ABSTRACT
Several approaches were used to probe aspects of the mechanism of acrylonitrile carcinogenicity in rats. Acrylonitrile did not appear to increase the rate of DNA synthesis resulting from tissue injury and regeneration in liver or brain, the latter being a target organ. The chemical did cause unscheduled DNA synthesis in rat liver but not brain. The epoxide of acrylonitrile, 2-cyanoethylene oxide, was formed in perfused rat liver; this metabolite accumulated in the perfusate as long as acrylonitrile was available to the organ. When 2-cyano[2,3-14C]ethylene oxide was administered to rats i.p., covalent binding to both liver and brain protein was found, but no covalent binding to nucleic acids could be detected at the level of 0.3 alkylations per 10(6) bases. No 1,N6-ethenoadenosine or 1,N6-ethenodeoxyadenosine was found in liver nucleic acids after administration of either acrylonitrile or 2-cyanoethylene oxide to rats, with the limits of detection being 0.3 alkylations per 10(6) RNA bases and 1 alkylation per 10(6) DNA bases. However, low levels of N7-(2-oxoethyl)guanine were detected in the livers of these rats by means of a radiometric derivative assay (0.014-0.032 alkylations per 10(6) DNA bases). In the brains of the treated rats the levels of N7-(2-oxoethyl)guanine were not above the limit of detection. These results show that acrylonitrile has some limited potential for genotoxicity in vivo and that the epoxide, with its ability to leave the liver and possibly to enter the brain, can interact with nucleic acids to a limited degree.
Subject(s)
Acrylonitrile/metabolism , DNA/metabolism , Ethylene Oxide/analogs & derivatives , Nitriles/metabolism , Acrylonitrile/toxicity , Alkylation , Animals , DNA Repair , DNA Replication/drug effects , Ethylene Oxide/metabolism , Liver/metabolism , Male , Mutagens , Perfusion , RNA/metabolism , Rats , Rats, Inbred F344ABSTRACT
The roles of various metabolic pathways in DNA and protein alkylation are discussed here. Simple vinyl halides are oxidized to 2-haloethylene oxides and 2-haloacetaldehydes, which alkylate DNA and proteins, respectively. Polysubstituted vinyl halides are oxidized with group transfer to yield halocarbonyl compounds which alkylate proteins. Oxidation of vic-dihaloalkanes results in protein alkylation while glutathione conjugates alkylate DNA. Acrylonitrile, without previous activation, alkylates proteins and glutathione. Oxidation of acrylonitrile yields a relatively stable epoxide which can react with DNA in vitro, but alkylation by this epoxide does not occur readily in vivo. Hard-soft acid-base theory is of some use in understanding why some adducts are formed in preference to others.
Subject(s)
Acrylonitrile/metabolism , DNA Damage , Hydrocarbons, Halogenated/metabolism , Nitriles/metabolism , Vinyl Compounds/metabolism , Acrylonitrile/toxicity , Adenosine/analogs & derivatives , Animals , Biotransformation , Cysteine/metabolism , Cytidine/analogs & derivatives , DNA/drug effects , Glutathione/metabolism , Hydrocarbons, Halogenated/toxicity , Microsomes, Liver/metabolism , Vinyl Compounds/toxicityABSTRACT
Several of the pathways of metabolism of the suspected carcinogen acrylonitrile (AN) were identified previously in this laboratory with the use of subcellular fractions and purified enzymes (Guengerich, F.P., Geiger, L.E., Hogy, L.L., and Wright, P.L., Cancer Res., 41: 4925-4933, 1981). In order to establish the relative contributions of the various pathways leading to activated and detoxicated products, we examined AN metabolism in isolated Fischer 344 rat hepatocytes as a model. Reduced glutathione (GSH) was depleted, and cell viability was lost in an AN concentration-dependent manner. The major GSH adduct formed at all AN concentrations was identified as S-(2-cyanoethyl)GSH using thin-layer and high-performance liquid chromatography. Acid hydrolysis and amino acid analysis of labeled hepatocellular, protein revealed S-(2-carboxyethyl)-cysteine as the major adduct formed, indicating direct alkylation of cysteinyl residues by AN. 2-Cyanoethylene oxide accumulated in the hepatocyte incubations but did not appear to contribute extensively to alkylation of GSH or protein. Cyanide, resulting from hydrolysis of 2-cyanoethylene oxide, appeared to be completely converted to thiocyanate, which was identified by gel exclusion chromatography and mass spectrometry of the methyl derivative. The concentration of thiocyanate formed was directly proportional to the concentration of AN used. Cyanide does not appear to play a role in AN-mediated cell death. Alkylation of hepatocellular DNA and RNA and extracellular DNA was not observed to an extent greater than one base in 3.5 X 10(5). The relative rates of the various pathways were compared, and more than 97% of the metabolites can be accounted for by the described reactions. These results are of use in evaluating the contribution of the various pathways and modes of binding of AN to toxicity and carcinogenicity in liver and extrahepatic target tissues.
