Covalent binding to proteins as a mechanism of chemical toxicity

Analytical discovery of water disinfection byproducts of toxicological relevance: highlighting halobenzoquinones

Analytical advancement enables discoveries in water research, and challenges in the identification and determination of a wide range of trace level toxic compounds in water drives the development of new analytical platforms and tools. The identification of toxic disinfection byproducts (DBPs) in disinfected drinking water is an excellent example. Water disinfection is necessary to protect the public from waterborne disease. However, an unintentional consequence is the formation of DBPs resulting from reactions of disinfectants with natural organic matter in source water. To date, regulated DBPs do not account for the increased bladder cancer risk estimated in epidemiological studies. The majority of halogenated DBPs remain unidentified and the discovery of unknown DBPs of toxicological relevance continues to be a major focus of current water research. This review will highlight halobenzoquinones as a class of DBPs that serves as an example of analytical development and toxicological studies. We will feature recent trends and gaps in analytical technologies for identification of unknown DBPs and bioassays for evaluation of the toxicological effects of specific DBPs and their mixtures.

Initial characterization of the major mouse cytochrome P450 enzymes involved in the reductive metabolism of the hypoxic cytotoxin 3-amino-1,2,4-benzotriazine-1,4-di-N-oxide (tirapazamine, SR 4233, WIN 59075)

The benzotriazine di-N-oxide SR 4233 (tirapazamine, WIN 59075) is currently in phase I clinical trials as the lead compound in a series of novel and highly selective antitumour hypoxic cytotoxins. Reductive bioactivation is thought to proceed via a one-electron reduced, oxidizing nitroxide radical and also forms the inactive single N-oxide SR 4317 via radical disproportionation or a second one-electron reduction. In mouse liver microsomes reductive metabolism is catalysed predominantly by cytochrome P450 (70%) and cytochrome P450 reductase (30%). The aim of the present study was to examine which cytochrome P450 isozymes may be involved. Reduction of SR 4233 to SR 4317 was monitored by HPLC analysis. Metabolism by microsomes from both control and dexamethasone-induced BALB/c male mice was 70% inhibited by carbon monoxide. The cytochrome P450 inhibitor SKF 525A, following aerobic preincubation, also inhibited SR 4233 reduction by 58%. Reduction was induced 2-3-fold by dexamethasone and was not accountable by increases in cytochrome P450 reductase or DT-diaphorase. The induction data and the greater degree of inhibition of SR 4233 reduction by metyrapone compared to alpha-naphthoflavone suggested a possible involvement of Cyp2b, Cyp2c and Cyp3a cytochrome P450 subfamilies. Both Cyp3a (7.4-fold) and Cyp2b (1.8-fold) type enzymes were shown by western immunoblot analysis to be induced by dexamethasone, the latter correlating more closely with increased SR 4233 reductase activity and also with the 2-fold induction of benzphetamine N-demethylase, a Cyp2b-type enzyme. No inhibition of SR 4233 reduction was seen with erythromycin or cyclosporin A which act as substrates/inhibitors for Cyp3a-type enzymes, but inhibition was seen with p-nitrophenol and tolbutamide which are substrates for Cyp2el- and Cyp2c-type enzymes, respectively (11% and 25% inhibition in induced microsomes). SR 4233 itself inhibited benzphetamine N-demethylase, which is catalysed by Cyp2b-type enzymes but not erythromycin N-demethylase which is catalysed by Cyp3a-type isoforms. Immunoinhibition studies with epitope specific monoclonal antibodies were consistent with the major involvement of phenobarbitone- and steroid-inducible products of the Cyp2b and Cyp2c subfamilies. These forms contributed at least 53% and 26%, respectively, of the cytochrome P450-associated SR 4233 reductase activity in the induced microsomes. The findings support our earlier conclusion that cytochrome P450 is the major SR 4233 reductase in mouse liver and provides leads as to the possible involvement of specific isoforms in human tumours and normal tissues.

An experimental approach to identifying the genotoxic risk from cooked meat mutagens

In order to define the toxicological risk to the human population from the chemical compounds formed during the process of cooking animal meat, which have been described as possessing mutagenic, genotoxic and carcinogenic activities, an extensive study was undertaken of cooked meat extract and two cooked meat mutagens, 2-amino-3-methylimidazo(4,5-f)quinoline (IQ) and 2-amino-3,8-dimethylimidazo(4,5-f)quinoxaline (MeIQx). The study involved toxicokinetics and mouse-tissue distribution studies of the two chemicals, in vitro and in vivo mutagenicity/genotoxicity analyses (i.e. the detection of gene mutations, chromosome aberrations and micronuclei in mouse bone marrow cells, and mouse urine and faeces mutagenicity tests), as well as in vivo protein and DNA binding assays. IQ and MeIQx were found to be positive for the induction of gene mutations in Salmonella typhimurium TA98, but not in Chinese hamster V79 cells; IQ only was found to be positive for the induction of chromosome aberrations in Chinese hamster ovary cells and cultured human lymphocytes. IQ and MeIQx were negative for the induction of micronuclei in mice treated with 40 mg chemical/kg body weight; the lowest effective dose administered to the mice that produced mutagenic urine was 0.4 mg IQ/kg body weight and 0.04 mg MeIQx/kg. A dose of 40 mg IQ/kg, given orally by gavage to mice, produced an excretion of 1-4% of the applied dose in the urine and 0.1-2% of the applied dose in the faeces, when evaluated chemically or mutagenically. The number of DNA adducts in the liver correlated with the dose of IQ or MeIQx administered to the mice. All the data have been used for defining a possible risk estimate to the human population as a consequence of a cooked meat diet.

An experimental approach to identifying the genotoxic risk by cooked meat mutagens

In order to define the toxicological risk for the human population derived from the chemical compounds formed during the process of cooking animal meat, which have been described to possess a mutagenic, genotoxic, and carcinogenic activity, an extensive study has been developed on cooked meat extract and two cooked meat mutagens, IQ and MeIQx. The study has been based on toxicokinetics and mouse tissue distribution of the two chemicals, on in vitro and in vivo mutagenicity/genotoxicity analyses (gene mutation, chromosome aberration, micronuclea in mouse bone marrow cells, mice urine and faeces mutagenicity test), as well as in vivo protein and DNA binding. The two chemicals have been found positive for the induction of gene mutation on Salmonella, but not in V-79 Chinese hamster cells; IQ only has been found positive for the induction of chromosome aberrations on CHO cells and cultured human lymphocytes. IQ and MeIQx were negative for the induction of micronuclea in mice treated with 40 mg/kg of the chemicals; the lowest effective administered dose to the mice which produced mutagenic urine was 0.4 mg/kg of IQ and 0.04 mg/kg of MeIQx. A dose of 40 mg/kg of IQ given by gavage to mice produced an excretion of 1-4% of the applied dose in the urine and 0.1-2% of the applied dose in the faeces, when evaluated chemically or mutagenically. The DNA adducts for the liver were correlated with the dose of the IQ and MeIQx administered to the mice. All the data have been used for defining a possible risk estimate derived to the human population as a consequence of a cooked meat diet.