Characterizing the binding interaction between dicyanogold(I) and human serum albumin

Gold-based drugs have been used in the treatment of rheumatoid arthritis for over 60 years, yet the mechanism of action or these drugs is not understood clearly. This study focuses on an early step in the transport mechanism of gold drugs, namely the interaction of dicyanogold(I) (a common bio-transformation product of gold drugs) with human serum albumin (the most abundant protein in blood serum). These binding studies involve both qualitative and quantitative characterization of the binding interaction between dicyanogold(I) and two forms of human serum albumin (HSA): HSA with cysteine-34 in the native form and HSA with a blocking group bound to the sulfur of cysteine-34 (HSA*AA). The blocked protein is HSA treated with iodoacetamide so that cysteine-34, a known binding site for gold drugs, is prevented from participating in other covalent interactions.;Each form of HSA was characterized using two mass spectrometry techniques. FT-ICR-MS with electrospray ionization uses an acidic environment to add up to 60 protons to the HSA, lowering the mass to charge ratio to a value that can be detected easily by the mass spectrometer so that the intact protein can be characterized. MALDI-TOF-MS forms predominantly singly charged ions, thus it is necessary to perform an enzymatic digest of the HSA before analysis. Subsequently, individual fragments were studied yielding information about the form of cysteine-34 (native vs. blocked).;A series of experiments were conducted in which the effect of changing the conditions of the system on the equilibrium constant, Keq, was studied. These experiments include varying the initial concentration of dicyanogold(I) used, the ionic strength of the buffer solution and the incubation temperature of the system. Additionally, a series of experiments that tested the reversibility of this binding were conducted. Reversed phase ion-pairing chromatography was used to determine that the gold species released from the protein after establishing equilibrium was dicyanogold(I). Throughout these experiments, both HSA and HSA*AA were used. Comparing the results of these experiments using different forms of the protein shows that there is a relationship between the binding site for dicyanogold(I) on HSA and cysteine-34, though covalent binding is not observed.