Reliable partitioning data are essential for assessing the bioaccumulation potential and the toxicity of chemicals. In contrast to neutral organic chemicals, the partitioning behavior of ionogenic ...organic chemicals (IOCs) is still a black box for environmental scientists. Partitioning to serum albumin, the major protein in blood plasma, strongly influences the freely dissolved concentration of many chemicals (including IOCs), which affects their transport and distribution in the body. Because consistent data sets for partitioning of IOCs are rarely available, bovine serum albumin-water partition coefficients (K BSA/w) were measured in this study for 45 anionic and 4 cationic organic chemicals, including various substituted benzoic and naphthoic acids, sulfonates and several pesticides and pharmaceuticals. The results of this study suggest that binding to BSA is substantially influenced by the three-dimensional structure of the chemicals and the position of substitutions on the sorbing molecules. For example, we found a difference of >1.5 log units between isomeric chemicals such as 3,4-dichlorobenzoic acid and 2,6-dichlorobenzoic acid, and 1-naphthoic acid and 2-naphthoic acid. Conventional modeling approaches (e.g., based on octanol–water partition coefficients) poorly predict log K BSA/w of organic ions (R 2 ≤ 0.5), partially because they do not capture the observed steric effects. Hence, alternative modeling strategies will be required for accurate prediction of serum albumin-water partition coefficients of organic ions.
The in vivo partitioning behavior of ionogenic organic chemicals (IOCs) is of paramount importance for their toxicokinetics and bioaccumulation. Among other proteins, structural proteins including ...muscle proteins could be an important sorption phase for IOCs, because of their high quantity in the human and other animals’ body and their polar nature. Binding data for IOCs to structural proteins are, however, severely limited. Therefore, in this study muscle protein–water partition coefficients (K MP/w) of 51 systematically selected organic anions and cations were determined experimentally. A comparison of the measured K MP/w with bovine serum albumin (BSA)–water partition coefficients showed that anionic chemicals sorb more strongly to BSA than to muscle protein (by up to 3.5 orders of magnitude), while cations sorb similarly to both proteins. Sorption isotherms of selected IOCs to muscle protein are linear (i.e., K MP/w is concentration independent), and K MP/w is only marginally influenced by pH value and salt concentration. Using the obtained data set of K MP/w a polyparameter linear free energy relationship (PP-LFER) model was established. The derived equation fits the data well (R 2 = 0.89, RMSE = 0.29). Finally, it was demonstrated that the in vitro measured K MP/w values of this study have the potential to be used to evaluate tissue-plasma partitioning of IOCs in vivo.
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•C18-SPME versatile method to measure PFAS binding to proteins, cells and plasma.•Anionic PFAS exhibit nonlinear protein binding with one high affinity binding site.•Nonspecific ...binding to proteins and membrane lipids very similar.•Concentration-dependent mass balance model explains binding to cells and plasma.•Concentrations in human plasma at levels where sorption is specific and very strong.
Perfluoroalkyl substances (PFAS) are persistent and pose a risk to human health. High throughput screening (HTS) cell-based bioassays may inform risk assessment of PFAS provided that quantitative in vitro to in vivo extrapolation (QIVIVE) can be developed. The QIVIVE ratio is the ratio of nominal (Cnom) or freely dissolved concentration (Cfree) in human blood to Cnom or Cfree in the bioassays. Considering that the concentrations of PFAS in human plasma and in vitro bioassays may vary by orders of magnitude, we tested the hypothesis that anionic PFAS bind to proteins concentration-dependently and therefore the binding differs substantially between human plasma and bioassays, which has an impact on QIVIVE. Solid phase microextraction (SPME) with C18-coated fibers served to quantify the Cfree of four anionic PFAS (perfluorobutanoate (PFBA), perfluorooctanoate (PFOA), perfluorohexane sulfonate (PFHxS) and perfluorooctane sulfonate (PFOS)) in the presence of proteins and lipid, medium components, cells and human plasma over five orders of magnitude in concentrations. The C18-SPME method was used to quantify the non-linear binding to proteins, human plasma and medium, and the partition constants to cells. These binding parameters were used to predict Cfree of PFAS in cell bioassays and human plasma by a concentration-dependent mass balance model (MBM). The approach was illustrated with a reporter gene assay indicating activation of the peroxisome proliferator-activated receptor gamma (PPARγ-GeneBLAzer). Blood plasma levels were collected from literature for occupational exposure and the general population. The QIVIVEnom ratios were higher than the QIVIVEfree ratios due to the strong affinity to proteins and large differences in protein contents between human blood and bioassays. For human health risk assessment, the QIVIVEfree ratios of many in vitro assays need to be combined to cover all health relevant endpoints. If Cfree cannot be measured, they can be estimated with the MBM and concentration-dependent distribution ratios.
