Mass-spectrometric characterization of cisplatin binding sites on native and denatured ubiquitin

Because interactions between cisplatin and plasma proteins contribute to drug efficacy and side effects, it is important to understand both the binding sites of cisplatin on the proteins and the formation of protein-cisplatin adducts. Previous results suggest that cisplatin preferentially binds to residues on the protein surface. The present work employed electrospray ionization mass spectrometry (MS) to identify such sites on both native and denatured ubiquitin (Ub). Fourier transform (FT) MS and tandem MS (MS/MS and MS(3)) enable analysis of Ub-cisplatin adduct digests to locate specific cisplatin binding sites. Results indicate that there are three such binding sites, i.e., M1, T12 and T14, and D32, on native Ub. The intensity of the relevant peaks in the FT-MS spectrum of the native Ub adduct digest demonstrates that residues T12 and T14 comprise the primary cisplatin binding site under the native conditions rather than residue M1 as reported in previous research studies. It is found in the present work, however, that M1 is the primary binding site on denatured Ub. Comparison of cisplatin binding sites on native and denatured Ub in this research demonstrates that the conformation of a protein significantly influences the preference of cisplatin for specific binding sites.