Impact of protein binding on the availability and cytotoxic potency of organochlorine pesticides and chlorophenols in vitro

The transport of DDT from chylomicrons to adipocytes does not mimic triacylglycerol transport

Despite being banned in the U.S., organochlorine toxins such as DDT are frequently detected in human adipose tissue. The main route of exposure is through the consumption of contaminated foods and subsequent intestinal packaging of DDT into chylomicrons. These chylomicrons, which also contain dietary triacylglycerol (TG), are delivered directly to peripheral tissues without first being metabolized by the liver. The physiological process by which these compounds are delivered from chylomicrons to adipose is not well understood, but is clinically relevant since it bypasses first-pass metabolism. Based on its highly lipophilic nature, it has been assumed that DDT is transferred to peripheral tissues similar to TG; however, this has not been measured. Here, we use the lymph fistula rat to isolate chylomicrons containing both DDT and TG. These chylomicrons are the in vivo DDT delivery vehicle. Using 3T3-L1 adipocytes, we investigated the rate at which DDT transfers from chylomicrons to adipocytes, and mediators of this process. This novel approach closely approximates the in vivo DDT exposure route. We show that: 1) DDT repartitions from chylomicrons to adipocytes, 2) this transport does not require hydrolysis of TG within the chylomicron, and is stimulated by the inhibition of LPL, 3) albumin does not inhibit DDT uptake, 4) DDT dissolved in DMSO does not appropriately mimic in vivo DDT transport; and most importantly, 5) DDT uptake from chylomicrons does not mimic the uptake of TG from the same particles. Understanding these factors is important for designing interventions for human populations exposed to DDT.

Multiple spectroscopic studies on the interaction between olaquindox, a feed additive, and bovine serum albumin

The interaction between olaquindox (OLA) and bovine serum albumin (BSA) was investigated using fluorescence, UV-vis absorption and circular dichroism (CD) spectroscopy. The results showed that the fluorescence quenching of BSA by OLA was a static quenching process induced by the formation of OLA-BSA complex. The binding constant of OLA-BSA complex was calculated to be 1.299 × 10(4)L mol(-1) (293K). The negative values of ΔH(0) and ΔS(0) indicated that hydrogen bond and van der Waals interactions played major roles in stabilizing the complex. Site probe competition experiments and number of binding sites (n) revealed that OLA could bind to site I in subdomain IIA of BSA, and the binding distance (r) was evaluated to be 3.643 nm according to Förster's non-radiative energy transfer theory. The results of CD and three-dimensional fluorescence spectra suggested some conformational changes of BSA after OLA binding.

In vitro investigation of the interaction between pentachlorophenol and alkaline phosphatase by spectroscopic methods

The interaction characteristics of alkaline phosphatase (ALP) with pentachlorophenol (PCP) were investigated using fluorescence, UV-vis absorption, and circular dichroism (CD) techniques. Results obtained from analysis of fluorescence intensity indicated that PCP has a strong ability to quench the intrinsic fluorescence of ALP through a static quenching procedure. The thermodynamic parameters ΔH and ΔS were observed to be -4.60kJmol(-1) and 54.59Jmol(-1)K(-1), respectively, and the value of ΔG was negative. These results indicate that the binding reactions were spontaneous, and both hydrophobic and electrostatic forces were involved in the interaction of PCP and ALP. Based on Forster's theory of non-radiation energy transfer, the binding distance, r, between the ALP and PCP was evaluated to be 2.50nm and the critical distance R(0) was 2.26nm. The CD spectra results showed that the α-helicity was decreased from 49.68% in native ALP to 47.28% in PCP-ALP systems, which indicate the secondary structure of ALP was changed slightly in the presence of PCP.

Quantifying processes determining the free concentration of phenanthrene in Basal cytotoxicity assays

Difficulties may arise when extrapolating in vitro derived toxicity data to in vivo toxicity data because of the high variability and occasional low sensitivity of in vitro results. Differences in the free concentration of a test compound between in vitro and in vivo systems and between different in vitro systems may in part explain this variability and sensitivity difference. The aim of this study was to determine what assay components influence the free concentration of phenanthrene in a Balb/c 3T3 and RTgill-W1 MTT assay. Partition coefficients of phenanthrene to serum, well plate plastic, cells, and headspace were measured and subsequently used to model the free concentration of the compound in vitro. The estimated free concentration was compared to the free concentration measured in the assays using solid phase microextraction (SPME). Results indicate that the free concentration of phenanthrene, a relatively volatile and hydrophobic compound, is significantly reduced in a typical in vitro setup as it binds to matrices such as serum protein and well plate plastic. A reduction in free concentration due to increasing serum protein levels is accompanied by an increase in the median effect concentration (EC(50)) and can be modeled, with the exception of evaporation, using the partition coefficients of the compound to assay components.

