A Roadmap to the Structure-Related Metabolism Pathways of Per- and Polyfluoroalkyl Substances in the Early Life Stages of Zebrafish ( Danio rerio )

Abstract

BACKGROUND: Thousands of per-and polyfluoroalkyl substances (PFAS) with diverse structures have been detected in the ambient environment. Apart from a few well-studied PFAS, the structure-related toxicokinetics of a broader set of PFAS remain unclear. OBJECTIVES: To understand the toxicokinetics of PFAS, we attempted to characterize the metabolism pathways of 74 structurally diverse PFAS samples from the U.S. Environmental Protection Agency's PFAS screening library. METHODS: Using the early life stages of zebrafish (Danio rerio) as a model, we determined the bioconcentration factors and phenotypic toxicities of 74 PFAS. Then, we applied high-resolution mass spectrometry-based nontargeted analysis to identify metabolites of PFAS in zebrafish larvae after 5 d of exposure by incorporating retention time and mass spectra. In vitro enzymatic activity experiments with human recombinant liver carboxylesterase (hCES1) were employed to validate the structure-related hydrolysis of 11 selected PFAS. RESULTS: Our findings identified five structural categories of PFAS prone to metabolism. The metabolism pathways of PFAS were highly related to their structures as exemplified by fluorotelomer alcohols that the predominance of b-oxidation or taurine conjugation pathways were primarily determined by the number of hydrocarbons. Hydrolysis was identified as a major metabolism pathway for diverse PFAS, and perfluoroalkyl carboxamides showed the highest in vivo hydrolysis rates, followed by carboxyesters and sulfonamides. The hydrolysis of PFAS was verified with recombinant hCES1, with strong substrate preferences toward perfluoroalkyl carboxamides. CONCLUSIONS:We suggest that the roadmap of the structure-related metabolism pathways of PFAS established in this study would provide a starting point to inform the potential health risks of other PFAS.

Invited Perspective: PFAS Bioconcentration and Biotransformation in Early Life Stage Zebrafish and Its Implications for Human Health Protection

“…Of the 36 evaluated PFAS, 28 accumulate in zebrafish larvae (Han et al 2021). Biotransformation, saturation, and column retention time, but not hydrophobicity (i.e., log K ow ), influence variation in PFAS accumulation (Han et al 2021). Interestingly, experimentally determined retention times, used as a proxy measurement for hydrophobicity, correlate well with PFAS accumulation and substantially outperformed widely used structure-based hydrophobicity predictions (Han et al 2021).…”

“…Of the 36 evaluated PFAS, 28 accumulate in zebrafish larvae (Han et al 2021). Biotransformation, saturation, and column retention time, but not hydrophobicity (i.e., log K ow ), influence variation in PFAS accumulation (Han et al 2021).…”

“…Biotransformation, saturation, and column retention time, but not hydrophobicity (i.e., log K ow ), influence variation in PFAS accumulation (Han et al 2021). Interestingly, experimentally determined retention times, used as a proxy measurement for hydrophobicity, correlate well with PFAS accumulation and substantially outperformed widely used structure-based hydrophobicity predictions (Han et al 2021). To place BCF data from Han et al (2021) in the context of previously published work, we compared PFAS accumulation data between seven early-life zebrafish and adult fish studies for a handful of well-studied PFAS, including PFOS, PFHxS, and PFOA (Figure 1 and Table 1).…”

“…In this issue of Environmental Health Perspectives, Han et al (2021) determined the bioconcentration factors and biotransformation potentials of 36 PFAS in the alternative early life stage zebrafish model. Bioconcentration factors (BCFs) are ratios between internal PFAS concentrations in whole body of 5-d zebrafish and PFAS exposure concentrations and therefore serve as an aggregate measure for key toxicokinetic processes: absorption, distribution, metabolism (i.e., biotransformation), and elimination, collectively referred to as ADME.…”

“…These same toxicokinetic principals orchestrate PFAS concentrations detected in specific biological matrices such as serum, blood plasma, and urine and therefore universally dictate PFAS accumulation across all organisms, including humans. In addition to measuring parent PFAS in 5-d zebrafish, Han et al (2021) performed rapid toxicokinetic profiling via nontarget analysis to identify biotransformation products. The study demonstrates that many PFAS accumulate in zebrafish and undergo biotransformation into a wide range of structurally diverse products, including those that persist for extensive periods of time in humans and the environment.…”