The Binding of Platinum (II) Complexes to Rabbit Skeletal Muscle G-Actin Induces Conformation Changes

The binding of cis-diamminedichloroplatinum (DDCP) and cis-diaquodiammine platinum (DADP) to rabbit skeletal muscle G-actin and the consequent conformation changes were studied as the function of the Pt/actin molar ratio (R) and time by intrinsic and NPM labeled fluorescence, CD spectra as well as gelfiltration chromatography. The results indicated that the unhydrolyzed DDCP can react with G-actin in presence of Cl(-) ion. The reaction differs from that of its hydrolysis product DADP in a higher specificity and a lower capacity. Both of them induced exposure of the tryptophane residues and labeled Cys374 and the increase in alpha-helix content depending on R, but the conformation changes caused by DADP are more significant than DDCP at the same R. These are related to the binding of DADP to groups other than thiols. The rate constants of conformation change suggested that DADP quenched the intrinsic fluorescence more rapid. The temporal change in fluorescence of NPM labeled actin has a biphasic feature: in the first 16 minutes, the fluorescence was quenched, then it recovered slowly, indicating a multi-step reaction including high affinity platinum binding --> labeled Cys374 moving to hydrophilic environment --> low affinity platinum binding --> Cys374-related conformation compacting in sequence.