Experimental characterization of the mechanism of perfluorocarboxylic acids' liver protein bioaccumulation: The key role of the neutral species

Abstract

Perfluorocarboxylic acids (PFCAs) of chain length greater than seven carbon atoms bioconcentrate in the livers of fish. However, a mechanistic cause for the empirically observed increase in the bioconcentration potential of PFCAs as a function of chain length has yet to be determined. To this end, recombinant rat liver fatty acid-binding protein (L-FABP) was purified, and its interaction with PFCAs was characterized in an aqueous system at pH 7.4. Relative binding affinities of L-FABP with PFCAs of carbon chain lengths of five to nine were established fluorimetrically. The energetics, mechanism, and stoichiometry of the interaction of perfluorooctanoic acid (PFOA) with L-FABP were examined further by isothermal titration calorimetry (ITC) and electrospray ionization combined with tandem mass spectrometry (ESI-MS/MS). Perfluorooctanoic acid was shown to bind to L-FABP with an affinity approximately an order of magnitude less than the natural ligand, oleic acid, and to have at least 3:1 PFOA:L-FABP stoichiometry. Two distinct modes of PFOA binding to L-FABP were observed by ESI-MS/MS analysis; in both cases, PFOA binds solely as the neutral species under typical physiological pH and aqueous concentrations of the anion. A comparison of their chemical and physical properties with other well-studied biologically relevant chemicals showed that accumulation of PFCAs in proteins as the neutral species is predictable. For example, the interaction of PFOA with L-FABP is almost identical to that of the acidic ionizing drugs ketolac, ibuprofen, and warfarin that show specificity to protein partitioning with a magnitude that is proportional to the K(OW) (octanol-water partitioning) of the neutral species. The experimental results suggest that routine pharmacochemical models may be applicable to predicting the protein-based bioaccumulation of long-chain PFCAs.

Equilibrium modeling: A pathway to understanding observed perfluorocarboxylic and perfluorosulfonic acid behavior

“…The observed uniqueness of PFC(A) physical properties is in the properties of the anionic species, not the neutral species. Thus, the agreement between observed tissue-water distributions and the standard BCF model applied to PFCAs could have been anticipated from recent experimental results that are consistent with the neutral species binding with proteins [31,43,52], and with earlier speculation to the same effect [43,53]. The D SW calculated by Equation 2 with an assumed typical sediment solids density of 2.4 kg/L [54,55] allowed the direct comparison of model results with all measured sediment-water distributions reported by Higgins and Luthy [37] in consistent units of L/kg.…”

“…We note that perfluorooctane sulfonamide is, unfortunately, elsewhere in the literature denoted by the acronym ''PFOSA''. However, in the absence of a standard convention for acronyms that distinguish between the acid and the anion, we follow the convention established for the PFC(A)s [20,42,43], that is, perfluorooctane sulfonic acid is designated as PFOSA, the sulfonate is exclusively designated as PFOS, and the pair, or when the identity of the species is unknown, is referred to as PFOS(A).…”

Activation Instead of Inhibition: Targeting Proenzymes for Small‐Molecule Intervention

“…The observed uniqueness of PFC(A) physical properties is in the properties of the anionic species, not the neutral species. Thus, the agreement between observed tissue–water distributions and the standard BCF model applied to PFCAs could have been anticipated from recent experimental results that are consistent with the neutral species binding with proteins 31, 43, 52, and with earlier speculation to the same effect 43, 53.…”

Bioavailability of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) in biosolids-amended soils to earthworms (Eisenia fetida)

“…The fluorinated carbon length of PFOS was longer than that of PFOA. In addition, the binding affinity by sulfonate was higher than carboxylate with the same fluorinated perfluorinated chain (Martin et al, 2003;Woodcroft et al, 2010;Liu et al, 2011;Zhao et al, 2013).…”