Biomedical subjects

H A Govers

Publications and source records attributed to H A Govers.

11 recordsLinked to original sources

PubMed2001

Determination and theoretical aspects of the equilibrium between dissolved organic matter and hydrophobic organic micropollutants in water (Kdoc).

Literature on the equilibrium constant for distribution between dissolved organic carbon (DOC) (Kdoc) data of strongly hydrophobic organic contaminants were collected and critically analyzed. About 900 Kdoc entries for experimental values were retrieved and tabulated, including those factors that can influence them. In addition, quantitative structure-activity relationship (QSAR) prediction equations were retrieved and tabulated. Whether a partition or association process between the contaminant and DOC takes place could not be fully established, but indications toward an association process are strong in several cases. Equilibrium between a contaminant and DOC in solution was shown to be achieved within a minute. When the equilibrium shifts in time, this was caused by either a physical or chemical change of the DOC, affecting the lighter fractions most. Adsorption isotherms turned out to be linear in the contaminant concentration for the relevant DOC concentration up to 100 mg of C/L. Eighteen experimental methods have been developed for the determination of the pertinent distribution constant. Experimental Kdoc values revealed the expected high correlation with partition coefficients over n-octanol and water (Kow) for all experimental methods, except for the HPLC and apparent solubility (AS) method. Only fluorescence quenching (FQ) and solid-phase microextraction (SPME) methods could quantify fast equilibration. Only 21% of the experimental values had a 95% confidence interval, which was statistically significantly different from zero. Variation in Kdoc values was shown to be high, caused mainly by the large variation of DOC in water samples. Even DOC from one sample gave different equilibrium constants for different DOC fractions. Measured Kdoc values should, therefore, be regarded as average values. Kdoc was shown to increase on increasing molecular mass, indicating that the molecular mass distribution is a proper normalization function for the average Kdoc at the current state of knowledge. The weakly bound fraction could easily be desorbed when other adsorbing media, such as a SepPak column or living organism, are present. The amount that moves from the DOC to the other medium will depend, among other reasons, on the size of the labile DOC fraction and the equilibrium constant of the other medium. Variation of Kdoc with temperature turned out to be small, probably caused by a small enthalpy of transfer from water to DOC. Ionic strength turned out to be more important, leading to changes of a factor of 2-5. The direction of this effect depends on the type of ion. With respect to QSAR relationships between Kdoc and macroscopic or molecular descriptors, it was concluded that only a small number of equations are available in the literature, for apolar compounds only, and with poor statistics and predictive power. Therefore, a first requirement is the improvement of the availability and quality of experimental data. Along with this, theoretical (mechanistic) models for the relationship between DOC plus contaminant descriptors on the one side and Kdoc on the other should be further developed. Correlations between Kdoc and Kow and those between the soil-water partition constant (Koc) and Kow were significantly different only in the case of natural aquatic DOC, pointing at substantial differences between these two types of organic material and at a high correspondence for other types of commercial and natural DOC.

Environmental Monitoring↗

PubMed1996

Ecological hazard assessment of dioxins: hazards to organisms at different levels of aquatic food webs (fish-eating birds and mammals, fish and invertebrates).

An ecological hazard assessment was performed for 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin in the aquatic environment, taking into account four different classes of species which differ with respect to their positions in aquatic food webs: invertebrates, fish, birds preying on aquatic organisms, and mammals preying on aquatic organisms. The most sensitive toxicological endpoints for each class were selected from the literature. These endpoints were extrapolated to no effect concentrations (NECs) in water with a two-level food chain model as indicator for secondary poisoning. From the results it is concluded that fish-eating birds and mammals experience larger hazards from dioxins and related compounds than fish. The limited data available appear to indicate that invertebrates are rather insensitive to dioxins. The study indicates that assessment of secondary poisoning affects environmental quality criteria setting and risk assessment.

Animals↗

PubMed1996

Determination of bioconcentration factors of eight tetrachlorobenzyltoluenes in the zebra mussel Dreissena polymorpha.

Accurate duplicate bioconcentration factors were determined for tetrachlorobenzyltoluenes (TCBTs). This is the first study that quantifies bioconcentration factors for single TCBT isomers. A continuous flow uptake experiment was performed during 21 days with the zebra mussel (Dreissena polymorpha) as a test organism. Bioconcentration factors (BCFs) and rate constants were derived by an iterative integration method. With this method BCFs and rate constants can be derived from experimental data of a bio-concentration test, even if the concentration in the water is not constant and steady state has not been reached. Mean log BCF values of TCBTs ranged from 4.43 +/- 0.05 to 5.19 +/- 0.15 liter.kg-1 on a wet weight basis. The experimental log BCF values were compared with log BCF values of TCBTs, calculated according to linear and polynomial relationships between log BCF and log Kow.

