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Concentration dependence of in vitro biotransformation rates of hydrophobic organic sunscreen agents in rainbow trout S9: Implications for bioaccumulation assessment
Citation:
Saunders, L., S. Fontanay, J. Nichols, AND F. Gobas. Concentration dependence of in vitro biotransformation rates of hydrophobic organic sunscreen agents in rainbow trout S9: Implications for bioaccumulation assessment. ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. Society of Environmental Toxicology and Chemistry, Pensacola, FL, 38(3):548-560, (2019). https://doi.org/10.1002/etc.4342
Impact/Purpose:
In-vitro biotransformation studies were performed to support the bioaccumulation assessment of three hydrophobic organic ultraviolet filters (UVFs), 4-methylbenzylidene camphor (4-MBC), 2-ethylhexyl-4-methoxycinnamate (EHMC), and octocrylene (OCT). In-vitro depletion rate constants (kdep) were determined for each UVF in rainbow trout liver S9 fractions. The concentration dependence of in vitro activity was evaluated to estimate Michaelis-Menten parameters (Km and Vmax) for each UVF. Measured kdep values were then extrapolated to apparent whole-body biotransformation rate constants using an in-vitro to in-vivo (IVIVE) extrapolation model. Bioconcentration factors (BCFs) calculated from kdep measured at substrate concentrations below Km were closer to empirical BCFs than those extrapolated from kdep measured at higher test concentrations. However, modeled BCFs were also sensitive to different in vitro binding assumptions employed in the IVIVE model, highlighting the need for further characterization of chemical binding effects on hepatic clearance. This study suggests that bioaccumulation (BCF = 2000 L kg-1) of these UVFs is unlikely, but consideration of appropriate in-vitro test concentrations and binding correction factors are important when using IVIVE methods to refine modeled BCFs. In vitro methods are being used with increasing frequency to provide information needed by regulators to perform chemical assessments under TSCA (F. Lautenberg Chemical Safety for the 21st Century Act) and FIFRA. The results of this study provide important guidance on performance of a biotransformation assay (trout S9) that is being used with increasing frequency to measure chemical biotransformation in fish.
Description:
In-vitro biotransformation studies were performed to support the bioaccumulation assessment of three hydrophobic organic ultraviolet filters (UVFs), 4-methylbenzylidene camphor (4-MBC), 2-ethylhexyl-4-methoxycinnamate (EHMC), and octocrylene (OCT). In-vitro depletion rate constants (kdep) were determined for each UVF in rainbow trout liver S9 fractions. Incubations performed with and without added cofactors showed complete (4-MBC) or partial (EHMC and OCT) dependence of activity on NADPH, suggesting that hydrolysis of EHMC and OCT by cofactor-independent enzymes (e.g. carboxylesterases) is an important metabolic route. The concentration dependence of in vitro activity was evaluated to estimate Michaelis-Menten parameters (Km and Vmax) for each UVF. Measured kdep values were then extrapolated to apparent whole-body biotransformation rate constants using an in-vitro to in-vivo (IVIVE) extrapolation model. Bioconcentration factors (BCF) calculated from kdep measured at concentrations well below Km were closer to empirical BCFs than those extrapolated from kdep measured at higher test concentrations. Modeled BCFs were also sensitive to different in vitro binding assumptions employed in the IVIVE model, highlighting the need for further characterization of chemical binding effects on hepatic clearance. This study suggests that bioaccumulation (BCF = 2000 L kg-1) of these UVFs is unlikely, but consideration of appropriate in-vitro test concentrations and binding correction factors are important when using IVIVE methods to refine modeled BCFs.
URLs/Downloads:
DOI: Concentration dependence of in vitro biotransformation rates of hydrophobic organic sunscreen agents in rainbow trout S9: Implications for bioaccumulation assessment