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

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. C-13 NMR spectroscopy, radioisotope tracers, chromatography, ultrafiltration, and atomic spec spectroscopy, employing Au((CN)-C-13)(2)(-) or Au((CN)-C-14)(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 degrees C) of K-1=5.5 (+/-1.1) x 10(4), and three additional equivalents bind with a constant of 7.0 (+/-0.1) x 10(3). Au((CN)-C-13)(2)(-) associated with albumin is characterized by a broad C-13 NMR resonance at delta(C)=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 (less than or equal to 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 inter action 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.