The interaction between 4-aminoantipyrine and bovine serum albumin: multiple spectroscopic and molecular docking investigations

4-Aminoantipyrine (AAP) is widely used in the pharmaceutical industry, in biochemical experiments and in environmental monitoring. AAP as an aromatic pollutant in the environment poses a great threat to human health. To evaluate the toxicity of AAP at the protein level, the effects of AAP on bovine serum albumin (BSA) were investigated by multiple spectroscopic techniques and molecular modeling. After the inner filter effect was eliminated, the experimental results showed that AAP effectively quenched the intrinsic fluorescence of BSA via static quenching. The number of binding sites, the binding constant, the thermodynamic parameters and binding subdomain were measured, and indicated that AAP could spontaneously bind with BSA on subdomain IIIA through electrostatic forces. Molecular docking results revealed that AAP interacted with the Glu 488 and Glu 502 residues of BSA. Furthermore, the conformation of BSA was demonstrably changed in the presence of AAP. The skeletal structure of BSA loosened, exposing internal hydrophobic aromatic ring amino acids and peptide strands to the solution.

The metabolism and analysis of isoflavones and other dietary polyphenols in foods and biological systems

Polyphenols in dietary and botanical matrices are usually present as simple and complex O-glycosides. In fermented dietary materials, the glycosidic moiety is removed and accompanied in some cases by more complex changes to the polyphenol. As for most xenobiotics, polyphenols undergo phase II conjugation in the intestinal wall during their absorption from the gut. In contrast, a few polyphenols, such as puerarin in the kudzu vine, are C-glycosides and are stable in the gut and during absorption, distribution and excretion. Large bowel bacteria reduce polyphenol aglycones, causing opening of the heterocyclic B-ring and ring cleavage. The products are mostly absorbed and enter the bloodstream. Phase I and II metabolism events occur in the intestine and the liver - most polyphenols predominantly circulate as β-glucuronides and sulfate esters with very little as the aglycones, the presumed active forms. In addition, metabolism can occur in non-hepatic tissues and cells including breast tumor cells that have variable amounts of cytochrome P450s, sulfatase and sulfotransferase activities. Inflammatory cells produce chemical oxidants (HOCl, HOBr, ONO(2)(-)) that will react with polyphenols. The isoflavones daidzein and genistein and the flavonol quercetin form mono- and dichlorinated products in reaction with HOCl. Genistein is converted to 3'-nitrogenistein in the lung tissue of lipopolysaccharide-treated rats. Whereas polyphenols that can be converted to quinones or epoxides react with glutathione (GSH) to form adducts, chlorinated isoflavones do not react with GSH; instead, they are converted to β-glucuronides and are excreted in bile. Analysis of polyphenols and their metabolites is routinely carried out with great sensitivity, specificity and quantification by LC-tandem mass spectrometry. Critical questions about the absorption and tissue uptake of complex polyphenols such as the proanthocyanins can be answered by labeling these polyphenols with (14)C-sucrose in plant cell culture and then purifying them for use in animal experiments. The (14)C signature is quantified using accelerator mass spectrometry, a technique capable of detecting one (14)C atom in 10(15) carbon atoms. This permits the study of the penetration of the polyphenols into the interstitial fluid, the fluid that is actually in contact with non-vascular cells.

Effects of solvents and dosing procedure on chemical toxicity in cell-based in vitro assays

Due to the implementation of new legislation, such as REACh, a dramatic increase of animal use for toxicity testing is expected and the search for alternatives is timely. Cell-based in vitro assays are promising alternatives. However, the behavior of chemicals in these assays is still poorly understood. We set out to quantify the exposure and associated toxicity of chemicals with different physicochemical properties toward a fish gill cell line when different solvents and procedural steps are used to introduce test chemicals to cells. Three chemicals with a range of hydrophobicity and volatility were selected and delivered in three different solvents using two common dosing procedures. Toxicity tests were coupled with chemical analysis to quantify the chemical concentrations within culture wells. The impact of solvents and dosing procedure was greatest for the most volatile and hydrophobic test chemical. We show that certain combinations of the test chemical, solvent, and procedural steps can lead to inhomogeneous distribution of the test chemical and thus differing degrees of bioavailability, resulting in quantitative differences in apparent toxicity.

Impact of carbon chain length on binding of perfluoroalkyl acids to bovine serum albumin determined by spectroscopic methods

Perfluoroalkyl acids (PFAAs), an emerging class of globally environmental contaminants, pose a great threat to humans with wide exposure from food and other potential sources. To evaluate the toxicity of PFAAs at the protein level, the effects of three PFAAs on bovine serum albumin (BSA) were characterized by fluorescence spectroscopy, synchronous fluorescence spectroscopy, and circular dichroism (CD). On the basis of the fluorescence spectra and CD data, we concluded that perfluoropentanoic acid (PFPA) had little effect on BSA. However, perfluorooctanoic acid (PFOA) and perfluorodecanoic acid (PFDA) exhibited remarkable fluorescence quenching, which was attributed to the formation of a moderately strong complex. The enthalpy change (DeltaH) and entropy change (DeltaS) indicated that van der Waals forces and hydrogen bonds were the dominant intermolecular forces in the binding of PFAAs to BSA. Furthermore, the BSA conformation was slightly altered in the presence of PFOA and PFDA, with a reduction of alpha helix. These results indicated that PFAAs indeed impact the conformation of BSA, and PFAAs with longer carbon chains were more toxic, especially at lower concentrations.

