(265f) Binding of Perfluoroalkyl Substances (PFAS) to Serum Albumins: A Comparison of Human and Bovine Serum Albumins

The biodistribution, bioaccumulation, and pharmacokinetics of poly- and perfluoroalkyl substances (PFAS) are all related in part to how strongly they bind to proteins. Although protein binding is vital to environmental and pharmaceutical models, current techniques to obtain protein binding constants for PFAS are varied and analytical techniques are not consistent. In addition, bovine serum albumin (BSA) has been proposed as an experimental substitute for human serum albumin (HSA) in numerous studies but a thorough comparison of the two has not yet been performed. The goal of this work was to compare PFAS binding to BSA with that of HSA using an array of experimental techniques.

Although BSA and HSA show a high degree of homology, one of the relevant differences is the number of intrinsic fluorescent amino acid residues- namely tryptophan. Within the hydrophobic binding pocket of BSA is a tryptophan residue that is monitored through fluorescence quenching as PFAS bind in that pocket. HSA does not have this same residue in the same location so the results from fluorescence quenching experiments differ. Thus, the use of additional experimental techniques is required to determine the strength of this binding interaction, as presented in this work. Multiple straight chain perfluorocarboxylates (C₆-C₁₀) as well as the replacement compound perfluoro(2-methyl-3-oxahexanoic) acid (Gen X) were examined to extrapolate beyond the legacy compounds perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). We established binding relationships for PFAS chain length and temperature, and showed that protein conformational changes play a clear role in determining the extent of entropic binding. Not only was specific binding within hydrophobic pockets of BSA investigated but so too was adsorption behavior of PFASs onto the protein surface. We then established relationships between octanol-water partitioning coefficient, an indicator of hydrophobicity, and extent of binding to aid in prediction tools for the wide range of PFAS compounds, derivatives, and precursors. Further, by employing C₆-C₁₀ hydrocarbon analogs, it was discovered that binding of PFASs is mechanistically different and much stronger than that of the hydrocarbons.