Formation of O6-ethylthioethylguanine in DNA by reaction with the sulfur mustard, chloroethyl sulfide, and its apparent lack of repair by O6-alkylguanine-DNA alkyltransferase

The reactions of the monofunctional sulfur mustard, chloro-ethylethyl sulfide, with calf thymus DNA have been studied in vitro.In addition to extensive alkylation of the 7 position of guanineand the 3 position of adenine, - 0.1% of the total alkylationis on the O⁶ position of guanine. Since chloroethyl ethyl sulfideis a model for S-adenosylmethionine and for the reactive intermediatesgenerated from ghitathione by certain environmental agents,this result establishes a route by which these compounds couldbe mutagenic. A DNA substrate modified for repair studies hasbeen produced by incubating calf thymus DNA with [¹⁴C]chloroethyl-labelledchloroethyl ethyl sulfide. This DNA contains 6 nmol/mg of alkylatedproducts including 6 pmol/mg O⁶-ethylthioethylguanine. Incubationof this substrate with up to a 20-fold excess of mammalian (O⁶-alkylguanine-DNAalkyltransferase has indicated that O⁶-ethylthioethylguanineis either not a substrate or a poor one for this DNA repairsystem.