Biokinetic modeling and in vitro-in vivo extrapolations

The introduction of in vitro methodologies in the toxicological risk assessment process requires a number of prerequisites regarding both the toxicodynamics and the biokinetics of the compounds under study. In vitro systems will need to be relevant for measuring those structural and physiological changes that are good indicators for adverse effects. Furthermore, the dose metric found to have an effect in the in vitro system should be relevant. One element in defining the appropriate dose metric is related to the kinetic behavior of the compound in the in vitro system: binding to proteins, binding to plastic, evaporation, and the interaction between the culture medium and the cells. Ways to measure and model "in vitro biokinetics" are described. Second, the appropriate dose metric in vitro, e.g., the effective concentration, will need to be extrapolated to relevant in vivo exposure scenarios. The application of physiologically based biokinetic modelling is essential in such extrapolations. The parameters needed to build these models often can be estimated based on nonanimal data, namely chemical properties (QSARs) and in vitro experiments.

Bioaccumulation, sublethal toxicity, and biotransformation of sediment-associated pentachlorophenol in Lumbriculus variegatus (Oligochaeta)

The xenobiotics accumulated in sediments represent a hazard to organisms. In order to study the toxic effects of xenobiotics in organisms, body residue has been proposed as a more relevant dose-metric than the environmental concentration of the chemical. In this study, the benthic oligochaetes Lumbriculus variegatus were exposed to sediment-spiked pentachlorophenol (PCP) in a chronic study at different exposure concentrations. The aim was to examine sublethal toxic effects in sediment-dwelling and sediment-ingesting organisms, and to link the effects with chemical body residues. Growth, reproduction, and egestion rate were used as sublethal endpoints. Bioaccumulation, sublethal toxic effects, and biotransformation of PCP were investigated by exposing organisms to both artificial and natural sediments with similar organic carbon content. Sediment characteristics were assumed to have an effect on toxicity since PCP retarded both growth and reproduction in L. variegatus in the artificial sediment. In natural sediment, growth, and reproduction was also reduced in control treatments, probably indicating poor nutritional quality. Most of the extracted chemicals in L. variegatus tissues were water-soluble metabolites, indicating that L. variegatus was capable of biotransforming PCP. The extractable parent PCP body residues (CBR(50)) for L. variegatus growth and reproduction were in agreement with the values estimated for respiratory uncouplers in the literature.

Comparison of the effect of phenol and its derivatives on protein and free radical formation in human erythrocytes (in vitro)

The effect of phenolic compounds: phenol, 2,4-dichlorophenol (2,4-DCP), 2,4-dimethylphenol (2,4-DMP) and catechol on human erythrocytes was studied. The level of fluorescent label - 6-carboxy-2',7'-dichlorodihydrofluorescein diacetate (H(2)DCFDA) oxidation by phenolic compounds in erythrocytes as well as the carbonyl group content and hemoglobin denaturation were monitored. H(2)DCFDA has been utilized extensively as a marker for studies of oxidative stress at the cellular level. We noted that 2,4-DCP, 2,4-DMP and catechol induced an increase in the concentration- and time-dependent H(2)DCFDA oxidation. We also observed an increase in carbonyl group content and the changes in parameter T (denaturation of hemoglobin) in erythrocytes incubated with 2,4-DCP, catechol and 2,4-DMP. The highest level of H(2)DCFDA oxidation was provoked by 2,4-DCP. The biggest changes of proteins in erythrocytes measured as the carbonyl group content were induced by 2,4-DMP, but measured as parameter T they were induced by catechol. It was observed that phenol did not oxidize H(2)DCFDA up to the concentration of 2.5 mM after 3 h of incubation. Phenol did not affect the carbonyl group content but decreased parameter T (induced denaturation of hemoglobin). To sum up, the kind of the substituent in a phenolic ring determines the molecular mechanism of action of the individual compound and the capacity of reactive oxygen species generation and thus damages the specified structures in human erythrocytes.

Influence of Albumin on Sorption Kinetics in Solid-Phase Microextraction: Consequences for Chemical Analyses and Uptake Processes

Because of its simplicity, solid-phase microextraction (SPME) is an increasingly popular technique to use in experiments measuring freely dissolved concentrations of compounds in biological and environmental samples. However, a number of studies have shown that sorption kinetics of compounds in such SPME systems is dependent on the presence of a binding matrix. This affects the interpretability of nonequilibrium SPME data. In this study, this phenomenon was investigated by measuring the rate of depletion of pyrene from a "loaded" poly(dimethylsiloxane) fiber into surrounding cell culture medium containing different concentrations of bovine serum albumin (BSA). The rate of depletion was found to steadily increase with increasing concentrations of BSA. It was postulated that BSA facilitated the transport of pyrene through the medium. This phenomenon was modeled by considering diffusion of BSA-bound pyrene in addition to diffusion of unbound pyrene in the aqueous boundary layer (BL) around the fiber. The model closely fit the experimental data and illustrated that diffusion in the BL was rate limiting because the analyte's affinity for the fiber was high and the BL thickness significant. The concentration of binding matrix and the analyte's affinity for the matrix further determined the extent to which BSA-facilitated transport contributed to the kinetics of the system.

