EFFECTS OF CAMPTOTHECIN ANALOGS ON DNA TRANSFORMATIONS MEDIATED BY CALF THYMUS AND HUMAN DNA TOPOISOMERASES I

Camptothecin (CPT) and 10 structural analogues were studied to charact erize their effects on specific rearrangements of DNA structure mediat ed by human and calf thymus DNA topoisomerases I. A 30 base pair DNA d uplex containing a single high-efficiency topoisomerase cleavage site was incubated with each of the enzymes in the presence of the inhibito rs. Individual inhibitors stabilized the covalent enzyme-DNA binary co mplex to different extents, as anticipated. However, for several of th e inhibitors, the extent of ternary complex formation differed substan tially for the human and calf thymus enzymes. In common with calf thym us topoisomerase I, the human enzyme was shown to mediate the rearrang ement of branched, nicked, and gapped DNA substrates that constitute m odels for illegitimate recombination. However, some of these rearrange ments proceeded with different rates and efficiencies in the presence of human topoisomerase I. When inhibition of three of the rearrangemen ts by CPT analogues was studied, most of the analogues exhibited diffe rential effects on a given transformation, depending on the source of the enzyme employed.