Picoplatin binding to proteins: X-ray structures and mass spectrometry data on the adducts with lysozyme and ribonuclease A.

The reactivity of the anticancer drug picoplatin (cis-amminedichlorido(2-methylpyridine)platinum(II) complex) with the model proteins hen egg white lysozyme (HEWL) and bovine pancreatic ribonuclease (RNase A) was investigated by electrospray ionisation mass spectrometry (ESI MS) and X-ray crystallography. The data were compared with those previously obtained for the adducts of these proteins with cisplatin, carboplatin and oxaliplatin under the same experimental conditions. ESI-MS data show binding of Pt to both proteins, with fragments retaining the 2-methylpyridine ligand and, possibly, a chloride ion. X-ray crystallography identifies different binding sites on the two proteins, highlighting a different behaviour of picoplatin in the absence or presence of dimethyl sulfoxide (DMSO). Metal-containing fragments bind to HEWL close to the side chains of His15, Asp18, Asp119 and both Lys1 and Glu7, whereas they bind to RNase A on the side chain of His12, Met29, His48, Asp53, Met79, His105 and His119. The data suggest that the presence of DMSO favours the loss of 2-methylpyridine and alters the ability of the Pt compound to bind to the two proteins. With both proteins, picoplatin appears to behave similarly to cisplatin and carboplatin when dissolved in DMSO, whereas it behaves more like oxaliplatin in the absence of the coordinating solvent. This study provides important insights into the pharmacological profile of picoplatin and supports the conclusion that coordinating solvents should not be used to evaluate the biological activities of Pt-based drugs.