SUPERHELICAL TORSION CONTROLS DNA INTERSTRAND CROSS-LINKING BY ANTITUMOR CIS-DIAMMINEDICHLOROPLATINUM(II)

Negatively supercoiled, relaxed and linearized forms of pSP73 DNA were modified in cell-free medium by cis-diamminedichloroplatinum(II) (cis platin). The frequency of interstrand cross-links (ICLs) formed in the se DNAs has been determined by: (i) immunochemical analysis; (ii) an a ssay employing NaCN as a probe of DNA ICLs of cisplatin; (iii) gel ele ctrophoresis under denaturing conditions. At low levels of the modific ation of DNA (<1 Pt atom fixed per 500 bp) the number of ICLs formed b y cisplatin was radically enhanced in supercoiled in comparison with l inearized or relaxed DNA. At these low levels of modification, the fre quency of ICLs in supercoiled DNA was enhanced with increasing level o f negative supercoiling or with decreasing level of modification. In a ddition, the replication mapping of DNA ICLs of cisplatin was consiste nt with these lesions being preferentially formed in negatively superc oiled DNA between guanine residues in both the 5'-d(GC)-3' and the 5'- d(CG)-3' sites. Among the DNA adducts of cisplatin the ICL has the mar kedly greatest capability to unwind the double helix. We suggest that the formation of ICLs of cisplatin is thermodynamically more favored i n negatively supercoiled DNA owing mainly to the relaxation of superco ils.