Time-dependence in mixture toxicity with soft-electrophiles: 2. Effects of relative reactivity level on time-dependent toxicity and combined effects for selected Michael acceptors

ArticleinJournal of Environmental Science and Health Part A 43(1):43-52 · February 2008with15 Reads
DOI: 10.1080/10934520701750371 · Source: PubMed
Toxicity assessments for organic chemical mixtures are often described as being approximately additive. Recent mixture studies with soft electrophiles have suggested that agents with less-than fully time-dependent toxicity (TDT) may actually induce toxicity by more than one mode of toxic action within the same series of concentrations. To evaluate this concept further, four Michael acceptor electrophiles, each with a different rate of in chemico reactivity and different level of TDT, were tested with each other and in sham combinations (a single chemical tested as if it were a binary mixture) using the Microtox system. For each binary combination, each agent was tested alone and in a mixture, with toxicity assessed as inhibition of bioluminescence at 15-, 30- and 45-min of exposure. Each single agent and mixture test included seven duplicated concentrations and a duplicated control treatment. To evaluate relative reactivity, each agent was also tested with the model nucleophile glutathione (GSH). Agents with greater in chemico reactivity (mean RC(50) mM) showed greater toxicity (mean 45-min EC(50) - mM) but these were inversely related to the TDT levels of the agents. Combined effects for the sham combinations, as quantified by additivity quotient values for the EC(50) of the mixture, tended to be close to 1.00 (i.e., the dose-addition EC(50)-AQ). For true binary combinations (i.e., two chemicals tested together), the EC(50)-AQ tended to be increasingly above 1.00 when TDT levels of the agents in the mixture were more disparate. The results of this study with Michael acceptors suggested that: (i) when reactivity was fast, there was most likely a single prominent mode of toxic action, i.e., electro(nucleo)philic reactivity, leading to time-dependent toxicity at the full or high levels, (ii) when the reaction rate for a chemical was slower, two modes of action, electro(nucleo)philic reactivity and narcosis, were apparent such that the time-dependent toxicity level was lower as well, (iii) mixtures of the former agents show a combined effect that was strictly dose-additive, whereas (iv) mixtures which included one (or more) agent with a lower reaction rate had a combined effect that was approximately additive rather than strictly dose-additive.
    • "There was an increasing number of published works that were more ambitious in their scope and applications. Dawson and coworkers [103, 342, 343] performed a series of basic mechanistic studies of binary mixtures of reactive and nonreactive chemicals (soft electrophiles and narcotics, respectively ) on V. fischeri. They generated a series of well-designed experiments oriented to reveal the molecular site(s) of action of these chemicals. "
    [Show abstract] [Hide abstract] ABSTRACT: This chapter deals with the use of bioluminescent microorganisms in environmental monitoring, particularly in the assessment of the ecotoxicity of pollutants. Toxicity bioassays based on bioluminescent microorganisms are an interesting complement to classical toxicity assays, providing easiness of use, rapid response, mass production, and cost effectiveness. A description of the characteristics and main environmental applications in ecotoxicity testing of naturally bioluminescent microorganisms, covering bacteria and eukaryotes such as fungi and dinoglagellates, is reported in this chapter. The main features and applications of a wide variety of recombinant bioluminescent microorganisms, both prokaryotic and eukaryotic, are also summarized and critically considered. Quantitative structure-activity relationship models and hormesis are two important concepts in ecotoxicology; bioluminescent microorganisms have played a pivotal role in their development. As pollutants usually occur in complex mixtures in the environment, the use of both natural and recombinant bioluminescent microorganisms to assess mixture toxicity has been discussed. The main information has been summarized in tables, allowing quick consultation of the variety of luminescent organisms, bioluminescence gene systems, commercially available bioluminescent tests, environmental applications, and relevant references.
    Full-text · Article · Nov 2014
    • "Thus, due to the different modes of action of NPN and S N Ar, the toxicities had dose-additive behaviour. A second part of this work (Dawson et al., 2008) studied the mixture toxicity for a series of Michael acceptors. They observed that there was a clear correlation between toxicity in the Microtox assay and GSH reactivity. "
    [Show abstract] [Hide abstract] ABSTRACT: The use of so-called “in chemico” methodology - abiotic assays that measure chemical reactivity - is gaining ground as relevant and reliable means of toxicity prediction. In this report we explain the basis of the in chemico approach to toxicity prediction and we review the studies that have developed the concept and its practical application since the 1930s, with special attention being paid to studies aimed at the development of Quantitative Structure-Activity Relationship (QSAR) models and read-across approaches. The studies covered in this review are limited to non-enzymatic experiments and to nucleophiles up to 50 amino acids. The main applications identified are related to the assessment of skin sensitisation, aquatic toxicity and hepatotoxicity. Various experimental measures of nucleophile depletion or adduct formation have been proposed as chemical reactivity descriptors, but no single protocol has emerged as the most generally useful. It is concluded that in chemico approaches provide a promising means of toxicity prediction within their applicability domains and should be further developed and investigated as alternative methods to animal testing, especially when used in the context of integrated testing strategies based on the use of multiple non-animal methods.
    Full-text · Technical Report · Nov 2012 · Toxicology
    • "It could be argued that the range of additivity quotient (AQ) values used herein for dose-addition (0.90 – 1.10) was too narrow (i.e., 1.00 ± 0.20 is often considered " additive " ; Hodges et al., 2006; Broderius et al., 1995). However, our previous results, which were derived from tests that have included using at least seven duplicated concentrations/curve and testing of " sham " combinations (the latter being rather rare among mixture toxicity assays), support the narrower designation (Dawson et al., 2011; 2010; 2008; Gagan et al., 2007). With that having been stated, using the broader range of AQ values (0.80 – 1.20) to represent a dose-additive combined effect had little effect on the total number of " different " combined effect designations between the 4PL and 5PL-1P functions (39 for the narrower dose-addition range, 38 for the broader range). "
    [Show abstract] [Hide abstract] ABSTRACT: In mixture toxicity, concentration-effect data are often used to generate conclusions on combined effect. While models of combined effect are available for such assessments, proper fitting of the data is critical to obtaining accurate conclusions. In this study an asymmetry parameter (s) was evaluated for data-fitting and compared with our previous approach. Inhibition of bioluminescence was assessed with Vibrio fischeri at 15, 30 and 45-min of exposure with seven or eight concentrations and a control (each duplicated) for each single-chemical (A or B) and mixture (A:B). Concentration-effect data were fitted to sigmoid curves using the four-parameter logistic function (4PL) and the five-parameter logistic minus one-parameter (5PL-1P) function. For the 4PL, parameters included minimum effect, maximum effect, EC(50) and slope, while for the 5PL-1P the minimum effect parameter was removed and an asymmetry parameter was added. A total of 72 mixture toxicity data sets were evaluated, representing 432 single-chemical and 216 mixture curves. Mean coefficients of determination (r(2)) for all 648 curves showed that the 5PL-1P gave better fitting (0.9982 ± 0.0018) than the 4PL (0.9973 ± 0.0030). For both functions, the sum-of-squares of the residuals (SS-Res) was determined for each curve. The 5-parameter rational regression best described the relationship between the decrease in sum-of-squares of the residuals (i.e., 4PL: SS-Res - 5PL-1P: SS-Res) and log s, with fitting improved the most at low values of s (s<0.8). This held even when curves with r(2) values ≤ 0.9970 were removed from the analyses. Subsequent review of the combined effects obtained via the 4PL and the 5PL-1P functions resulted in a change in the interpretation of combined effect in 39/216 (18%) cases.
    Full-text · Article · Dec 2011
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