Glendon D. Sinks

Structure–toxicity relationships for selected halogenated aliphatic chemicals

Toxicity to the ciliate Tetrahymena pyriformis (log(IGC(50)(-1))) for 39 halogen-substituted alkanes, alkanols, and alkanitriles were obtained experimentally. Log(IGC(50)(-1)) along with the hydrophobic term, logK(ow) (1-octanol/water partition coefficient) and the electrophilic parameter, E(lumo) (the energy of the lowest unoccupied molecular orbital) were used to develop quantitative structure-activity relationships (QSARs). Two strong hydrophobic dependent relationships were obtained: one for the haloalkanes and a second for the haloalcohols. The relationship for the haloalkanes [log(IGC(50)(-1))=0.92 (logK(ow))-2.58; n=4, r(2)=0.993, s=0.063, f=276, Pr>f=0.0036] was not different from baseline toxicity. With the rejection of 1,3-dibromo-2-propanol as a statistical outlier, the relationship [log(IGC(50)(-1))=0.63(logK(ow))-1.18; n=19, r(2)=0.860, s=0.274, f=104, Pr>f=0.0001] was observed for the haloalcohols. No hydrophobicity-dependent model (r(2)=0.165) was observed for the halonitriles. However, an electrophilicity-dependent model [log(IGC(50)(-1))=-1.245(E(lumo))+0.73; n=15, r(2)=0.588, s=0.764, F=18.6, Pr>f=0.0009] was developed for the halonitriles. Additional analysis designed to examine surface-response modeling of all three chemical classes met with some success. Following rejection of statistical outliers, the plane [log(IGC(50)(-1))=0.60(logK(ow))-0.747(E(lumo))-0.37; n=34, r(2)=0.915, s=0.297, F=162, Pr>F=0.0001] was developed. The halogenated alcohols and nitriles tested all had observed toxicity in excess of non-reactive baseline toxicity (non-polar narcosis). This observation along with the complexity of the structure-toxicity relationships developed in this study suggests that the toxicity of haloalcohols and halonitriles is by multiple and/or mixed mechanisms of action which are electro(nucleo)philic in character.

Quinone-induced toxicity to Tetrahymena: structure-activity relationships

Abstract The aquatic toxicities (log(IGC50−1)) of 22 quinones were evaluated in the static Tetrahymena pyriformis population growth assay. A toxicity in excess of baseline was associated with each chemical eliciting a response. The ortho-quinones were the most toxic; internal quinones, with benzenoid substitution on both sides of the quinoid rings, were not toxic at saturation. Ring substitution of 1,4-benzoquinone resulted in a graded toxic response. Substitution by electron-donating, methoxy or hydroxy groups decreased the toxicity. The position of ring substitution was also important, with 2,5-substitution being less toxic than 2,6-substitution. The fully substituted tetramethyl and tetrachloro derivatives, quinones which cannot conjugate or arylate protein thiols, were significantly less toxic than 1,4-benzoquinone. Due to their molecular structure, quinones may act as soft electrophiles and/or redox cyclers. This mixture of mechanisms of toxic action precludes high-quality, quantitative, structure-toxicity relationships. The toxic potency was found to be independent of the 1-octanol/water partition coefficient, one-electron redox potential and lowest unoccupied molecular orbital energy.

Structure-Toxicity Relationships for Aliphatic Compounds Encompassing a Variety of Mechanisms of Toxic Action to Vibrio fischeri

Abstract QSARs based upon the logarithm of the octanol-water partition coefficient, logP, and energy of the lowest unoccupied molecular orbital, ELUMO were developed to model the toxicity of aliphatic compounds to the marine bacterium Vibrio fischeri. Statistically robust, hydrophobic-dependent QSARs were found for chloroalcohols and haloacetonitriles. Modelling of the toxicity of the haloesters and the diones required the use of terms to describe both hydrophobicity and electrophilicity. The differences in intercepts, slopes, and fit of these models suggest different electrophilic mechanisms occur between classes, as well as within the diones and haloesters. In order to model globally the toxicity of aliphatic compounds to V. fischeri, all the data determined in this study were combined with those determined previously for alkanones, alkanals, and alkenals. A highly predictive two-parameter QSAR [pT15 = 0.760(log P) −0.625(E LUMO) −0.466; n = 63, s = 0.462, r 2 = 0.846, F = 171, Pr > F = 0.0001] was developed for the combined data that models across classes and is independent of mechanisms of action. The toxicity of these compounds to V. fischeri compares well to the toxicity (50% population growth inhibition) to the ciliate Tetrahymena pyriformis (r 2 = 0.850).

