DIM (3,3′-di-indolylmethane), an abundant dietary component of cruciferous vegetables, exhibits a wide spectrum of pharmacological properties. In the present study, we show that DIM is a potent inhibitor of Leishmania donovani topoisomerase I with an IC50 of 1.2 μM. Equilibrium dialysis shows that DIM binds strongly to the free enzyme with a binding constant of 9.73×10−9 M. The binding affinity of DIM to the small subunit is 8.6-fold more than that of the large subunit of unusual LdTOP1LS (bi-subunit L. donovani topoisomerase I). DIM stabilizes topoisomerase I–DNA cleavage complexes in vitro and also in vivo. Like CPT (camptothecin), DIM inhibits the religation step when the drug was added to preformed topoisomerase I–DNA binary complex. Hence, DIM is similar to CPT with respect to its ability to form the topoisomerase I-mediated ‘cleavable complexes’ in vitro and in vivo. But unlike CPT, DIM interacts with both free enzyme and substrate DNA. Therefore DIM is a non-competitive class I inhibitor of topoisomerase I. DIM also inhibits the relaxation activity of the CPT-resistant mutant enzyme LdTOP1Δ39LS (N-terminal deletion of amino acids 1–39 of LdTOP1LS). The IC50 values of DIM in simultaneous and enzyme pre-incubation relaxation assays were 3.6 and 2.9 μM respectively, which are higher than that of wild-type topoisomerase I (LdTOP1LS), indicating that the affinity of DIM to LdTOP1Δ39LS is less than that for LdTOP1LS. This is the first report on DIM as an L. donovani topoisomerase I poison. Our study illuminates a new mode of action of enzyme inhibition by DIM that might be exploited for rational drug design in human leishmaniasis.

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