Animals↗

PubMed1996

Trace metals in populations of freshwater isopods: influence of biotic and abiotic variables.

Trace metal levels in water, sediments and freshwater isopods from 28 different water systems in the Netherlands were measured during the period of 1986 to 1989. Distinct element-specific internal distribution patterns were present, with Cd and Cu stored mainly in the hepatopancreas (30-60% of total body burden) and Pb and Zn in the hindgut and exoskeleton with hemolymph. Mean whole-body concentrations of the non-essential elements Cd and Pb in individually analyzed isopods varied over three orders of magnitude between populations. The variability of Zn and Cn were within one order of magnitude difference. The variability of trace metal levels between populations exceeded within-population variability. Within-population variability was related to seasonal and biological factors such as body weight, sex, reproductive state, and species abundance. The highest concentrations were found in small juvenile animals compared to adults, females compared to males, and Proasellus meridianus compared to Asellus aquaticus. However, after correction for size effects using a power-curve regression model no significant differences remained between sex and species. Seasonal fluctuations accounted for 33-79% of the within population variability. Trace metal levels in isopods were predicted from concentrations in water and sediments in combination with aqueous Ca, Cl-, DOC, and sediment characteristics (Org-C, clay, CaCO3) using a multiple regression model. With this predictive model 42-63% of the variance could be explained. In situ determined partitioning coefficients (apparent BCF, biota-sediment BSAF, and sediment-water distribution coefficient Kd) varied between locations and covaried with factors related to trace metal bioavailability (aqueous Ca, Cl- and DOC, sediment Org-C, clay, and CaCO3). Especially for Cd and Cu field-derived BCF values were in agreement with previously reported experimental studies. It is concluded, that A. aquaticus may be a suitable candidate-organism for biomonitoring available trace metal levels in littoral freshwater systems. Finally, some practical recommendations are given for future field surveys with freshwater isopods with respect to sample size, allometric standardization, period of sampling and statistical design.

Analysis of Variance↗

PubMed1996

Comparison of bioconcentration factors of tetrachlorobenzyltoluenes in the guppy (Poecilia reticulata) and zebra mussel (Dreissena polymorpha).

Preliminary bioconcentration factors (BCFs) were determined of six tetrachlorobenzyltoluenes (TCBTs) in guppies (Poecilia reticulata). BCFs and rate constants were derived by an iterative integration method. With this method BCFs and rate constants can be derived from experimental data of a bioconcentration test, even if the concentration in the water is not constant and steady state has not been reached. Log BCF values of TCBTs in guppies ranged from 1.67 to 2.68 L.kg-1 on a wet weight basis. These values are much lower than the reported log BCF values of TCBTs in zebra mussels (Dreissena polymorpha) determined with the same method. Biotransformation of TCBTs in guppies as an explanation for the higher log BCF values in zebramussel is discussed.

Animals↗

PubMed1996

Effect of fatty acids and the aqueous diffusion barrier on the uptake and transport of polychlorinated biphenyls in Caco-2 cells.

Polychlorinated biphenyls (PCBs) dissolved in dietary fat are absorbed in the gastrointestinal tract by the enterocytes in combination with the fatty acids proceeding from the lipid hydrolysis in the gut lumen. The effect of fatty acid absorption on the uptake and transport of 14 PCBs in enterocytes was studied using monolayers of the human intestinal Caco-2 cell line as a model system. The diffusive resistance of the unstirred water layer and the facilitating role of mixed bile salt micelles on the PCB uptake were examined by varying the thickness of the unstirred water layer adjacent to the apical membrane. In additional experiments, the polarity of the PCB uptake and transport in Caco-2 cells was determined. The solubility of PCBs in the mixed bile salt-fatty acid micelles was 2.7 to 4.8-fold higher than the solubility in plain bile salt micelles. Both the uptake and transport of PCBs in Caco-2 cells were significantly higher (up to 10-fold) when the PCBs were presented mixed with fatty acids. Reducing the thickness of the unstirred water layer resulted in an increased uptake of PCBs. The PCB uptake in Caco-2 cells exceeded the uptake as expected from monomer diffusion only, indicating that bile salt micelles facilitate the PCB transport over the unstirred water layer. Concentrations of dichlorobiphenyls accumulating in the basolateral medium stayed unexpectedly low, suggesting that Caco-2 cells might possess the capability of metabolizing lower chlorinated biphenyls. Uptake of PCBs into the Caco-2 cells was not significantly different whether the PCBs were presented at the apical side or at the basolateral side. However, transport of PCBs over the cell monolayer was significantly higher when the PCBs were presented at the apical side as compared to the basolateral side, suggesting that the unidirectional transport of lipids and lipoproteins affected the PCB transport as well. Our studies indicate that monolayers of the Caco-2 cell line offer a useful model system for studying the intestinal uptake and transport processes of hydrophobic xenobiotics such as polychlorinated biphenyls.

