Nuclease P1-mediated enhancement of sensitivity of 32P-postlabeling test for structurally diverse DNA adducts

Exceedingly sensitive procedures are required to detect the presence of covalent DNA adducts in humans exposed to environmental genotoxicants because of low levels of such derivatives (1 adduct in 10(8)-10(10) DNA nucleotides). A 32P-postlabeling assay for detection and quantitation of carcinogen--DNA adducts with a sensitivity limit of 1 adduct in 10(7)-10(8) nucleotides has been described previously. In the standard procedure, DNA is enzymatically digested to 3'-phosphorylated normal and adducted mononucleotides, which are quantitatively 32P-labeled at their 5'-hydroxyl groups by T4 polynucleotide kinase-catalyzed [32P]phosphate transfer from [gamma-32P]ATP. 32P-labeled derivatives are resolved by t.l.c., detected by autoradiography and quantitated by counting. We now report that a minor modification of this procedure, entailing the postincubation of DNA digests with Penicillium citrinum nuclease P1 before 32P-labeling, enhanced the technique's sensitivity to 1 adduct in approximately 10(10) nucleotides for a 10-micrograms DNA sample. Nuclease P1 cleaves deoxyribonucleoside 3'-monophosphates of normal nucleotides to deoxyribonucleosides which do not serve as substrates for polynucleotide kinase, while most adducted nucleotides were found to be totally or partially resistant to the 3'-dephosphorylating action of nuclease P1. The additional enzymatic step enabled specific labeling of adducts in up to 20 micrograms of DNA with excess carrier-free [gamma-32P]ATP. The enzymatic digestion conditions were standardized to afford optimal adduct recovery. The new procedure was found to be simple, highly reproducible, and applicable to the detection and measurement of aromatic or bulky non-aromatic DNA adducts formed with such structurally diverse carcinogens as benzo[a]pyrene, 7,12-dimethyl-benz[a]anthracene, dibenzo[c,g]carbazole, 4-aminobiphenyl, safrole and mitomycin C; most adducts were recovered quantitatively with a 500- to 1000-fold increase in 32P-count rates as compared with the standard procedure.