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

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. c/sDichlorodiammineplatinum(ll), a potent antitumor agent, pro duced a marked enhancement in fluorescence intensity when reacted with double-stranded DNA. This can be explained by the c/s-dichlorodiammineplatinum(ll) 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 ob served. Helical polynucleotides such as poly(deoxyguanylic acid-deoxycytidylic acid) displayed large increases in fluores cence, whereas those with little or no helical structure resulted in decreased fluorescence. An increase in terbium fluores cence was also observed with polydisperse linear DNA frag ments from Escherichia coli and calf thymus as well as with defined-length Hind\\\ restriction enzyme fragments of <£x1 74 DNA and supercoiled pBR322 DNA. However, the inactive frans-dichlorodiammineplatinum(ll) 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, how ever, displayed marked decreases in fluorescence, particularly with the frans-isomer. Analysis of the treated RNAs on agarose gels showed decreased ethidium bromide staining with both isomers.