Nitroaniline Isomers Interaction with Bovine Serum Albumin and Toxicological Implications

The interactions of 2-nitroaniline (2-NA), 3-nitroaniline (3-NA) and 4-nitroaniline (4-NA) with bovine serum albumin (BSA) have been investigated by means of fluorescence spectrometry, synchronous fluorescence spectrometry and UV absorption spectrometry under the simulative physiological conditions. Association constants (K(A)) were estimated by the remarkable static quenching effect of 2-NA, 3-NA and 4-NA to the intrinsic fluorescence of BSA, and thermodynamic parameters such as enthalpy change (DeltaH) and entropy change (DeltaS) were calculated according to van't Hoff equation. The results show that hydrophobic force plays a main role in the interaction of nitroanilines to BSA, nitroanilines have high affinity to BSA and the affinity order is as follows: 4-NA > 2-NA > 3-NA. On the basis of this study, it is found that percents of the binding of nitroanilines to BSA are almost no relative to the concentrations of nitroanilines, and correlation between K(A) and logK (ow) is disclosed. In the meantime, relationships between the combination of nitroanilines with BSA and toxicological implications were also discussed. In addition, synchronous fluorescence method was used to study the interaction mechanisms between nitroanilines and BSA, and energy transfer distances from BSA to nitroanilines were estimated based on the Förster's non-radiation energy transfer theory. The results suggest that the binding site for nitroanilines on BSA is close to the sub-domain IIA where Trp 214 is located.

Bioavailability in dose and exposure assessment of organic contaminants in (eco)toxicology

The dose is an essential element in toxicology and risk assessment. In most cases, the dose is expressed as a concentration in the external environment. The internal dose is a more direct measure for the exposure in toxicological assays, because it takes differences in bioavailability into account. Because the internal dose is often not measurable, the effective free concentration in a medium or the environment is a useful alternative. This short review discusses the advantages of free concentration measurements of organic compounds for interpretation of effects in sediment and soil tests as well as for in vitro assays.

The improvement of in vitro cytotoxicity testing for the assessment of acute toxicity in fish

The use of fish cell line cytotoxicity tests as alternatives to acute lethality tests with fish is hampered by the clearly lower sensitivity of the fish cell line tests. Recently, it has been shown that this is not a unique feature of fish cells. In fact, the sensitivity of mammalian and human cell lines toward the cytotoxic actions of chemicals, in general, is comparable to that of fish cell lines. Reviewing some of our recent investigations, the objective of this paper is to show that the sensitivity of in vitro cytotoxicity testing and the correspondence between in vitro cytotoxic and acute fish toxic concentrations (LC50) can be increased, if: a) inhibition of cell growth instead of cell death is used as the endpoint; and b) the bioavailable free cytotoxic concentration (ECu50) of chemicals in vitro, instead of the nominal cytotoxic concentration (EC50), is used as the measure of cytotoxic potency. Based on these results, a pragmatic in vitro testing strategy for estimating the minimal aquatic toxic potency of chemicals is proposed.

Long-term exposure to dieldrin reduces γ-aminobutyric acid type A and N-methyl-D-aspartate receptor function in primary cultures of mouse cerebellar granule cells

The organochlorine pesticide dieldrin is a persistent organic pollutant that accumulates in the fatty tissue of living organisms. In mammals, it antagonizes the GABA(A) receptor, producing convulsions after acute exposure. Although accumulation in human brain has been reported, little is known about the effects of long-term exposure to dieldrin in the nervous system. Homeostatic control of the balance between excitation and inhibition has been reported when neuronal activity is chronically altered. We hypothesized that noncytotoxic concentrations of dieldrin could decrease glutamatergic neurotransmission as a consequence of a prolonged reduction in GABA(A) receptor function. Long-term exposure of primary cerebellar granule cell cultures to 3 microM dieldrin reduced the GABA(A) receptor function to 55% of control, as measured by the GABA-induced (36)Cl(-) uptake. This exposure produced a significant reduction (approximately 35%) of the NMDA-induced increase in [Ca(2+)](i) and of the [(3)H]MK-801 binding, which was not accompanied by a reduction in the NMDA receptor subunit NR1, as determined by Western blot. Consistent with the decreased NMDA receptor function, dieldrin-treated cultures were insensitive to an excitotoxic stimulus induced by exposure to high potassium. In summary, we report that the chronic reduction of GABA(A) receptor function induced by dieldrin decreases the number of functional NMDA receptors, which may be attributable to a mechanism of synaptic scaling. These effects could underlie neural mechanisms involved in cognitive impairment produced by low-level exposure to dieldrin.

Does lipophilicity of toxic compounds determine effects on drought tolerance of the soil collembolan Folsomia candida?

