Direct determination of the primary binding site of cisplatin on cytochrome C by mass spectrometry.

Protein-cisplatin interactions lie at the heart of both the effectiveness of cisplatin as a therapeutic agent and side effects associated with cisplatin treatment. Because a greater understanding of the protein-cisplatin interactions at the molecular level can inform the design of cisplatin-like agents for future use, mass spectrometric determination of the binding site of cisplatin on a model protein, cytochrome c, was undertaken in this paper. The monoadduct cytochrome c-Pt(NH(3))(2)(H(2)O) is found to be the primary adduct produced by the cytochrome c-cisplatin interactions under native conditions. To locate the primary binding site of cisplatin, both free cytochrome c and the cytochrome c adducts underwent trypsin digestion, followed by Fourier transform mass spectrometry (FT-MS) to identify unique fragments in the adduct digest. Four such fragments were found in the adduct digest. Tandem mass spectrometry (MS/MS and MS(3) indicates that two fragments are Pt(NH(3))(2)(H(2)O) bound peptides (Gly56-Glu104 and Asn54-Glu104) with one water associated at the peptide bond Lys79-Met80, and the other two fragments are heme containing peptides (acety1-Gly1-Lys53 and acety1-Gly1-Lys55). The product-ion spectra of the four fragments reveal that Met65 is the primary binding site of cisplatin on cytochrome c.