The antitumor agent coralyne and a number of structural analogues were found to be inhibitors of DNA topoisomerase I and were characterized biochemically. Several of these analogues stabilized the covalent binary complex formed between calf thymus topoisomerase I and pSP64 plasmid DNA; coralyne and 5,6-dihydrocoralyne had the greatest potency as inhibitors in this assay. In common with camptothecin, the effects of coralyne and 5,6-dihydrocoralyne were reversed in the presence of increasing salt concentration or temperature, consistent with the interpretation that both functioned mechanistically in a fashion analogous to camptothecin. The sequence specificity of DNA cleavage by coralyne and 5,6-dihydrocoralyne was also studied in comparison with camptothecin using a 471-bp DNA duplex as a substrate for topoisomerase I. Seven sites of cleavage were apparent, four of which were shared in common by coralyne, 5,6-dihydrocoralyne and camptothecin. Coralyne and 5,6-dihydrocoralyne produced cleavage at one sequence, 5′-TCTC↓GTAA-3′, that was not apparent in the presence of camptothecin; correspondingly, two cleavage bands appeared only when camptothecin was present. Coralyne and 5,6-dihydrocoralyne also inhibited topoisomerase I-mediated relaxation of supercoiled plasmid DNA. Coralyne was the most potent inhibitor of DNA relaxation; the effects of camptothecin and 5,6-dihydrocoralyne were roughly equal. At high concentrations, coralyne completely suppressed the formation of the topoisomerase I-DNA covalent binary complex.
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