Topoisomerase II is a cellular target for antiproliferative cobalt salicylaldoxime complex

Topoisomerase II is a cellular target for a number of clinically relevant a ntitumor drugs. To elucidate the possible cellular target for the antiproli feration activity of cobalt salicylaldoxime (CoSAL), which inhibits 50% of leukemic cell proliferation at a concentration of 60 mu M, DNA binding stud ies and studies of the action of this complex on topoisomerase II catalytic activities were carried out. The results from. DNA binding studies show th at CoSAL binds DNA strongly with a stoichiometric ratio of two drug molecul es for live nucleotide bases and shows a mode of interaction similar to tha t of DNA groove binding agents. The results from topoisomerase II inhibitio n studies show that the complex inhibits the relaxation activity of topoiso merase II in a dose-dependent manner and poisons its activity through cleav age complex formation. To see if the hydroxyl group present on imine nitrog en is involved in topoisomerase II poisoning, we synthesized an analogue of CoSAL in which the hydroxyl group was replaced with semicarbazone. This co mplex too binds DNA with an affinity similar to that of CoSAL, but with a s mall difference in the mode of interaction; however, it marginally inhibits leukemic cell proliferation and does not inhibit topoisomerase II activity , which suggests:the involvement of a hydroxyl group. An immunoprecipitatio n assay was conducted which showed that the cleavage complex formed in the presence of CoSAL contained 75% of the complex, while the other complex sho ws only 7.65%. Cyclic voltametric spectra of the complexes in the presence of DNA show that they do not oxidize DNA. These results suggest that CoSAL shows a bidirectional mode of interaction with enzyme and DNA and inhibits topoisomerase II activity by forming a drug-mediated cleavage complex. Our data strongly suggest that topoisomerase II may be one of the cellular targ ets for antiproliferation activity of CoSAL. (C) 1999 Academic Press.