Endogenous apurinic/apyrimidinic sites in genomic DNA of mammalian tissues

Apurinic/apyrimidinic (AP) sites are one of the most frequent lesions in DNA. Using a highly sensitive slot blot assay, we determined the number and condition of endogenous AP sites in normal tissues of rats and human liver. The number of AP sites (50,000-200,000 per mammalian cell) was greatest in brain, followed by colon and heart, and then liver, lung, and kidney. The majority of endogenous AP sites were cleaved 5' to the AP site. These data suggest that removal of the deoxyribosyl phosphate moiety is the rate-limiting step in base excision and AP site repair in vivo.