The ability of Collembola to survive drought stress is crucial for their distribution in the terrestrial environment. Previous studies have suggested that several toxic compounds affect the drought tolerance of Folsomia candida in a synergistic manner and that these compounds have the feature in common that they elicit their toxicity by causing membrane damage. We hypothesised that the detrimental effect of toxic chemicals on drought tolerance in F. candida depends on the lipophilicity (log K(ow)) of the compound because a higher log K(ow) would mean a closer interaction with membranes. In this study the three chemicals 4-nonylphenol, pyrene and p,p'-DDE were tested. Surprisingly, 4-nonylphenol, with the lowest log K(ow), was the most potent with respect to reducing drought tolerance followed by pyrene, suggesting that interactions between drought tolerance and chemical stress do not depend on lipophilicity alone.

Pesticides and their binary combinations as P-glycoprotein inhibitors in NIH 3T3/MDR1 cells

The purpose of the present study was to assess do selected pesticides as well as their binary combinations act as inhibitors of P-glycoprotein (P-gp) activity of NIH 3T3 mouse fibroblasts stably transfected with human MDR1 gene (NIH 3T3/MDR1). As a result of P-gp inhibition, the increase of intracellular accumulation of a model P-gp substrate fluorescent calcein acetoxymethyl ester was measured. Pesticide and verapamil individual dose-response data were scaled and expressed as percent of maximum effect. Results showed that out of 14 pure pesticides tested, endosulfan, phosalone and propiconazole were nearly as potent as model inhibitor verapamil (EC(50)=1.5μM), while diazinon showed a lower potency of inhibiting P-gp transport activity (EC(50)=58.4μM). Concentrations of pesticides that produced the same inhibiting effect (isoboles) were combined binary. Results calculated using the isobole method revealed that diazinon caused synergistic effect in inhibiting P-gp transport activity in all combinations.

Validation of a prediction model for estimating serum concentrations of chemicals which are equivalent to toxic concentrations in vitro

The objective of the present study was to evaluate the validity of a recently developed extrapolation model for the prediction of concentrations of chemicals in serum which are equivalent to in vitro effective nominal concentrations. Necessary input data are in vitro toxic concentrations and distribution relevant system and substance specific parameters, e.g. lipid volume fractions and albumin concentrations, octanol/water partition coefficients and specific binding to albumin. It was investigated whether the influence of human and bovine serum, respectively, on nominal cytotoxic potencies (EC(50)-values) of selected chemicals in vitro can be properly predicted using this algorithm. Cytotoxicity was determined as growth inhibition of proliferating Balb/c 3T3 cells after exposure for 72 h. Concentration-effect relationships were measured in the presence of 2% foetal bovine serum (FBS) and, additionally, 18% FBS or human serum (HS), or 1% (w/v) bovine (BSA) or human (HSA) albumin, respectively. Addition of HSA and BSA increased the EC(50)-values of the different chemicals by factors of 2.1 - 22 and 1.7 - 29, respectively. From these measurements values for the specific binding of the test compounds to BSA and HSA were derived. Addition of 18% HS increased the EC(50)-values by factors between 4.2 and 52, while addition of 18% FBS resulted only in 1.5 - 10.4-fold increases. A comparison of experimentally determined and calculated EC(50)-values revealed that the differing influence of human and bovine serum was quite well predicted by the extrapolation model. Deviations did not exceed the factor 3 and were in most cases lower than 2. It is concluded that the extrapolation model is quite well suited to predict equivalent concentrations in serum from in vitro effective concentrations.

Proposal to improve vertebrate cell cultures to establish them as substitutes for the regulatory testing of chemicals and effluents using fish

Cultures of vertebrate cells are widely applied in mechanistic studies in human toxicology as well as in toxicity identification in ecotoxicology. As in vitro models, they display many advantages over whole animal experimentation, pertaining to such characteristics as availability, reproducibility and costs. As well, they satisfy the societal desire to reduce the number of animals in toxicology. For these reasons vertebrate cell models also appear to be a desirable replacement for animals in regulatory tests. Several vertebrate cell models are now accepted for regulatory purposes in human health sciences, with the test for photocytotoxicity using the 3T3 mouse cell line being one example. However, an in vitro alternative to whole animal tests has not yet been established for regulatory risk assessment in ecotoxicology. This review sets out to outline why such a replacement has not yet been possible and explores avenues to improve vertebrate cell cultures so that a replacement of whole animal tests could more likely be achieved. Inasmuch as fish is the most widely used non-mammalian vertebrate in risk assessment and regulation, focus will be on the replacement, by in vitro vertebrate models, of fish.

Measurement of the free concentration of octylphenol in biological samples with negligible depletion-solid phase microextraction (nd-SPME): Analysis of matrix effects

A negligible depletion-solid phase microextraction (nd-SPME) method is presented to measure free concentrations of octylphenol in biological samples. Potential confounding factors, such as matrix effects, are studied as well. Fouling of the fibre appears to occur, but it does not seem to reduce or enhance the measured uptake of octylphenol. In the setup applied here, without any agitation, it has also been found that there is a large effect of protein presence on the kinetics of octylphenol uptake. In addition, an apparent affinity constant of octylphenol for bovine serum albumin was determined.

