Fig. 5. Comparison of the AP endonuclease activity in XRCC1-proficient or deficient whole-cell extracts. XRCC1-proficient (AA8) or XRCC1-deficient (EM9, EMC11 and EMC12) CHO cell lines were used to prepare extracts. Reactions were carried out as described in Materials and methods with increasing amounts of total extract protein as indicated. An oligonucleotide harboring a single THF site was used as substrate. (A) Plot of the percentage of DNA substrate cleaved by the AP endonuclease activities of the four CHO cell line extracts versus total amount of protein used in the reaction. Each point represents the average and standard deviation from two independent experiments. (B) APE1 protein levels in 50 µg of each cell extract were determined by western blotting.