to humans. Other parts of the problem, such as the definition of variation within human populations and the development of methods for designating groups or individuals at significantly greater risk, cannot be carried out on surrogate animals, and the data must be derived from humans. The relatively recent availability of human cell.fractions, such as microsomes, cytosol, etc., human cells such as primary hepatocytes, recombinant human enzymes, and their isoforms and polymorphic variants has enabled a significant start to be made in developing the human data needed. These initial studies have examined the human metabolism by cytochrome P450, other phase I enzymes, and their isoforms and, in some cases, their polymorphic variants of compounds such as chlorpyrifos, carbaryl, DEET, permethrin, and pyridostigmine bromide, and, to a lesser extent, other chemicals from the same chemical and use classes, including solvents, jet fuel components, and sulfur mustard metabolites. A number of interactions at the metabolic level have been described both with respect to other xenobiotics and to endogenous metabolites. Probably the most dramatic have been seen in the ability of chlorpyrifos to inhibit not only the metabolism of other xenobiotics such as carbaryl and DEET but also to inhibit the metabolism of steroid hormones.

Drug-metabolizing enzymes in the skin of man, rat, and pig

The mammalian skin has long been considered to be poor in drug metabolism. However, many reports clearly show that most drug metabolizing enzymes also occur in the mammalian skin albeit at relatively low specific activities. This review summarizes the current state of knowledge on drug metabolizing enzymes in the skin of human, rat, and pig, the latter, because it is often taken as a model for human skin on grounds of anatomical similarities. However only little is known about drug metabolizing enzymes in pig skin. Interestingly, some cytochromes P450 (CYP) have been observed in the rat skin which are not expressed in the rat liver, such as CYP 2B12 and CYP2D4. As far as investigated most drug metabolizing enzymes occur in the suprabasal (i.e. differentiating) layers of the epidermis, but the rat CYP1A1 rather in the basal layer and human UDP-glucuronosyltransferase rather in the stratum corneum. The pattern of drug metabolizing enzymes and their localization will impact not only the beneficial as well as detrimental properties of drugs for the skin but also dictate whether a drug reaches the blood flow unchanged or as activated or inactivated metabolite(s).

Thiodiglycol, the hydrolysis product of sulfur mustard: Analysis of in vitro biotransformation by mammalian alcohol dehydrogenases using nuclear magnetic resonance

Thiodiglycol (2,2'-bis-hydroxyethylsulfide, TDG), the hydrolysis product of the chemical warfare agent sulfur mustard, has been implicated in the toxicity of sulfur mustard through the inhibition of protein phosphatases in mouse liver cytosol. The absence of any inhibitory activity when TDG was present in assays of pure enzymes, however, led us to investigate the possibility for metabolic activation of TDG to inhibitory compound(s) by cytosolic enzymes. We have successfully shown that mammalian alcohol dehydrogenases (ADH) rapidly oxidize TDG in vitro, but the classic spectrophotometric techniques for following this reaction provided no information on the identity of TDG intermediates and products. The use of proton NMR to monitor the oxidative reaction with structural confirmation by independent synthesis allowed us to establish the ultimate product, 2-hydroxyethylthioacetic acid, and to identify an intermediate equilibrium mixture consisting of 2-hydroxyethylthioacetaldehyde, 2-hydroxyethylthioacetaldehyde hydrate and the cyclic 1,4-oxathian-2-ol. The intermediate nature of this mixture was determined spectrophotometrically when it was shown to drive the production of NADH when added to ADH and NAD.

NMR analysis of thiodiglycol oxidation by Mammalian alcohol dehydrogenases

Nuclear magnetic resonance spectroscopy (NMR) is a powerful technique for elucidating the metabolism of xenobiotics, as it allows for the least ambiguous assignment of chemical structure when compared to other forms of spectroscopy. In addition, it is a sensitive technique that can reveal the presence of transient species that otherwise would not be detected by utilizing either large-scale batch processes or other forms of spectroscopic analyses. The primary focus of this unit describes the use of NMR to identify metabolites arising from the oxidation of thiodiglycol by equine and human variants of alcohol dehydrogenase (ADH). Given that it is often risky to base metabolism studies on a single form of spectroscopy, a spectrophotometric method is also presented. In addition, incorporation of independent organic syntheses in conjunction with the spectroscopic studies to further solidify structural identification of the ADH metabolites is presented.

The metabolic activation of abacavir by human liver cytosol and expressed human alcohol dehydrogenase isozymes

Abacavir (ZIAGEN) is a reverse transcriptase inhibitor marketed for the treatment of HIV-1 infection. A small percentage of patients experience a hypersensitivity reaction indicating immune system involvement and bioactivation. A major route of metabolism for abacavir is oxidation of a primary betagamma unsaturated alcohol to a carboxylic acid via an aldehyde intermediate. This process was shown to be mediated in vitro by human cytosol and NAD, and subsequently the alphaalpha and gamma2gamma2 human isoforms of alcohol dehydrogenase (ADH). The alphaalpha isoform effected two sequential oxidation steps to form the acid metabolite and two isomers, qualitatively reflective of in vitro cytosolic profiles. The gamma2gamma2 isozyme generated primarily an isomer of abacavir, which was minor in the alphaalpha profiles. The aldehyde intermediate could be trapped in incubations with both isozymes as an oxime derivative. These metabolites can be rationalized as arising via the aldehyde which undergoes isomerization and further oxidation by the alphaalpha enzyme or reduction by the gamma2gamma2 isozyme. Non-extractable abacavir protein residues were generated in cytosol, and with alphaalpha and gamma2gamma2 incubations in the presence of human serum albumin (HSA). Metabolism and residue formation were blocked by the ADH inhibitor 4-methyl pyrazole (4-MP). The residues generated by the alphaalpha and gamma2gamma2 incubations were analyzed by SDS-PAGE with immunochemical detection. The binding of rabbit anti-abacavir antibody to abacavir-HSA was shown to be dependent on metabolism (i.e. NAD-dependent and 4-MP sensitive). The mechanism of covalent binding remains to be established, but significantly less abacavir-protein residue was detected with an analog of abacavir in which the double bond was removed, suggestive of a double bond migration and 1,4 addition process.