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Cyclo(L-prolyl-L-phenylalanyl) [cyclo(pro-phe)] was isolated from Streptomyces sp. AMLK-335 and found to inhibit DNA topoisomerase Ⅰ activity. In a DNA relaxation assay using supercoiled pBR322 DNA, cyclo(pro-phe) inhibited the DNA topoisomerase activity more strongly than camptothecin, a known topoisomerase inhibitor. However, at a concentration of 10 ㎛, cyclo(pro-phe) produced a lower degree of DNA relaxation than camptothecin, therefore, the inhibition topoisomerase Ⅰ activity by cyclo(pro-phe) was also found to be dose dependent. Accordingly, the current results suggest that cyclo(pro-phe) may be a novel inhibitor of topoisomerase Ⅰ.
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