Investigation of 6-thiodeoxyguanosine alkylation products and their role in the potentiation of BCNU cytotoxicity.

The principal products of the reaction of 6-thio-2'-deoxyguanosine (6-TdGuo) with dimethylsulfate are S6-methyl-6-thiodeoxyguanosine and 7-methyl-6-thiodeoxyguanosine, identified by ultraviolet and mass spectrometry. To study the reactions of 6-TdGuo in DNA, cells were treated with 6-thioguanine, which is incorporated into DNA during S-phase; DNA was purified from cell lysates and reacted with 3H-methylnitrosourea. In addition to the expected methylated purines (O6-methylguanine, 7-methylguanine and 3-methyladenine), 0.6% of the total product was S6-methyl-6-thioguanine. On the basis of thioguanine content, the formation of S6-methyl-6-thioguanine occurs 70-fold more efficiently than O6-methylguanine, which indicates that 6-thioguanine incorporated into DNA is very susceptible to chemical modification by alkylating agents. The ultraviolet and mass spectra of two of the major products of the reaction between 6-TdGuo and 2-chloroethyl methanesulfonate suggest that the structures are (1,S6-ethano)-6-thiodeoxyguanosine and (S6,7-ethano)-6-thiodeoxyguanosine, which are presumably formed by an internal cyclization reaction that proceeds through a sulfonium ion intermediate. In cells, this intermediate could react with DNA nucleophiles to form both DNA intra- and interstrand cross-links.