A simple and fast extraction method for organochlorine pesticides and polychlorinated biphenyls in small volumes of avian serum

A solid-phase extraction (SPE) method was developed using 8M urea to desorb and extract organochlorine pesticides (OCs) and polychlorinated biphenyls (PCBs) from avian serum for analysis by capillary gas chromatography with electron capture detection (GC-ECD). The analytes were efficiently extracted from the denatured serum-lipoprotein-analyte complex by one passage through an Oasis((R)) hydrophilic-lipophilic-balanced (HLB) SPE cartridge. No further clean-up was necessary, the entire extraction procedure and GC-ECD analysis can be accomplished in less than 3h. Serum volumes ranged from 100 microL to 1 mL with absolute recoveries of 90-101% for PCBs and 74% to 101% for the OC pesticides.

Comparison of the cytotoxicity of the MEIC reference chemicals measured in protein free and in complete culture medium

In order to investigate if a protein free cytotoxicity assay could improve the prediction of human acute toxicity, the cytotoxicity of 40 MEIC reference chemicals was measured by the neutral red uptake inhibition after 24h in protein free culture medium on rat hepatoma-derived Fa32 cells. The results were compared with the corresponding values obtained in complete culture medium, including 10% fetal calf serum. Potassium cyanide, arsenic trioxide, mercuric chloride, hexachlorophene and pentachlorophenol were much more cytotoxic in PF medium, as was the case to a lower extent for 16 other chemicals. The cytotoxicity of 8 chemicals was only changed to a limited extent when tested in PF medium, suggesting that serum proteins do not strongly interact with their cytotoxicity. Eleven other chemicals were less cytotoxic in PF medium, maybe because of too poor physiological conditions. Although a large number of differences in cytotoxicity were observed in function of the medium used for the assay, a good correlation was observed between both series of data (r(2)=0.946). The correlation between the cytotoxicity in PF medium and the human acute toxicity is lower (r(2)=0.647) than that in complete medium (r(2)=0.746). The results show that further research is necessary in order to improve the in vitro/in vivo correlations by introducing protein-dependent considerations.

Impact of bioavailability on the correlation between in vitro cytotoxic and in vivo acute fish toxic concentrations of chemicals

The lower sensitivity of in vitro cytotoxicity assays currently restricts their use as alternative to the fish acute toxicity assays for hazard assessment of chemicals in the aquatic environment. In vitro cytotoxic potencies mostly refer to nominal concentrations. The main objective of the present study was to investigate, whether a reduced availability of chemicals in vitro can account for the lower sensitivity of in vitro toxicity test systems. For this purpose, the bioavailable free fractions of the nominal cytotoxic concentrations (EC50) of chemicals determined with a cytotoxicity test system using Balb/c 3T3 cells and the corresponding free cytotoxic concentrations (ECu50) were calculated. The algorithm applied is based on a previously developed simple equilibrium distribution model for chemicals in cell cultures with serum-supplemented culture media. This model considers the distribution of chemicals between water, lipids and serum albumin. The algorithm requires the relative lipid volume of the test system, the octanol-water partition coefficient (K(ow)) and the in vitro albumin-bound fraction of the chemicals. The latter was determined from EC50-measurements in the presence of different albumin concentrations with the Balb/c 3T3 test system. Organic chemicals covering a wide range of cytotoxic potency (EC50: 0.16-527000 microM) and lipophilicity (logK(ow): -5.0-6.96) were selected, for which fish acute toxicity data (LC50-values) from at least one of the three fish species, medaka, rainbow trout and fathead minnow, respectively, were available. The availability of several chemicals was shown to be extensively reduced either by partitioning into lipids or by serum albumin binding, or due to both mechanisms. Reduction of bioavailability became more important with increasing cytotoxic potency. The sensitivity of the Balb/c 3T3 cytotoxicity assay and the correspondence between in vivo and in vitro toxic potencies were increased when the free cytotoxic concentrations instead of the nominal cytotoxic concentrations were used as measure of cytotoxic potency. The few remaining prominent differences between cytotoxic and acute toxic concentrations can be explained by a more specific mechanism of acute toxic action than basal cytotoxicity. It is concluded that the frequently observed low sensitivity of in vitro cytotoxicity test systems, compared to fish acute toxicity assays, at least in part, can be explained by differences in the availability of chemicals in vitro and in vivo. Moreover, neglecting these differences systematically causes a bias of the correlation between in vivo and in vitro toxic potencies of chemicals. Taking them into account, however, increases the predictivity of the in vitro assays.

The prediction of human acute systemic toxicity by the EDIT/MEIC in vitro test battery: the importance of protein binding and of partitioning into lipids

