32P-postlabeling of 7-methylguanine adducts in DNA

The 32P-postlabeling technique introduced by Randerath and co-workers has been particularly successful with stable and nonpolar DNA adducts, but the assay has not been used to any large extent to detect 7-alkylguanine derivatives. In the present communication, we have investigated the phosphorylation reaction by T4 polynucleotide kinase using 7-methyl-3'-dGMP, ring-opened 7-methyl-3'-dGMP and enzyme-digested methylated DNA as substrates. The methylated substrates were detected at femtomol (fmol) sensitivities. 7-methyl-3'-dGMP was quantitatively phosphorylated at these low concentrations. The efficiency of phosphorylation of the ring-opened product was less. It was shown that ring-opened 7-methyl-3'-dGMP was resistant to digestion with nuclease P1, making alkali-treatment and enzyme digestion of DNA possible approaches to the determination of 7-methylguanine in DNA.