Species Difference? Bovine, Trout, and Human Plasma Protein Binding of Per- and Polyfluoroalkyl Substances

Abstract

Per- and polyfluoroalkyl substances (PFAS) strongly bind to proteins and lipids in blood, which govern their accumulation and distribution in organisms. Understanding the plasma binding mechanism and species differences will facilitate the quantitative in vitro-to-in vivo extrapolation and improve risk assessment of PFAS. We studied the binding mechanism of 16 PFAS to bovine serum albumin (BSA), trout, and human plasma using solid-phase microextraction. Binding of anionic PFAS to BSA and human plasma was found to be highly concentration-dependent, while trout plasma binding was linear for the majority of the tested PFAS. At a molar ratio of PFAS to protein ν < 0.1 mol_PFAS/mol_protein, the specific protein binding of anionic PFAS dominated their human plasma binding. This would be the scenario for physiological conditions (ν < 0.01), whereas in in vitro assays, PFAS are often dosed in excess (ν > 1) and nonspecific binding becomes dominant. BSA was shown to serve as a good surrogate for human plasma. As trout plasma contains more lipids, the nonspecific binding to lipids affected the affinities of PFAS for trout plasma. Mass balance models that are parameterized with the protein-water and lipid-water partitioning constants (chemical characteristics), as well as the protein and lipid contents of the plasma (species characteristics), were successfully used to predict the binding to human and trout plasma.

Keywords: PFAS; plasma binding mechanism; proteins and lipids; solid-phase microextraction; specific and nonspecific protein binding.

Figure 1

Bovine serum albumin (BSA) binding of (a–d) HFPO–DA and (e–h) PFNA. (a, e) Data points were fitted linearly with the Freundlich-type model (eq 8, solid line); the dotted line refers to fixed n_Fr = 1 for comparison. (b, f) The concentration-dependent distribution ratios between BSA and water, log D_BSA/w, were fitted linearly (eq 11). (c, g) Experimental data points were fitted nonlinearly with the combined binding/partition model (eq 15). (d, h) The saturable specific binding in the low concentration range was derived with eq 13. Results of this study were compared with literature data^, (green triangle and crosses).

Figure 2

Human plasma (HP) and trout plasma (TP) binding isotherms of 11 anionic PFAS. Curves were fitted linearly with the Freundlich-type model (eq 8) or nonlinearly with the combined binding/partitioning model (eq 16). The selection of models was based on whether the binding isotherms were concentration-dependent (Table 1).

Figure 3

Nonspecific or average BSA binding, log D_BSA/w (pH = 7.4) of perfluoroalkyl carboxylic acids (PFCAs, magenta circle), sulfonic acids (PFSAs, blue diamond), and average BSA binding of fluorotelomer alcohols (FTOHs, gold triangle). log D_BSA/w of HFPO–DA (empty circle), 6:2 FTSA, and PFOSA (empty diamond) were excluded from the regression but plotted for comparison.

Figure 4

Prediction of plasma–water distribution ratios, log D_plasma/w (pH = 7.4), of 16 PFAS. log D_plasma/w of (a) human plasma or (b) trout plasma were measured experimentally (Exp) and compared with the D_plasma/w predicted by a mass balance model (MBM) from protein binding constants, log D_BAS/w (pH = 7.4) and lipid binding constants, log D_lip/w and log D_oil/w, as well as the volume fractions of proteins and lipids in plasmas (eq 23).