Bis(triethylphosphine)gold(I) gold(I) Chloride: Ionization in Aqueous Solution, Reduction in Vitro of the External and Internal Disulfide Bonds of Bovine Serum Albumin, and Antiarthritic Activity

Bis(triethylphosphine)gold(I) chloride (1) is an orally active antiarthritic agent in the adjuvant-induced arthritic rat model at doses of 10 and 20 (mg of Au/kg)/day.³¹P NMR and conductivity measurements show that 1 ionizes in aqueous solution, forming [(Et₃P)₂Au⁺] and Cl-. 1 reacts with bovine serum albumin (BSA) at cysteine-34 of mercaptalbumin (AlbSH), producing AlbSAuPEt₃and free Et₃P, which reduces both the “external” disulfide bonds between Cys-34 and free cysteine or glutathione and some of the “internal” disulfide bonds of the albumin tertiary structure. The cysteines generated from the internal disulfide bonds can be titrated with 5,5’-dithiobis(2-nitrobenzoic acid) (DTNB) and also react with Et₃PAuCl, forming (Et₃PAu)„-(HS)₂_,BSA. The³¹P NMR chemical shifts of Et₃PAu⁺bound at Cys-34 and the newly reduced “internal” cysteines can distinguish the two sites: δ_P= 38.8 ppm for AlbSAuPEt₃and 35–36 ppm for (Et₃PAuS)_x(HS)₂__xBSA, relative to internal trimethyl phosphate (TMP). Modified albumin samples, in which the cysteine-34 residue is converted into a cysteine disulfide (Cy-BSA) or a thioether (Ac-BSA, prepared by using ICH₂CONH₂), also react with 1. The mercaptalbumin adduct is extensively regenerated from Cy-BSA by reduction of the “external” disulfides. For Ac-BSA, the regeneration of AlbSH is limited by the extent that the naturally occurring disulfides of cysteine and glutathione were present before the modification. After saturating the free sulfhydryl groups of albumin, Et₃PAuCl also populates weaker binding sites characterized by chemical shifts between 28 and 23 ppm and previously shown to be nitrogen donor groups. Simultaneously, the AlbSAuPEt₃ moiety is reversibly transformed into a new species characterized by a resonance at 36 ppm and assigned as AlbS(AuPEt₃)₂⁺.