This journal is © The Royal Society of Chemistry 2014 Metallomics, 2014, 6, 117--125 | 117 Cite this: Metallomics, 2014, 6, 117 Effect of oxindolimine copper(II) and zinc(II) complexes on human topoisomerase I activity† Prafulla Katkar, a Andrea Coletta, a Silvia Castelli, a Gustavo L. Sabino, b Ricardo A. Alves Couto, b Ana M. da Costa Ferreira b and Alessandro Desideri* a The ability of oxindolimine copper(II) and zinc(II) complexes, known to have antitumor activity, to inhibit human topoisomerase IB has been tested through enzymatic kinetic assays and molecular docking simulations. These copper and zinc compounds are able to inhibit remarkably the cleavage reaction and only partially the religation step, the copper compound being more efficient than the zinc one. A complete inhibition activity of the cleavage is only obtained when the enzyme is pre-incubated with the compound, the inhibition being irreversible and reversible for the copper and zinc compounds, respectively. The relative stability of such complexes was estimated by competitive equilibria with human serum albumin (HSA), monitored by CD spectroscopy. The copper species shows a log K CuL = 17.2, while the analogous zinc complex exhibits a log K ZnL = 7.2. Molecular docking simulation studies show that the almost square planar geometry of the copper compound allows a direct coordination of the metal with two amino acids (Glu492, Asp563) of the enzyme at variance of the zinc compound which has a more tetrahedral geometry. Altogether, the data indicate that the different coordination geometry achieved by the two transition metal ions has an important role in modulating their efficiency as topoisomerase I inhibitors. 1. Introduction Human topoisomerases are ubiquitous enzymes that catalyze the topological changes of DNA during replication, transcription, recombination, repair, chromatin assembly and chromosome segregation by triggering, breaking and re-joining of the DNA strand. 1 Strand cleavage by all topoisomerase involves nucleo- philic attack by the catalytic tyrosine residue on the scissile phosphodiester bond that culminates in the formation of a covalent bond between the enzyme and one end of the broken strand. 2 There are two types of human topoisomerase enzymes i.e. type I and type II. Type II enzymes are dimeric and cleave both DNA strands, while type I enzymes mediate the catalytic reaction creating a transient single strand break in the backbone of DNA. 3 The catalytic cycle of the topoisomerase I enzyme begins at the active site tyrosine (tyr 723 ) through a nucleophilic attack on DNA, creating breakage in one strand with the enzyme covalently attached to the 3 0 -phosphate to form the cleaved complex. After changing the linking number, a second nucleo- philic attack, driven by the 5 0 hydroxyl DNA end, restores an intact double stranded DNA, and the enzyme is released. Human topoisomerase I consists of 765 amino acids with four distinct domains: the N-terminal, the core, the linker and the C-terminal domain. 4 The bilobed structure of the enzyme formed by the core and C-terminal domains fully wraps around the DNA during the catalytic cycle. 5 Topoisomerase IB is the target of a vast number of drugs that depending on their action are divided as poisons and inhibitors. 6,7 The poisons reversibly bind to the DNA–enzyme cleavage complex, camptothecin is the best characterized repre- sentative. Camptothecin converts topoisomerase I into a DNA damaging agent by prolonging the lifetime of the cleavage intermediate, thus creating an obstacle to the advancement of replication machinery that ultimately results into DNA damage and cell death. 8–10 Two main analogs of camptothecin, topotecan and irinotecan, have been successfully used in the treatment of several human cancers and have been approved by the US food and Drug administration for the clinical purpose. 11,12 Catalytic inhibitors are compounds that prevent the binding of topoisome- rase I to DNA or inhibit the cleavage reaction of the enzyme and so inhibit the DNA relaxation. 13–16 Some compounds are able to inhibit both cleavage and religation. 17,18 A few metal complexes have also been found to target topoisomerases, including essential and non-essential metals. 19–23 The here presented compounds are particularly interesting since their geometrical arrangement can be finely tuned by the coordination properties of each metal and structural features of each ligand. Some oxindole–Schiff base copper(II) complexes have been synthesized and characterized, a Dipartimento di Biologia, Universita ` Tor Vergata di Roma, 00173 Roma, Italy. E-mail: desideri@uniroma2.it b Instituto de Quı ´mica, Universidade de Sa ˜o Paulo, 05508-000 Sa ˜o Paulo, SP, Brazil † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3mt00099k Received 27th March 2013, Accepted 14th October 2013 DOI: 10.1039/c3mt00099k www.rsc.org/metallomics Metallomics PAPER Published on 30 October 2013. Downloaded on 01/10/2014 16:35:11. View Article Online View Journal | View Issue