High-throughput in vitro bioassays are becoming increasingly important in the risk characterization of anthropogenic chemicals. Large databases gather nominal effect concentrations (C nom) for ...diverse modes of action. However, the biologically effective concentration can substantially deviate due to differences in chemical partitioning. In this study, we modeled freely dissolved (C free), cellular (C cell), and membrane concentrations (C mem) in the Tox21 GeneBLAzer bioassays for a set of neutral and ionogenic organic chemicals covering a large physicochemical space. Cells and medium constituents were experimentally characterized for their lipid and protein content, and partition constants were either collected from the literature or predicted by mechanistic models. The chemicals exhibited multifaceted partitioning to proteins and lipids with distribution ratios spanning over 8 orders of magnitude. Modeled C free deviated over 5 orders of magnitude from C nom and can be compared to in vivo effect data, environmental concentrations, and the unbound fraction in plasma, which is needed for the in vitro to in vivo extrapolation. C cell was relatively constant for chemicals with membrane lipid–water distribution ratios of 1000 or higher and proportional to C nom. Representing a sum parameter for exposure that integrates the entire dose from intracellular partitioning, C cell is particularly suitable for the effect characterization of chemicals with multiple target sites and the calculation of their relative effect potencies. Effective membrane concentrations indicated that the specific effects of very hydrophobic chemicals in multiple bioassays are occurring at concentrations close to baseline toxicity. The equilibrium partitioning model including all relevant system parameters and a generic bioassay setup is attached as an excel workbook to this paper and can readily be applied to diverse in vitro bioassays.
Sorption to the polystyrene (PS) of multiwell plates can affect the exposure to organic chemicals over time in in vitro and in vivo bioassays. Experimentally determined diffusion coefficients in PS ...(D PS) were in a narrow range of 1.25 to 8.0 · 10–16 m2 s–1 and PS-water partition constants (K PS/w) ranged from 0.04 to 5.10 log-units for 22 neutral organic chemicals. A kinetic model, which explicitly accounts for diffusion in the plastic, was applied to predict the depletion of neutral organic chemicals from different bioassay media by sorption to various multiwell plate formats. For chemicals with log K ow > 3, the medium concentrations decreased rapidly and considerably in the fish embryo toxicity assay but medium concentrations remained relatively constant in the cell-based bioassays with medium containing 10% fetal bovine serum (FBS), emphasizing the ability of the protein- and lipid-rich medium to compensate for losses by multiwell plate sorption. The PS sorption data may serve not only for exposure assessment in bioassays but also to model the contaminant uptake by and release from plastic packaging material and the chemical transport by PS particles in the environment.
Controlling the exposure of chemicals in in vitro mammalian cell assays is an important prerequisite for the application of in vitro methods in risk and hazard assessment of chemicals. Existing ...models require numerous physicochemical and system parameters to quantify the effective concentration in the assay. Synthesizing these studies, this article briefly communicates how the protein-rich supplement in the medium can be utilized to adjust constant and quantifiable exposure concentrations without the need for measurements and complex modeling. We present a simplified mass balance equation based on chemical properties and system parameters from openly accessible databases, which can be used to adjust the dose of chemicals in the exposure medium, leading to defined and stable freely dissolved concentrations (C free). The proposed framework prevents experimental artifacts associated with the use of cosolvents and medium oversaturation and enables the conversion of in vitro effect data to freely dissolved effect concentrations (ECfree), which can directly be applied in quantitative in vitro to in vivo extrapolation models and compared to other exposure scenarios.
Suspended particulate matter (SPM) plays an important role in the fate of organic micropollutants in rivers during rain events, when sediments are remobilized and turbid runoff components enter the ...rivers. Under baseflow conditions, the SPM concentration is low and the contribution of SPM-bound contaminants to the overall risk of organic contaminants in rivers is assumed to be negligible. To challenge this assumption, we explored if SPM may act as a source or sink for all or specific groups of organic chemicals in a small river. The concentrations of over 600 contaminants and the mixture effects stemming from all chemicals in in vitro bioassays were measured for river water, SPM, and the surface sediment after solid-phase extraction or exhaustive solvent extraction. The bioavailable fractions of chemicals and mixture effects were estimated after passive equilibrium sampling of enriched SPM slurries and sediments in the lab. Dissolved compounds dominated the total chemical burden in the water column (water plus SPM) of the river, whereas SPM-bound chemicals contributed up to 46% of the effect burden even if the SPM concentration in rivers was merely 1 mg/L. The equilibrium between water and SPM was still not reached under low-flow conditions with SPM as a source of water contamination. The ratios of SPM-associated to sediment-associated neutral and hydrophobic chemicals as well as the ratios of the mixture effects expressed as bioanalytical equivalent concentrations were close to 1, suggesting that the surface sediment can be used as a proxy for SPM under baseflow conditions when the sampling of a large amount of water to obtain sufficient SPM cannot be realized.
