Thiol competition for Et3PAuS-albumin: a nonenzymatic mechanism for Et3PO formation

Thiols (RSH = 2,3,4,6-tetra-O-acetyl-beta-1-D-thioglucose, beta-1-D-thioglucose, and glutathione) can displace either the albumin or the triethylphosphine from the protein-gold complex, AlbSAuPEt3. The albumin is displaced in preference to triethylphosphine, but irreversible oxidation of the latter eventually shifts the equilibria toward Et3PO and AlbSAuSR. Albumin disulfide bonds are the probable oxidants. Neither O2 nor oxidized glutathione substantially enhanced the rate or extent of Et3PO formation. The labilization of the phosphine in AlbSAuPEt3 is attributed to a strong trans effect of the albumin thiolate, Cys-34. The 31P NMR chemical shifts of various thiolato(triethylphosphine)gold(I) complexes are correlated directly with the affinity of the thiols for gold and inversely with their pKSH values. Deacetylated auranofin (1-thio-beta-D-glucopyranosato-S) (triethylphosphine)gold(I) reacts with the mercaptalbumin and oxidized mercaptalbumin (putatively AlbSOH) forms of bovine serum albumin to form AlbSAuPEt3 with displacement of the thioglucose ligand.