Equilibrium binding constants and facile dissociation of novel serum albumin-dicyanoaurate(I) complexes

Abstract

Dicyanoaurate(I), Au(CN)2–, an important metabolite of chrysotherapy agents (anti-arthritic gold drugs), contains two tightly bound cyanide ligands which render it relatively unreactive toward ligand exchange reactions with potential gold-binding ligands. The extent and nature of its binding to bovine serum albumin (BSA), which may modulare the in vivo activity of Au(CN)2–, were investigated to determine whether Au(CN)2– might be more bioavailable than other gold complexes. 13C NMR spectroscopy, radioisotope tracers, chromatography, ultrafiltration, and atomic spectroscopy, employing Au(13CN)2– or Au(14CN)2– as appropriate, revealed two distinct binding mechanisms. The dominant reaction is reversible association (non-specific binding) of intact Au(CN)2– ions to form BSA·[Au(CN)2–]n adducts. Approximately one equivalent binds with an equilibrium binding constant (pH 7.4, 25 °C) of K1=5.5 (±1.1)×104, and three additional equivalents bind with a constant of 7.0 (±0.1)×103. Au(13CN)2– associated with albumin is characterized by a broad 13C NMR resonance at δC=154.7 ppm compared to the sharp resonance of the free complex at 156.4 ppm. The BSA·[Au(CN)2–]n adducts readily dissociate during gel exclusion chromatography and are therefore underestimated, but are retained and accurately quantitated by ultrafiltration methods. The second binding mechanism is a ligand exchange reaction at Cys-34, to form AlbSAuCN, which accounts for only a small fraction (≤11%) of the bound gold. The small extent of the latter interaction differentiates Au(CN)2– from the gold drugs such as auranofin, aurothiomalate (Myochrysin) and aurothioglucose (Solganol), which undergo ligand exchange at Cys-34 of albumin to form tightly bound gold-protein complexes. The weak interaction at Cys-34 and the facile dissociation of bound, intact Au(CN)2– are consistent with its putative role as a gold metabolite that can be accumulated intracellularly.