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.imagesfigure 2.figure 5 .