A New Structural Transition of Serum Albumin Dependent on the State of Cys34

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Abstract
1 Reactions of fatty-acid-free bovine serum albumin and recombinant human albumin with a range of antiarthritic gold(I) complexes [auranofin, deacetylated auranofin, triethylphosphinegold(I) chloride] and related thiols (thioglucose, tetraacetylthioglucose, glutathione, dithiothreitol) have been investigated using 1H-NMR spectroscopy. 2 In reactions of albumin with auranofin, tetraacetylthioglucose and dithiothreitol, release of cystine was detected, whereas for deacetylated auranofin, thioglucose and glutathione, mixed disulphides with cysteine were produced. It has been previously proposed that Cys34 of human and bovine serum albumins is partly blocked by disulphide formation with cysteine and glutathione. The above reactions lead to deblocking by thiol–disulphide interchange reactions. No release of glutathione from albumin was detected. 3 Changes in the His Hɛ1 regions of the 1H-NMR spectra show that albumin exists in two structural forms dependent on whether the side-chain of Cys34 is a free thiolate, or blocked by gold(I)triethylphosphine, by disulphide formation with cysteine or by another form of oxidation. We propose that Cys34 is either in a buried or in an exposed environment; the possible molecular basis of the structural change is discussed. 4 The relationship between reactions at Cys34, cysteine release, and the observed structural transition are discussed in terms of chrysotherapy, albumin metabolism and the use of gold(I) as a heavy atom derivative in X-ray crystallographic studies of albumins.