Cross-linking of DNA induced by chloroethyinitrosourea is prevented by O6-methyiguanine-DNA methyttransferase

Abstract

The DNA repair enzyme 0⁶-methylguanine-DNA methyltransferase has been used as a reagent to analyse the initial reaction sites of alkylatlng agents such as chloroethy In itrosourea that cross-link DNA. The transferase can be employed for this purpose because it removes substituted ethyl groups from DNA, as shown by its ability to act on 0⁶-hydroxyethylguanine residues In DNA. The enzyme counteracts the formation of interstrand cross-links Induced by bls-chloroethylnltrosourea, but not those induced by nitrogen mustard. Once formed, chloroethyl-nitrosourea-induced cross-links are not broken by the enzyme. In agreement with deductions from experiments with living cells, it is concluded that chloroethylnitrosoureas act by forming reactive monoadducts at the 0⁶ position of guanine and/or the 0⁴ position of thymine, which subsequently generate -CH₂CH₂- bridges to the complementary DNA strand. A new method for quantitatlng Interstrand cross-links in DNA has been employed.