Alkylation of 2'-deoxythymidylyl(3'----5')-2'-deoxythymidine with 2-hydroxyethylnitrosourea and stability of the resulting phosphotriester.

One of the main targets of RNA and DNA alkylation is the phosphate group. In contrast to RNA phosphotriesters, DNA phosphotriesters are relatively stable. Introduction of 2-hydroxyethyl phosphotriesters into DNA, however, has been reported to decrease its stability towards hydrolytic cleavage considerably. 2-Chloroethylnitrosoureas (CNUs) have been found to form predominantly 2-hydroxyethyl adducts of DNA. In the present study, we have determined the stability of 2'-deoxythymidylyl-(3'----5')-2'-deoxythymidine (dTpdT) after alkylation with various nitrosoureas, including N-methyl-N-nitrosourea (MNU) and 2-hydroxyethylnitrosourea (HENU) at alkaline and neutral pH (at 37 degrees C). The half-lives (t1/2) at pH 12.5 were, for example, 2.8 h for di(2'-deoxythymidine)methylphosphotriester [dTp(me)dT] but less than 1 min for di(2'-deoxythymidine) (2-hydroxyethyl)-phosphotriester [dTp(he)dT]. At pH 7.0, dTp(me)dT was stable for more than three days, whereas the t1/2 for dTp(he)dT was only 1.0 h at pH 7.0 (27 min at pH 9.0). The marked lability of dTp(he)dT, in comparison to other phosphotriester analogues, can be explained by intermediate formation of a dioxaphospholane ring resulting in triester bond breakage. Our data strongly support the hypothesis that a great proportion of DNA single-strand breaks induced in DNA by HENU or CNUs can be attributed to intermediate formation of 2-hydroxyethyl phosphotriesters.