Properties of a purified nuclear topoisomerase from L1210 cells

A nuclear type I topoisomerase from mouse leukemia L1210 cells has been partially purified and characterized. The sedimentation coefficient of the enzyme by velocity sedimentation is 4.3 S, consistent with a globular protein of 68 kDa. Enzyme activity is stimulated 20-fold in the presence of magnesium over that achieved in KCl alone. The enzyme is completely inhibited in the presence of the berenil congeners HOE 13548 and 15030 while berenil itself caused only partial inhibition at concentrations below 200 micrograms/ml. An acid soluble protein of 30 kDa (by SDS-polyacrylamide gel electrophoresis) co-purified with the topoisomerase but could be separated by precipitation in a low salt buffer. This protein, as well as a protein of similar characteristics, histone H1, stimulated topoisomerase activity over a narrow concentration range. The role of topoisomerase in the DNA strand scission observed in L1210 cells following exposure to intercalating agents remains conjectural as the purified enzyme did not produce nicks in plasmid DNA in the presence of adriamycin.