Effect of Platinum Antitumor Agents on DNA and RNA Investigated by Terbium Fluorescence¹ Free

Terbium, a fluorescent probe, which fluoresces primarily with guanine and xanthine bases, has been used to investigate the interaction of platinum complexes with DNA and RNA. cis-Dichlorodiammineplatinum(II), a potent antitumor agent, produced a marked enhancement in fluorescence intensity when reacted with double-stranded DNA. This can be explained by the cis-dichlorodiammineplatinum(II) producing an opening up of the helix, allowing accessibility of terbium to the guanine base which in turn produces fluorescence enhancement. When guanine nucleosides and nucleotides were reacted with the drug, however, little to no change in fluorescence was observed. Helical polynucleotides such as poly(deoxyguanylic acid-deoxycytidylic acid) displayed large increases in fluorescence, whereas those with little or no helical structure resulted in decreased fluorescence. An increase in terbium fluorescence was also observed with polydisperse linear DNA fragments from Escherichia coli and calf thymus as well as with defined-length HindIII restriction enzyme fragments of φχ174 DNA and supercoiled pBR322 DNA. However, the inactive trans-dichlorodiammineplatinum(II) isomer produced no change in terbium fluorescence. Analysis of the binding data showed that the increase in terbium fluorescence was not accompanied by marked changes in the number of binding sites or association constants. Transfer and ribosomal RNA from E. coli MRE 600 and single-stranded fd viral DNA, however, displayed marked decreases in fluorescence, particularly with the trans-isomer. Analysis of the treated RNAs on agarose gels showed decreased ethidium bromide staining with both isomers.