The freely dissolved concentration in the assay medium (C free) and the total cellular concentration (C cell) are essential input parameters for quantitative in vitro-to-in vivo extrapolations ...(QIVIVE), but available prediction tools for C free and C cell have not been sufficiently validated with experimental data. In this study, medium–water distribution ratios (D FBS/w) and cell–water distribution ratios (D cell/w) for four different cells lines were determined experimentally for 12 neutral and five ionizable chemicals. Literature data for seven organic acids were added to the dataset, leading to 24 chemicals in total. A mass balance model based on bovine serum albumin–water (D BSA/w) and liposome–water distribution ratios (D lip/w) of the chemicals was used to calculate D FBS/w and D cell/w. For all neutral and basic test chemicals, the mass balance model predicted D FBS/w and D cell/w within a factor of 3 and 3.4, respectively, indicating that existing models can reliably predict C free and C cell for these chemicals. For organic acids, a further refinement of the model will be required as large deviations between modeled and measured binding to assay medium and cells of up to a factor of 370 were found. Furthermore, saturation of medium proteins should be further explored for organic acids and neutral chemicals with moderate hydrophobicity.
The effects measured with in vitro cell-based bioassays are typically reported as nominal effect concentrations (C nom), but the freely dissolved concentration in the exposure medium (C w) and the ...total cellular concentration (C cell) are considered more quantitative dose metrics that allow extrapolation to the whole-organism level. To predict C w and C cell, the partitioning of the test chemicals to medium proteins and lipids and cells has to be known. In this study, we developed a solid-phase microextraction (SPME) method based on C18-coated fibers to quantify the partitioning of diclofenac, 2,4-dichlorophenoxyacetic acid (2,4-D), ibuprofen, naproxen, torasemide, warfarin, and genistein to bovine serum albumin (BSA), phospholipid liposomes, fetal bovine serum (FBS), and cells. For ibuprofen, 2,4-D, naproxen, and warfarin, the partitioning to the SPME fibers was found to be concentration dependent, which had to be considered for the calculation of distribution ratios to biological materials. The sorption isotherms to FBS were nonlinear for diclofenac, 2,4-D, ibuprofen, naproxen, and warfarin. The FBS isotherms could be described by assuming that the total amount of chemical bound to FBS is the sum of the amount specifically bound to the binding sites of albumin and nonspecifically bound to all medium proteins and lipids. The determined cell-water distribution ratios (D cell/w) differed considerably between four different cell lines (up to 1.83 log-units) and also between different batches of the same cell line (up to 0.48 log-units). The relative importance of protein and lipid content for D cell/w was evaluated with a mass balance model and different types of cellular proteins and lipids as input parameters. Existing in vitro mass balance models may underestimate C w because they do not account for saturable protein binding and overestimate C cell for organic acids, if BSA is used as surrogate for cellular proteins.
Cellular uptake kinetics are key for understanding time-dependent chemical exposure in in vitro cell assays. Slow cellular uptake kinetics in relation to the total exposure time can considerably ...reduce the biologically effective dose. In this study, fluorescence microscopy combined with automated image analysis was applied for time-resolved quantification of cellular uptake of 10 neutral, anionic, cationic, and zwitterionic fluorophores in two reporter gene assays. The chemical fluorescence in the medium remained relatively constant during the 24-h assay duration, emphasizing that the proteins and lipids in the fetal bovine serum (FBS) supplemented to the assay medium represent a large reservoir of reversibly bound chemicals with the potential to compensate for chemical depletion by cell uptake, growth, and sorption to well materials. Hence FBS plays a role in stabilizing the cellular dose in a similar way as polymer-based passive dosing, here we term this process as serum-mediated passive dosing (SMPD). Neutral chemicals accumulated in the cells up to 12 times faster than charged chemicals. Increasing medium FBS concentrations accelerated uptake due to FBS-facilitated transport but led to lower cellular concentrations as a result of increased sorption to medium proteins and lipids. In vitro cell exposure results from the interaction of several extra- and intracellular processes, leading to variable and time-dependent exposure between different chemicals and assay setups. The medium FBS plays a crucial role for the thermodynamic equilibria as well as for the cellular uptake kinetics, hence influencing exposure. However, quantification of cellular exposure by an area under the curve (AUC) analysis illustrated that, for the evaluated bioassay setup, current in vitro exposure models that assume instantaneous equilibrium between medium and cells still reflect a realistic exposure because the AUC was typically reduced less than 20% compared to the cellular dose that would result from instantaneous equilibrium.
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