32P-postlabeling assay for carcinogen-DNA adducts: nuclease P1-mediated enhancement of its sensitivity and applications.

Exceedingly sensitive assays are required for the detection of DNA adducts formed in humans exposed to low levels of environmental genotoxicants and therapeutic drugs. A 32P-postlabeling procedure for detection and quantitation of aromatic carcinogen-DNA lesions with a sensitivity limit of 1 adduct in 10(7) to 10(8) nucleotides has been described previously. In the standard procedure, DNA is enzymatically digested to 3'-phosphorylated normal and adducted mononucleotides, which are 32P-labeled at 5'-hydroxyl groups by T4 polynucleotide kinase-catalyzed [32P]phosphate transfer from [gamma-32P]ATP. 32P-labeled derivatives are resolved by TLC, detected by autoradiography, and quantitated by counting. This assay has been recently utilized for the determination and partial characterization of DNA adducts formed in somatic and reproductive tissues of rats given the clinically used anticancer drug, mitomycin C. The drug exhibits similar levels of covalent binding to DNA in most tissues. Further studies have revealed that adducted nucleotides are primarily guanine derivatives that are resistant to 3'-dephosphorylation by Penicillium citrinum nuclease P1. The latter observation has been utilized to enhance the 32P-assay's sensitivity to 1 adduct in 10(10) nucleotides for a 10-micrograms DNA sample by postincubation of DNA digests with nuclease P1 before 32P-labeling. The enzyme dephosphorylates the normal nucleotides but not most aromatic and bulky nonaromatic adducts, so that only the latter serve as substrates for the kinase-catalyzed labeling reaction. The new assay has also shown utility in the analysis of very low levels of age- and tissue-related DNA modifications, which might arise from dietary or endogenous compounds, in untreated rats and in humans.