Kinetic analysis of the in vitro binding of radioactive cis- and trans-dichlorodiammineplatinum(II) to DNA

Abstract

The binding of cis(c)- and trans(t)-Pt(NH3)2Cl2 to DNA at platinum/DNA-nucleotide ratios (Ri) of 0.1 or less has been studied by means of radioactive 195mPt-labeled compounds. Kinetic data are consistent with the following scheme: (Formula: see text). At 25 degrees C and pH 5-6 in 5 mM NaClO4, the values for the rate constants in the above scheme for the c-isomer are k2 = 2.2 X 10(-5) sec -1, k7 = 0.32 (sec M)-1, and k8 = 143 (sec M)-1; for the t-isomer the values are k2 less than 0.5 X 10(-5) sec-1 and k7 = 0.95 (sec M)-1. Platinum-DNA adducts do not undergo detectable exchange for 3 days at 37 degrees C, indicating the absence of a dynamic equilibrium. For both isomers the rate of binding is the same for single- and double-stranded DNA. The conclusions derived from Ag+ and H+ titration studies are consistent with binding at guanine N(7) for Ri less than 0.1. The reaction rate is competitively inhibited by various salts and buffers and is suppressed by raising the pH (50% inhibition of initial rates at pH 7.3). At 37 degrees C and pH 7 in 0.15 M NaCl, 6-8% of both the c- and t-isomers bind to DNA in 24 h, suggesting that both compounds should bind to DNA under biological conditions.