Models for risk assessment of reactive chemicals in aquatic toxicology
Year: 2000
Type: dissertation
Abstract: A quantitative structure property relationship (QSPR) for a,b-unsaturated carboxylates\n(mainly acrylates and methacrylates) was established in chapter 2. Chemical reaction rate\nconstants were measured for 12 different chemicals with three different nucleophiles, namely\nH 2 O, OH - and glutathione (GSH). Relatively small differences were found in hydrolysis\nrates (reaction with H 2 O and OH - ). At an elevated pH (8.8) the hydrolysis half-life of the\ncompound ranged between 7 and 40 days, with exception of diethyl fumarate (0.4 day). A\nseparation in two groups was observed for the reaction with GSH (Michael addition), where\nacrylates reacted approximately 100 times as fast as methacrylates. This difference was con-\nsistent with differences found in electronic structure, which was determined by quantum-\nchemical calculations. Because no single parameter could describe the electrophilic charac-\nter of the unsaturated carboxylates satisfactory, four descriptors were pooled, using a\nmultivariate correlation (partial least squares regression, PLS). The resulting QSPR for\nMichael addition was able to predict the reactivity of structurally related, unsaturated\ncarboxylates.\nAcute fish toxicity of a set of acrylates and methacrylates was evaluated in chapter 3.\nPublished four-day LC 50 data for fathead minnow were compared to the chemical reactivity\nof the compounds towards GSH, because Michael addition was expected to be the mecha-\nnism that causes harmful binding to essential biological thiol-sites in the fish (e. g. proteins\nand enzymes). A simple equation was used to model the interaction of electrophilic chemi-\ncals with GSH. The degree of GSH depletion, which was used to estimate the toxic effect,\nwas found to be related to the product of aqueous exposure concentration and chemical\nreaction rate of the reactive compound. Although, all acrylates and methacrylates poten-\ntially could react with GSH, narcosis was judged to be an alternative mode of toxic action\nresponsible for the observed acute toxicity. Potencies for GSH depletion and narcosis were\ncompared on the bases of critical body residues and critical depletion rates. Five out of 12\ncompounds were thereby identified as narcosis chemicals on the bases of their high calcu-\nlated lethal body burden. It was concluded that, although the tested chemicals all contained\na similar functional group, their mode of action regarding acute fish toxicity was not the\nsame. Therefore, a correlation between chemical reaction rate and LC 50 for the whole test set\nof chemicals would not be meaningful.\nThe results from chapter 3 indicated, that narcosis was an interfering mode of action in\nQSARÕs for fish toxicity of reactive chemicals. To evaluate this hypothesis, data of reactive\nchemicals from three different classes (unsaturated carboxylates, organophosphorus esters?126 Chapter 8 Summary and General Discussion\nand nitrobenzenes) were taken from the literature and subjected to an analysis for multiple\nmodes of action (chapter 4). The Toxic ratio, being the ratio between the observed LC 50 and\nthe LC 50 , predicted for the same compound by a narcosis QSAR was used to estimate the\nprobability of a compound to act by narcosis. In total, 40 % of the 61 compounds tested were\nidentified as Òprobably acting by narcosisÓ. For these compounds, a narcosis QSAR using\nthe octanol/water partitioning coefficient (K OW ) as sole descriptor was found to describe\nthe toxicity. QSARÕs using reactivity descriptors, which in earlier work had been found\ninsufficient to describe the toxicity of these classes of compounds improved considerably, if\nthe Ònarcotic chemicalsÓ were excluded from the data sets. It was concluded, that narcosis\nshould always be considered as a possible alternative cause of death in acute fish toxicity\ntest, even if the chemicals seem to have a very specific mode of action. Additionally, it was\nshown, that QSARÕs should only be established for sets of chemicals with an identical mode\nof action. Modes of action clearly should not be confused with functional groups.\nThe toxic effect of acrylates and methacrylates on a cellular level were investigated in\nchapter 5. Cellular glutathione (GSH) concentrations were recorded in isolated cells of rat\nlivers. These cells have a continuous high expression of GSH and a broad range of metabo-\nlism. Potentially toxic metabolites of the acrylates and methacrylates were therefore likely\nto be produced in these cells. Furthermore, the additivity of the toxic effect of these chemi-\ncals was investigated in this in-vitro test. For each chemical, an EC 50 for GSH depletion was\ndetermined and used as an effect equivalent to compare their potencies. By testing two\nmixtures, each containing six individual chemicals, it could be shown that the depletion of\nGSH was dose-additive. This means that in a mixture of acrylates and methacrylates each\nindividual chemical will contribute to the total toxic effect of the mixture. As expected, the\ncompounds were metabolized by the hepatocytes. For one of them, allyl methacrylate, the\nvery toxic metabolic product acrolein could be identified in the cell-culture medium. The\nproduction of this metabolite is most probably responsible for the high toxicity of this spe-\ncific compound towards the liver cells as well as towards fish (chapter 3).\nA preliminary physiologically based pharmacokinetic and -dynamic model (PBPK-PD)\nfor ethyl acrylate (EA) was presented in chapter 6. It was based on an existing PBPK model\nfor inert compounds in fish, which had been established by the US-EPA in Duluth, MN ( 1,\n2). The model was adapted to be used with EA by adding elimination processes in several\ntissue compartments. Elimination rates of EA, which had been measured in-vitro, were\nextrapolated to whole organs. The turnover of GSH in the gills was modeled separately and\nwas used to describe the toxic effect of EA on biological targets. Once the model was estab-?lished, several aspects of an aqueous exposure scenario were investigated. The uptake of\nEA in different organs of the fish was predicted to occur very rapidly (steady state concen-\ntrations reached in minutes to a few hours) with exception of the fat tissue. The metabolic\nelimination of EA in the gills was not sufficient to cause a notable first pass effect. Conse-\nquently, the EA concentration in the gill tissue was predicted to be almost instantaneously\nat equilibrium with the aqueous exposure concentration. The EA concentration in the gills\nwas subsequently used in the biological effect sub-model to describe the depletion of GSH.\nFor a simulated exposure scenario close to a lethal aqueous concentration, the GSH concen-\ntration in the gills decreased by 60 % during the first 6 hours. This forecast was in agree-\nment with experimental observations. In contrast to an existing rat model for EA, the trout\nmodel did not predict a first pass elimination of EA and therefore a systemic distribution\ncan be expected in the fish. In both models, however, a local depletion of the GSH level at\nthe site of adsorption was evident.\nIn chapter 7, several findings from the previous chapters were combined to postulate an\nelementary approach to model toxic effects of reactive chemicals in aquatic organisms. The\nmost important simplification of this approach was, to disregard the pharmacokinetics of\nmoderately hydrophobic reactive chemical in aquatic organisms. This resulted in a elemen-\ntary pharmacodynamic model (EPD), which describes a target and the interaction of a reac-\ntive chemical with this target. This approach can be used to describe time and concentration\ndependent toxicological effects. Models, based on this approach were found to give excel-\nlent description of experimental data on acetylcholine-esterase inhibition due to OP-esters\nin several aquatic animals. The approach was also able to predict time dependent effect\nconcentrations (e. g. LC 50 ). Under certain conditions, the EPD model can be reduced to an\nequivalent of HaberÕs Law, which states that the product of concentration and exposure\ntime will be constant. In addition to this, the EPD model can give a rational interpretation of\nthreshold concentration, which are often observed in toxicity experiments.
Author: Andreas P. Freidig
Language: English
Subfield: Health, Toxicology and Mutagenesis
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W1560874561