Biological Transport↗

PubMed1995

Solubility and micelle-water partitioning of polychlorinated biphenyls in solutions of bile salt micelles.

Experimental data are presented on the enhanced solubilities and micelle/water partition coefficients of 14 polychlorinated biphenyls (PCBs) in solutions of the bile salts sodium taurocholate (TC) and sodium taurodeoxycholate (TDC) below and above the critical micellar concentration (CMC). Solubilities of PCBs in micellar solutions above the CMC depend linearly on bile salt concentration and the solubility enhancement is up to four orders of magnitude. PCB partition coefficients (log Kmw) are independent of bile salt concentrations and are systematically higher in solutions of TDC than in solutions of TC, ranging from 4.26 to 6.24 and from 3.89 to 5.89 respectively. The partitioning behavior of the pertinent extremely hydrophobics between the aqueous and micellar phases is not completely comparable with simple organic solvent/water systems and the correlation between log Kow and log Kmw is non-linear.

Micelles↗

PubMed1991

Descriptors for isomer resolution of (bio-) distribution of chlorinated aromatic compounds.

Both Solubility Parameter (SOLPAR) and Linear Solvation Energy Relationship (LSER) theory utilize molar volume and energies as descriptors for distribution phenomena. They provide powerful methods for the prediction of the GLC retention indices of chlorobenzenes (CBzs) and tetrachlorodibenzo[p]dioxins (TCDDs). Almost complete isomer resolution is obtained with correlation coefficients (r) of greater than or equal to 0.9984 (CBzs) and greater than or equal to 0.9839 (TCDDs). The n-octanol/water partition coefficient (log Kow) of CBzs can be calculated with a standard error of regression of 0.04, which compares favourably with standard deviations of 0.03-0.26 in mean values based on four different experimental methods. The HPLC capacity factor of TCDDs on C18 columns can be predicted by SOLPAR with moderate accuracy (r = 0.9470), allowing for a preliminary calculation of log Kow. A simplified LSER method fails.

Chlorobenzenes↗

PubMed1991

QSARs and PARs for biodegradation of PCBs.

Relationships between the biodegradation rate constants of a number of polychlorinated biphenyls (PCBs) and hydrophobic and electronic structural parameters are compared. There is no simple relationship with octanol-water partition coefficients, indicating that the biodegradation rates of PCBs are probably not determined by their rates of permeation through the bacterial membranes. Biodegradation rate constants correlated much better with both the electronic and hydrophobic properties of the chlorine substituents, which suggests that the reactivity and possibly enzyme binding of PCBs control their biodegradation rates.

Acinetobacter↗

PubMed1990

Quantitative structure-activity relationships for biodegradation.

Quantitative structure-activity relationships (QSARs) between biodegradation rates of organic compounds and chemical structure parameters are reviewed. Although a number of such relationships have been developed, they in general only apply to restricted ranges of compounds, limiting their value as predictors of biodegradation rates. For many of these classes of chemicals relationships have been reported with different structural descriptors, varying from macroscopic physical properties to molecular structure parameters. More information on the mechanism and rate-determining steps of biodegradation, which can lead to a better-founded choice of descriptors, and more biodegradation rate data are required to further develop QSARs for biodegradation.

Biodegradation, Environmental↗

PubMedDate not supplied

Aerobic degradation of polychlorinated biphenyls by Alcaligenes sp. JB1: metabolites and enzymes.

In contrast to the degradation of penta- and hexachlorobiphenyls in chemostat cultures, the metabolism of PCBs by Alcaligenes sp. JB1 was shown to be restricted to PCBs with up to four chlorine substituents in resting-cell assays. Among these, the PCB congeners containing ortho chlorine substituents on both phenyl rings were found to be least degraded. Monochloro-benzoates and dichlorobenzoates were detected as metabolites. Resting cell assays with chlorobenzoates showed that JB1 could metabolize all three monochlorobenzoates and dichlorobenzoates containing only meta and para chlorine substituents, but not dichlorobenzoates possessing an ortho chlorine substituent. In enzyme activity assays, meta cleaving 2,3-dihydroxybiphenyl 1,2-dioxygenase and catechol 2,3-dioxygenase activities were constitutive, whereas benzoate dioxygenase and ortho cleaving catechol 1,2-dioxygenase activities were induced by their substrates. No activity was found for pyrocatechase II, the enzyme that is specific for chlorocatechols. The data suggest that complete mineralization of PCBs with three or more chlorine substituents by Alcaligenes sp. JB1 is unlikely.

Aerobiosis↗