Isodiospyrin is a natural product from the plant Diospyros morrisiana, which consists of an asymmetrical 1,2-binaphthoquinone chromophore. Isodiospyrin exhibits cytotoxic activity to tumor cell lines but very little is known about its cellular target and mechanism of action. Unlike the prototypic human topoisomerase I (htopo I) poison camptothecin, isodiospyrin does not induce htopo I-DNA covalent complexes. However, isodiospyrin antagonizes camptothecin-induced, htopo I-mediated DNA cleavage. Binding analysis indicated that isodiospyrin binds htopo I but not DNA. These results suggest that isodiospyrin inhibits htopo I by direct binding to htopo I, which limits htopo I access to the DNA substrate. Furthermore, isodiospyrin exhibits strong inhibitory effect on the kinase activity of htopo I toward splicing factor 2/alternate splicing factor in the absence of DNA. Thus, these findings have important implications on naphthoquinone and its derivatives' cellular mode of actions, i.e. these novel DNA topoisomerase I inhibitors can prevent both DNA relaxation and kinase activities of htopo I.
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Ting, C. Y., Hsu, C. T., Hsu, H. T., Su, J. S., Chen, T. Y., Tarn, W. Y., Kuo, Y. H., Whang-Peng, J., Liu, L. F., & Hwang, J. (2003). Isodiospyrin as a novel human DNA topoisomerase I inhibitor. Biochemical Pharmacology, 66(10), 1981-1991. https://doi.org/10.1016/j.bcp.2003.07.003