Mutation spectrum and sequence alkylation selectivity resulting from modification of bacteriophage M13mp18 DNA with S-(2-chloroethyl)glutathione: Evidence for a role of S-(2-(N⁷-guanyl)ethyl)glutathione as a mutagenic lesion formed from ethylene dibromide

The major DNA adduct (>95% total) resulting from the bioactivation of ethylene dibromide by conjugation with GSH is S-(2-(N⁷-guanyl)ethyl)GSH. The mutagenic potential of this adduct has been uncertain, however, because the observed mutagenicity might be caused by other adducts present at much lower levels, e.g. S-(2-(N^-1-adenyl)ethyl)GSH. To assess the formation of other potential adducts, S-(2-(N³-deoxycytidyl)ethyl)GSH, S-(2-(O⁶-deoxyguanosyl)ethyl)GSH, and S-(2-(N²-deoxyguanosyl)ethyl)GSH were prepared and used as standards in the analysis of calf thymus DNA modified by treatment with [1,2-¹⁴C]ethylene dibromide and GSH in the presence of rat liver cytosol; only minor amounts (<0.2%) were found. A forward mutation assay in (repair-deficient) Salmonella typhimurium TA100 and sequence analysis were utilized to determine the type, site, and frequency of mutations in a portion of the lacZ gene resulting from in vitro modification of bacteriophage M13mp18 DNA with S-(2-chloroethyl)GSH, an analog of the ethylene dibromide-GSH conjugate. An adduct level of ∼8 nmol (mg DNA)^-1 resulted in a 10-fold increase in mutation frequency relative to the spontaneous level. The spectrum of spontaneous mutations was quite varied, but the spectrum of S-(2-chloroethyl)GSH-induced mutations consisted primarily of base substitutions, of which G:C to A:T transitions accounted for 75% (70% of the total mutations). All available evidence implicates S-(2-(N⁷-guanyl)ethyl)GSH as the cause of these mutations inasmuch as the levels of the minor adducts are not consistent with the mutation frequency observed in this system. The sequence selectivity of alkylation was determined by treatment of end-labeled lac DNA fragments with S-(2-chloroethyl)GSH, cleavage of the DNA at adduct sites, and electrophoretic analysis. Comparison of the sequence selectivity with the mutation spectrum revealed no obligate relationship between the extent of adduct formation and the number of mutations which resulted at different sites. We suggest that the mechanism of mutagenesis involves DNA sequence-dependent alterations in the interaction of the polymerase with the (modified) template and incoming nucleotide.