TOPOISOMERASE-II BINDS TO ELLIPTICINE IN THE ABSENCE OR PRESENCE OF DNA - CHARACTERIZATION OF ENZYME DRUG-INTERACTIONS BY FLUORESCENCE SPECTROSCOPY

Although a number of drugs currently in use for the treatment of human cancers act by stimulating topoisomerase II-mediated DNA breakage, little is known regarding interactions between these agents and the enzyme. To further define the mechanism of drug action, interactions between ellipticine (an intercalative drug with clinical relevance) and yeast topoisomerase II were characterized. By utilizing a yeast genetic system, topoisomerase II was identified as the primary cellular target of the drug. Furthermore, ellipticine did not inhibit enzyme-mediated DNA religation, suggesting that it stimulates DNA breakage by enhancing the forward rate of cleavage. Finally, ellipticine binding to DNA, topoisomerase II, and the enzyme . DNA complex was assessed by steady-state and frequency domain fluorescence spectroscopy. As determined by changes in fluorescence intensity and emission maximum wavelength, and by lifetime analysis, only the protonated species of ellipticine bound to a double stranded 40-mer oligonucleotide containing a topoisomerase II cleavage site (K-D approximate to 65 nM). In contrast, predominantly deprotonated ellipticine bound to the enzyme . DNA complex (K-D approximate to 1.5 mu M) or to the enzyme in the absence of nucleic acids (K-D approximate to 160 nM). These findings suggest that ellipticine interacts directly with topoisomerase II and that the enzyme dictates the ionic state of the drug in the ternary complex. A model is presented in which the topoisomerase II . ellipticine DNA complex is formed via initial drug binding to either the enzyme or DNA.