CHEMICAL ANALYSIS OF DNA ALTERATIONS .4. REACTIONS OF OLIGODEOXYNUCLEOTIDES WITH MONOFUNCTIONAL ALKYLATING AGENTS LEADING TO BACKBONE BREAKAGE

The mechanisms leading to inactivating DNA alterations by the monofunctional alkylating agents methyl and ethyl methane sulfonate have been studied using oligodeoxythymidylic and oligodeoxyadenylic acids of different chain length as well as mononucleotides and their components. Thymine did not react and deoxythymidylic acid was alkylated exclusively at the phosphate group, about twice as fast as guanine at the 7-position. The alkylation of oligodeoxythymidylic acids produced products, separated by chromatography, which proved the existence of triester breakage. During alkylation of dGMP and dAMP, the amount of free alkylated guanine and adenine both increased with the square of time, which shows a two-step reaction of first alkylation and then depurination. The identification of certain reaction products (dAMP) of oligodeoxyadenylic acid furthermore proved the existence of a small but significant amount of backbone breakage which accompanied alkylation at neutral pH. A quantitative evaluation of the results concluded that triester breakage and depurination of DNA should occur with about equal frequency, both increasing with the square of the time during treatment and linearly afterwards. In contrast, backbone breakage caused by depurination should increase with the third power of time during treatment and with the square of the time afterwards. The results are significant for the explanation of the effects of deoxyribonucleases or repair mechanisms on alkylated DNA. © 1969.