Effects of thiamine deficiency on hepatic cytochromes P450 and drug-metabolizing enzyme activities

The effect of nutrition on chemical toxicity

The toxicity of chemicals, and of reactive oxygen species (ROS), are both affected by nutrition and diet. Calorific excess (continuous feeding), or deficiency (fasting), may increase production of ROS, which are also formed by interaction of toxic chemicals with cytochromes P450 (CYP2E or futile cycling). Both ROS (GSH reductase and peroxidase) and toxic chemicals (S-transferases) are detoxified by GSH enzymes; ROS are scavenged by a system comprising GSH, ascorbic acid and tocopherols, which may be regenerated by NADPH. Dietary protein is necessary for GSH or enzyme replacement, lipids are required for polyunsaturated fatty acids (PUFAs) and prostanoid biosynthesis, lipotropes and phospholipids for synthesis of endoplasmic reticulum, and folate is needed for dug metabolizing activity. Among required minerals, Se is necessary as the essential component of the antioxidant enzyme, glutathione peroxidase. Other dietary factors considered are the natural toxicants, gossypol, lathyrogens, glucosinolates, and saponins, and toxicants from food spoilage, food intoxication and food processing.

Acute liver failure after recommended doses of acetaminophen in patients with myopathies

OBJECTIVE

To determine the likelihood that recommended doses of acetaminophen are associated with acute liver failure in patients with myopathies.

DESIGN

Retrospective analysis.

SETTING

Level III pediatric intensive care unit.

PATIENTS

Two pediatric patients with myopathies and acute liver failure.

CLINICAL INVESTIGATIONS

We determined acetaminophen protein adduct levels, in combination with a literature review and systematic evaluation of the cases, using the Roussel Uclaf Causality Assessment Method for drug-induced liver injury to assess causality between recommended acetaminophen dosing and acute liver failure in two children with myopathies.

MAIN RESULTS

The serum adduct levels were consistent with the values previously reported in children with acute liver injury following acetaminophen overdose. We found four similar cases of acute liver failure in pediatric and adult patients with myopathies following recommended acetaminophen doses in the literature (n = 3) and personal communication (n = 1). The Roussel Uclaf Causality Assessment Method suggested a probable relationship between acetaminophen use at recommended doses and acute liver failure in our myopathy patients.

CONCLUSION

Our data suggest that some patients with myopathies who are receiving recommended doses of acetaminophen may be at increased risk for the development of toxicity resulting in acute liver failure. More studies are needed to corroborate these findings. In the meantime, we would advise physicians to be alert in these patients while taking acetaminophen, especially when critically ill or postoperative.

Methylglyoxal toxicity in mammals

Although the interest is growing towards glyoxalases and methylglyoxal, their role in metabolism is still an enigma. In this paper, the effects of methylglyoxal in both in vivo and in vitro mammalian systems are reviewed and correlated with its interaction with macromolecules (nucleic acids, proteins). The theories on the role of methylglyoxal and glyoxalases are also discussed. Recently, data obtained have focused attention on the possible role of disturbed methylglyoxal metabolism in the development of diabetic complications and it is hoped that the contribution of methylglyoxal to pathological events can be ascertained in the near future.

Effects of caloric restriction on rodent drug and carcinogen metabolizing enzymes: implications for mutagenesis and cancer

Caloric restriction in rodents results in increased longevity and a decreased rate of spontaneous and chemically induced neoplasia. The low rates of spontaneous neoplasia and other pathologies have made calorically restricted rodents attractive for use in chronic bioassays. However, caloric restriction also alters hepatic drug metabolizing enzyme (DME) expression and so may also alter the biotransformation rates of test chemicals. These alterations in DME expression may be divided into two types: (1) those that are the direct result of caloric restriction itself and are detectable from shortly after the restriction is initiated; (2) those which are the result of pathological conditions that are delayed by caloric restriction. These latter alterations do not usually become apparent until late in the life of the organism. In rats, the largest direct effect of caloric restriction on liver DMEs is an apparent de-differentiation of sex-specific enzyme expression. This includes a 40-70% decrease in cytochrome P450 2C11 (CYP2C11) expression in males and a 20-30% reduction of corticosterone sulfotransferase activity in females. Changes in DME activities that occur late in life in calorically restricted rats include a stimulation of CYP2E1-dependent 4-nitrophenol hydroxylase activity and a delay in the disappearance of male-specific enzyme activities in senescent males. It is probable that altered DME expression is associated with altered metabolic activation of chemical carcinogens. For example the relative expression of hepatic CYP2C11 in ad libitum-fed or calorically restricted rats of different ages is closely correlated with the amount of genetic damage in 2-acetylaminofluorene- or aflatoxin B1-pretreated hepatocytes isolated from rats of the same age and caloric intake. This suggests that altered hepatic drug and carcinogen metabolism in calorically restricted rats can influence the carcinogenicity of test chemicals.

Bioactivation of halogenated hydrocarbons by cytochrome P4502E1

Numerous halogenated hydrocarbons of the alkane, alkene, and alkyne classes are metabolized by P450 enzymes to products that elicit cytotoxic and/or carcinogenic effects. Such halogenated hydrocarbons include anesthetics (e.g., halothane and enflurane) and industrial solvents (e.g., carbon tetrachloride, chloroform, and vinylidine chloride). Formation of reaction intermediates from these compounds occurs via P450-promoted dehalogenation, reduction, or reductive oxygenation, with certain hydrocarbons undergoing all three reaction types. Of the multiple forms of P450 present in liver microsomes, P4502E1 has been identified as the primary catalyst of hydrocarbon bioactivation in animals and, most likely, in humans as well. As hepatic concentrations of this P450 enzyme are highly inducible by ethanol and similar agents, prior exposure to 2E1-inducing compounds can play a pivotal role in halogenated hydrocarbon toxicity. Considering that metabolism governs the cytotoxicity and carcinogenicity of halogenated hydrocarbons, an understanding of the mechanism(s) underlying 2E1 induction in man becomes all the more important.