Reactions of Trimethylphosphine Analogues of Auranofin with Bovine Serum Albumin

The reactions of bovine serum albumin (BSA) with (trimethylphosphine)(2,3,4,6-tetra-O-acetyl-1-thio-β-D-glucopyranosato-S)-gold(I), Me₃PAuSAtg, and its chloro analogue, Me₃zPAuCl, were studied to develop insights into the role of the phosphine ligand the serum chemistry of the related antiarthritic drug auranofin ((triethylphosphine)(2,3,4,6-tetra-O-acetyl-1 -thio-β-D-glucopyranosato- S)gold(I)). ³¹P NMR spectroscopy, protein modification, and gel-exclusion chromatography methods were employed. Comparison of the reactions of the methyl derivatives to the previously reported reactions of auranofin and Et₃PAuCl with BSA demonstrated that similar chemical species are formed but revealed three major differences: (1) The smaller and less basic trimethylphosphine is displaced to form the corresponding phosphine oxide (Me₃PO) more readily than is triethylphosphine. (2) (Trimethylphosphine)gold(I) chloride forms the bis((trimethylphosphine)gold(I))-μ-thiolato complex, Alb-S(AuPMe₃)₂ ⁺, at cysteine-34 in the albumin crevice more readily than (triethylphosphine)gold(I) chloride forms Alb-S(AuPEt₃)₂ ⁺. (3) The multiple weak binding sites for R₃PAu⁺ are more easily resolved for the methyl derivative than for the ethyl derivative. Despite these differences, the results for the methyl analogues provide important confirmation for previously developed chemical models of auranofin reactions in serum. Me₃PO was not observed in reaction mixtures lacking tetraacetylthioglucose (AtgSH); this result affirms the role of AtgSH, displaced by the reaction of Me₃PAuSAtg at Cys-34, in the generation of the phosphine oxide (an important metabolite in vivo). The weak binding sites on albumin react with Me₃zPAuCl, but not Me₃PAuSAtg, demonstrating importance of the strength and reactivity of the anionic ligand-gold bond on the reactions of auranofin analogues. The gold binding capacity of albumin is enhanced after Me₃PO is formed, consistent with the reductive cleavage of albumin disulfide bonds by trimethylphosphine.