The aim of the two studies presented in this paper was to further improve the predictability of the original Multicentre Evaluation of In Vitro Cytotoxicity (MEIC) in vitro test battery for acute systemic toxicity. In the first study, whether a protein-free cytotoxicity assay could improve the prediction of human acute systemic toxicity was investigated. The cytotoxicity of 39 MEIC reference chemicals was measured by the neutral red uptake inhibition test after 30 minutes in phosphate-buffered saline (PBS), with hepatoma-derived Fa32 cells. The results were compared with the corresponding values obtained in complete culture medium, including 10% fetal calf serum. Mercuric chloride and hexachlorophene were much more cytotoxic in PBS, as was the case, to a lesser extent, for seven other chemicals. Potassium cyanide and eight other chemicals were less cytotoxic in PBS than in complete culture medium, probably because of poor physiological conditions. The correlation between the cytotoxicity measured in PBS and human acute toxicity was rather low, but became of the same order as for other assays, when mercuric chloride and hexachlorophene were withdrawn from the comparison. In the second study, modelling of human lethal blood concentrations by using the results of the three cell line tests of the original MEIC test battery were complemented by logP (octanol-water partition coefficient) values. The introduction of logP into the modelling did not improve the correlations, but some improvement of both R(2) and Q(2) was obtained by expanding the logP values with logP(2) values. The highest R(2) (0.84) and Q(2) (0.80) values were obtained for a model in which both experimental and calculated (ambiguous) logP values were used. When only experimental logP values were used, the corresponding values were 0.80 and 0.78. These two studies showed that including protein binding and the partition of chemicals in the MEIC in vitro test battery is important, in order to improve the predictability of the results obtained.

Partitioning of Tetrachlorophenol into Lipid Bilayers and Sarcoplasmic Reticulum: Effect of Length of Acyl Chains, Carbonyl Group of Lipids and Biomembrane Structure

We report results of a partitioning study of 2,3,4,6-tetrachlorophenol (TeCP). In the study we explored (1) the effect of the length of acyl chains of lipids (C16:1 - C24:1) and alkanes (C6-C16), (2) the role of the carbonyl group of lipids, and (3) the effect of molecular structure of the sarcoplasmic reticulum membrane on TeCP partitioning. Mole fraction partition coefficients have been measured using equilibrium dialysis for un-ionized (HA), and ionized (A) species, Kp(x) (HA), Kp(x) (A). Their values are concentration-dependent. Partition coefficients were analyzed in terms of a model that accounts for saturation of membrane associated with the finite area of partition site, and electrostatic interactions of (A-) species with charged membrane. Limiting values of partition coefficients, corresponding to infinite dilution of solute, Kp(x0) (HA), Kp(x0) (A) were obtained. Kp(x0) (HA) and Kp(x0) (A ) measure the strength of solute-membrane interactions. Studies were done with single-layered vesicles of lipids with variable chain length: 1,2-dipalmitoleoyl-sn-glycero-3-phosphocholine (C16:1), 1,2-dioleoyl-sn-glycero-3-phosphocholine (C18:1), 1,2-dierucoyl-sn-glycero-3-phosphocholine (C22:1), and 1 ,2-dinervonoyl-sn-glycero-3-phosphocholine (C24:1), and egg-PC. Kp(x0) for transfer of TeCP from water into lipid membranes was found to be independent of the length of acyl chains, whereas Kp(x0) for transfer from water into alkanes increased with the length of alkane. The effect of the carbonyl CO group of lipids on partitioning was measured using 1,2-di-o-octadecenyl-sn-glycero-3-phosphocholine (CO absent) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (CO present) liposomes. Carbonyl groups, known to change dipolar potential, had no effect on partitioning. Partition coefficients of un-ionized and ionized forms of TeCP were invariant to the presence of proteins and other membrane components of sarcoplasmic reticulum (SR) membrane.

Cytotoxic potency of trialkyltins to C6 glioma cells in vitro: impact of exposure conditions

Because of a possible role of astrocytes in trialkyltin-induced neurotoxicity in vivo various studies have been performed using cultures of astrocytes or glioma cells in vitro. With respect to cytotoxic potencies of trialkyltins these studies gave rather divergent results. Therefore the aim of the present study was to clarify whether variations of experimental conditions could be responsible for the differences of the cytotoxic activities of trimethyltin (TMT), triethyltin (TET) and tributyltin (TBT). Experiments were performed with rat C6 glioma cells. Toxicity was determined by measuring the reduction of the cell protein content. Cultures of proliferating and growth-arrested cells did not differ in their sensitivity. Exposure duration (1-72 h) had a strong but differing influence on the cytotoxic potency of the trialkyltins. After short exposure times the potencies differed largely (TMT < TET < TBT), whereas they became more and more similar with increasing exposure duration. The potency-time relationships for TMT and TET could be described by the equation: EC50 = k x t(-n), while for TBT an incipient value (EC50, infinity) had to be included: EC50 = EC50, infinity + k x t(-n). Addition of serum albumin to the culture medium decreased the cytotoxic potency of the trialkyltins. However, the impact of protein binding on their bioavailability was relatively low. The cytotoxic potency of the alkyltins was not dependent on the concentration of C6 cells. Taken together, neither differences in exposure conditions nor in the proliferative status of the cells are sufficient to account for the discrepancies in published results for trialkyltin cytotoxicity to astrocytes. Instead they may--at least partially--be explained by differing sensitivities of the endpoints used. Furthermore, C6 glioma cells respond considerably more sensitively to trialkytins than primary astrocytes, which questions their applicability as models for astrocyte toxicity.

