The sensitivity to camptothecin of DNA topoisomerase I in L5178Y-S lymphoma cells.

Sensitivity to camptothecin (CPT) of type I DNA topoisomerases isolated from two L5178Y (LY) sublines was examined in reaction media containing either aspartate or chloride. The enzyme from LY-S cells was sensitive to the drug in the presence of 120 mM K-aspartate, but the sensitivity was markedly reduced in the presence of 120 mM KCl. The enzyme from LY-R cells was similarly sensitive to camptothecin in the presence of either aspartate or chloride. The optimum ionic strength for the relaxation reaction catalyzed by both LY-R and LY-S type I DNA topoisomerases was similar. We suggest that sensitivity of the LY-S enzyme to CPT depends on the amount of cleavable complex formed, which in turn depends on the ionic conditions of the assay.