The possibility that glutathione (GSH) S-transferases may affect microsome-mediated methylation of DNA by dimethylnitrosamine (DMN) in vitro has been investigated using aflatoxin B1 (AFB1) as a positive control. Hamster liver microsomes were incubated with either [14C]DMN or [3H]AFB1 and calf thymus DNA, with or without GSH and hamster cytosol. Although a significant amount of DMN was metabolized, GSH alone or in conjunction with cytosol or purified GSH S-transferases did not affect the binding of 14C to DNA and the amount of 7-methylguanine formed. However with AFB1, a significant reduction in both its binding to DNA and in the formation of AFB1-N7Gua adduct with a concomitant increase in AFB1-GSH conjugation was observed, suggesting that the test system was functioning effectively.

Effects of catechin, a plant phenolic flavonoid, and of the commonly used organic solvents dimethyl sulfoxide (DMSO) and ethanol (EtOH) on the microsome-mediated metabolism of two hepatocarcinogens, N-nitrosodimethylamine (NDMA) and aflatoxin B1 (AFB1), are presented. Using hamster liver microsomes as a source of mixed-function oxidases, it was shown that catechin at 0.1-0.2 mM levels had no effect on the oxidation of either carcinogen. However, at 1-5 mM levels it caused a concentration-dependent inhibition (38-70%) of the formation of formaldehyde from NDMA, and at the 5 mM level it caused a 40% inhibition of AFB1-DNA binding. DMSO and EtOH totally inhibited NDMA demethylase activity but had little effect on the binding of AFB1 to DNA. These observations indicate that the mixed-function oxidases (cytochrome P450) essential for the metabolic activation of these carcinogens exhibit different sensitivities to different inhibitors.

Rat and hamster liver cytosolic glutathione (GSH) S-transferases purified by GSH-affinity chromatography have been examined for their effects on the microsome mediated binding of aflatoxin B1 (AFB1) to DNA and on the conjugation of AFB1 -2,3-epoxide with GSH. Like previous studies with cytoslic preparations (Raj et al. (1984) Carcinogenesis 5, 879), our present study with purified GSH S-transferases showed 2–3-fold more inhibitory activity of AFB1-DNA binding with hamster than that with the rat. Concomitant with the inhibition of AFB1-DNA binding, increase in AFB1-GSH conjugation occurred. Subunit compositions of GSH S-transferases indicate preponderance of Yb and Ya subunits in the hamster and rat, respectively. The role of GSH S-transferases in modulating AFB1-DNA binding and AFB1 induced hepatocarcinogenesis is discussed.

The mutagenic properties of hydrazine and its mono- and di-methyl derivatives were compared by direct microbial tests with the tryptophan auxotroph Escherichia coli as indicator organism. The methyl- and dimethyl-hydrazine were also tested with metabolic activation both by the Ames plate test and by the host-mediated assay with the histidine auxotroph Salmonella typhimurium tester strains. Only hydrazine and methylhydrazine were mutagenic in direct tests, hydrazine being a far more potent mutagen than methylhydrazine. Neither methylhydrazine nor dimethylhydrazines gave positive results in the Ames tests. In host-mediated assays, symmetrical dimethylhydrazine was clearly mutagenic, whereas methylhydrazine showed marginal mutagenic activity, and unsymmetrical dimethylhydrazine was negative. Evidently the mutagenic actions of different hydrazine derivatives, though these compounds are chemically closely related, depend on different reaction mechanisms.

1. Administration of a large dose (500mg/kg body wt.) of (3)H-labelled l-ethionine to rats resulted in the incorporation of a small amount of radioactivity into the liver DNA. Considerable evidence that this radioactivity was not due to contamination of the isolated DNA with labelled protein, RNA, S-adenosyl-l-ethionine or l-ethionine was obtained. 2. After acidic hydrolysis of the DNA isolated from the livers of rats treated with labelled l-ethionine, virtually all of the radioactivity present in the DNA was found in a fraction with similar chromatographic properties to 7-ethylguanine. 3. Treatment of rats with comparable doses of l-methionine did not lead to the formation of 7-methylguanine in the liver DNA. 4. These results are discussed in relation to the induction of liver tumours by ethionine.

This chapter provides an overview of some of the relevant chemistry (general chemistry, preparation methods, and analytical methods) of the nitroso carcinogenic compounds and discusses the pathological lesions induced by these compounds and their mutagenic activity. Experimental studies on compounds like dimethylnitrosamine showed that they cause liver necrosis in rats, accompanied by hemorrhages into the liver and lungs and frequently an associated hemorrhagic ascites and blood in the lumen of the gut. Acute toxicity of other dialkyl and related nitrosamines cause liver damage, hemorrhagic lung lesions, convulsions, and coma. The neoplastic changes in the body organs (live, kidney, bladder, nose and nasal sinuses, lungs and bronchi, alimentary canal, nervous system, and skin) and the development of the malignant lesion caused by carcinogenic nitroso compounds are also illustrated in the chapter with the help of animal models. The variation in the response of different organs to the carcinogenic nitroso compounds is of interest in relation to the biochemical changes that may be essential for the initiation of a carcinogenic change. Many carcinogenic nitroso compounds are mutagenic. The nitroso mutagens act by the alkylation of the genetic material. The chapter also discusses the metabolism of nitroso carcinogens both in vivo and in vitro, along with their biochemical effects. The induction of cancers by single doses of rapidly eliminated nitroso carcinogens implies an interaction between the carcinogen and/or a product of its decomposition with some component or components of the cells, which must occur within a short time after administration. The nature of the proximate carcinogen and some of its possible interactions with cellular components and some serious carcinogenic hazards caused by the nitroso compounds are also discussed in the chapter.