Interaction of cisplatin drug with RNase A

cis-Pt(NH3)(2)Cl-2 (cisplatin) is an antitumor drug with many severe toxic side effects including enzymatic structural changes associated with its mec hanism of action. This study is designed to examine the interaction of cisp latin drug with ribonuclease A (RNase A) in aqueous solution at physiologic al pH, using drug concentration of 0.0001 mM to 0.1 mM with final protein c oncentration of 2% w/v. Absorption spectra and Fourier transform infrared ( FTIR) spectroscopy with its self-deconvolution, second derivative resolutio n enhancement and curve-fitting procedures were used to characterize the dr ug binding mode, association constant and the protein secondary structure i n the cisplatin-RNase complexes. Spectroscopic results show that at low drug concentration (0.0001 mM), no i nteraction occurs between cisplatin and RNase, while at higher drug concent rations, cisplatin binds indirectly to the polypeptide C=O, C-N (via H2O or NH3 group) and directly to the S-H donor atom with overall binding constan t 5.66 x 10(3)M(-1) At high drug concentration, major protein secondary str uctural changes occur from that of the alpha-helix 29% (free enzyme) to 20% and beta-sheet 39% (free enzyme) to 45% in the cisplatin-RNase complexes. The observed structural changes indicate a partial protein unfolding in the presence of cisplatin at high drug concentration.