Evidence for delayed cytotoxicity effects following exposure of rat hepatoma-derived Fa32 cells: implications for predicting human acute toxicity

The delayed cytotoxicity of the Multicentre Evaluation of In vitro Cytotoxicity (MEIC) reference chemicals was investigated in rat hepatoma-derived Fa32 cells. The cells were treated for 24 h with the test chemicals, and were than further cultured for 5 days in normal culture medium. The cytotoxicity was measured by the neutral red uptake inhibition, and the results were quantified by determining the NI50del. This is the concentration of test compound required to decrease the neutral red uptake with 50% compared with control cells. The results were compared with the acute NI50, the corresponding value measured immediately after 24 h treatment of the cells. On a total of 44 chemicals, nine showed delayed cytotoxicity (NI50del lower than or equal to NI50), 11 a probably delayed, and 24 no delayed cytotoxicity (NI50del more than 1.5 x NI50). When the NI50del was compared with human toxicity, a correlation coefficient r2=0.761 was obtained. For the same series of 44 chemicals this correlation was clearly higher than that for human hepatoma-derived Hep G2 cells (r2=0.695). The Hep G2 assay was the best acute in vitro assay for the prediction of human toxicity within the MEIC study. Consequently, the delayed cytotoxicity assay on cultured Fa32 cells has the best prediction value so far obtained for the human toxicity.

In vitro-in vivo extrapolation: estimation of human serum concentrations of chemicals equivalent to cytotoxic concentrations in vitro

In the present study an extrapolation model for estimating serum concentrations of chemicals equivalent to in vitro effective concentrations is developed and applied to median cytotoxic concentrations (EC(50)) determined in vitro. Nominal concentrations of a chemical in serum and in vitro are regarded as equivalent, if they result in the same aqueous concentration of the unbound form. The algorithm used is based on equilibrium distribution and requires albumin binding data, the octanol-water partition coefficient (K(ow)), and the albumin concentrations and lipid volume fractions in vitro and in serum. The chemicals studied cover wide ranges of cytotoxic potency (EC(50): 2.5-530,000 microM) and lipophilicity (logK(ow): -5 to 7). Their albumin binding characteristics have been determined by means of an in vitro cytotoxicity test as described previously. The equivalent serum concentrations of 19 of the 33 compounds investigated, having high protein binding and/or lipophilicity, were substantially higher than the EC(50)-values, by factors of 2.5-58. Prominent deviations between the equivalent nominal concentrations in serum and in vitro were largely restricted to chemicals with higher cytotoxic potency (EC(50)< or =1000 microM). The results suggest that estimates of equivalent serum concentrations based on in vitro data are robust for chemicals with low lipophilicity (logK(ow)< or =2) and low potency (EC(50)>1000 microM). With more potent chemicals or those with higher lipophilicity partitioning into lipids and/or binding to serum proteins have to be taken into account when estimating in vivo serum concentrations equivalent to in vitro effective concentrations.

Serum albumin binding at cytotoxic concentrations of chemicals as determined with a cell proliferation assay

The aim of the present study was to measure the influence of albumin binding on cytotoxic concentrations of chemicals and to determine binding parameters which can be used for quantitative in vitro-in vivo extrapolations. Protein binding parameters were determined from cytotoxic potencies measured with Balb/c 3T3 cells cultured in the presence of 18 and 600 microM bovine serum albumin (BSA). A subset of 27 chemicals from the Multicenter Evaluation of In Vitro Cytotoxicity (MEIC) project was investigated. At 18 microM BSA the EC(50)-values ranged from 2.54 microM (As(III)) to 527 mM (ethylene glycol). Increasing the BSA concentration either decreased the cytotoxic potency (12 compounds) by factors up to 34 (pentachlorophenol), had no effect (14 compounds), or increased the cytotoxicity (paraquat). Calculated molar ratios of binding ranged from 0.05 (Hg(2+)) to 4.8 moles per mole albumin (acetylic salicylic acid). At 18 microM BSA fractional binding of most of these compounds was low (<25%) but increased up to > or =90% (hexachlorophene, mercuric chloride, thioridazine, pentachlorophenol) at 600 microM BSA. The results obtained in general were compatible with available protein binding data and can be used to calculate equipotent concentrations of chemicals in biological systems containing different albumin concentrations.

Negligible depletion solid-phase microextraction with radiolabeled analytes to study free concentrations and protein binding: an example with [3H]estradiol

A new method is presented that enables sensitive measurement of free concentrations of radiolabeled ligands. Additionally, protein binding of radiochemicals in complex matrixes can be determined with this new technique that combines negligible depletion solid-phase microextraction (nd-SPME) with liquid scintillation counting (LSC) as detection. [3H]Estradiol was taken as an example compound. Possible matrix effects of protein on fiber uptake kinetics were studied. No matrix effect was found, either by fouling of the fiber, or by changed uptake kinetics. The validity of the method was shown in the determination of the affinity constant (Ka) of estradiol for human serum albumin (HSA). The Ka was estimated at 8.9 x 10(4) M(-1), which corresponds well with literature values. This study shows that nd-SPME is suitable to study the free concentration and protein binding of [3H]estradiol. The method described in this paper combines the advantages of nd-SPME with the advantages of radiolabeled analytes, creating a timesaving, simple, and sensitive analytical tool that will be particularly useful in complex matrixes containing many potential interferences for chromatographic methods.