Effect of trans-diamminedichloroplatinum(II) on human serum albumin: Conformational changes through partial disulfide bond cleavage

Abstract

trans-Diamminedichloroplatinum(II) ( trans-DDP), the trans isomer of cis-diamminedichloroplatinum(II) ( cis-DDP), was bound to human serum albumin (HSA) in pH 7.4 buffer containing 0.1 M NaCl at 37°C. The amount of bound trans-DDP per mol of HSA was found to be 21.4 mol when the protein was incubated with a 40-fold excess of trans-DDP for 6 days. In trans-DDP-treated HSA, 3.4 disulfide (S-S) bonds were cleaved, where one HSA molecule contains 17 S-S bonds. The spectral characteristics of trans-DDP-treated HSA were examined in terms of the fluorescence spectrum of its lone tryptophan (Trp-214), and molar ellipticity. The relative fluorescence intensity of platinum-bound HSA decreased to 32.4% of that of the native state, suggesting that perturbation around the Trp-214 residue took place. This was confirmed by the destruction of the warfarin-binding site containing Trp-214 observed in the metal-bound HSA. Analysis of circular dichroism (CD) spectra showed a decreasing helix content from 50.5% in the native state to 30.6% in the metal-bound HSA. These conformational changes observed in HSA may be attributed to the S-S bond rupture induced by trans-DDP. Comparison with cis-DDP, which has already been shown to cleave S-S bonds in HSA, revealed that trans-DDP binds to HSA and cleaves S-S bonds more readily than the cis isomer.