Reproducibility of toxicity across mode of toxic action in the Tetrahymena population growth impairment assay

Toxicity data collected in a laboratory setting are the primary source of potency information used for regulatory, modeling, or risk assessment purposes. However, the relative reproducibility of such toxicity data is rarely discussed. This study investigated the reproducibility of growth impairment data for the freshwater ciliate Tetrahymena pyriformis exposed to a structurally diverse group of chemicals of varying hydrophobicity within different modes of toxic action, either non-covalent narcosis or covalent electro(nucleo)philicity. The proportions of chemicals representing each mode of toxic action, or mechanism of action within each mode, were not chosen to emulate the occurrence of manufactured chemicals or chemicals within the TETRATOX database. Chemicals for which prior toxicity data existed were re-tested and reproducibility was evaluated. The toxic potency values of the selected chemicals were largely reproducible after re-testing of the toxic potency, as 98% of the chemicals had re-test toxicity values within one log unit of the original potency value. To further scrutinize the reproducibility of toxicity values, differences between values were investigated by mode of toxic action. A stringent criterion for reproducibility was enforced, which dictated that the re-tested toxicity value must be encompassed by the fiducial interval (FI) of the original toxicity value and vice versa for the chemical to be considered reproducible. Toxicity values of 28 of the 50 re-tested chemicals conformed to the criterion set for reproducible values. Of the nonreproducible chemicals, seven were narcotics: four nonpolar or neutral narcotics and three other narcotics (e.g. polar narcotics). However, four of these seven narcotics did have toxicity values encompassed by one FI, but not the other FI. The remaining chemicals that did not have reproducible potency measurements were electro(nucleo)philic in nature. Certain toxicophores were highly represented among these chemicals. These included quinone derivatives, electron releasing amino and hydroxyl moieties, and electron withdrawing nitro substituents, often in tandem with strong leaving groups (i.e. halogens), and unsaturated alcohols. Lack of reproducibility was common among the chemicals that elicited toxicity after either abiotic or biotic transformation. There was no clear trend between hydrophobicity and lack of reproducibility. While data are limited, these results suggest that toxic potency values of chemicals acting via the electro(nucleo)philic mode of toxic action could be more susceptible to non-reproducibility. Ramifications of such lack of reproducibility could manifest in predictive toxicology models and their use in regulatory and risk assessment endeavors.

STRUCTURE-TOXICITY RELATIONSHIPS FOR ALKANONES AND ALKENONES

The relative toxicity (log IGC-1(50)) of 54 selected alkanones, both aliphatic and aromatic, as well as, alkenones and alkynones was evaluated in the static Tetrahymena pyriformis population growth assay. Excess toxicity, an indicator of bioreactivity, was associated only with the alpha-beta unsaturated alkenones and alkynones. Moreover, the alkynones were found to be more toxic than corresponding alkenones. A high quality 1-octanol/water partition coefficient (log Kow) dependent structure-toxicity relationship, log IGC-1(50) = 0.86 (log Kow) - 2.27; r2 = 0.955, was developed for alkanones. This QSAR represented the nonpolar narcosis mechanism of toxic action. Toxicity of alkenones was predicted by the highest-occupied-molecular-orbital energy (HOMO), log IGC-1(50) = -3.474 (HOMO) -35.357; r2 = 0.897, and the difference between HOMO and the lowest-unoccupied-molecular-orbital energy (LUMO), log IGC-1(50) = -3.559 (HOMO-LUMO gap) - 36.106; r2 = 0.903. The alpha-beta unsaturated ketones are considered soft electrophiles. Moreover, the toxicity of the aliphatic alkanones and alkenones was predicted by log Kow and LUMO, log IGC-1(50) = 0.69 (log Kow) - 2.55 (LUMO) + 0.05; r2 = 0.852.

Acclimation to sublethal exposures to a model nonpolar narcotic: population growth kinetics and membrane lipid alterations in Tetrahymena pyriformis

Abstract Tetrahymena pyriformis has been shown to acclimate (i.e. phenotypic change) to the presence of sublethal levels of hydrophobic organic chemicals considered to act via nonpolar narcosis mode of toxic action. The theoretical site of action for narcosis is the cellular membrane. The objective of this work was to parallel population growth kinetics with molecular toxicology to investigate acclimation. To this end naive and pre-exposed populations of ciliates were exposed to sublethal concentrations of the model nonpolar narcotic, 1-octanol. A control, solvent control, and three sublethal concentrations of 1-octanol were tested. Naive populations exposed to 1-octanol exhibited a concentration dependent lag phase in growth where there was no growth followed by growth at rates similar to control populations. Pre-exposed populations transferred to the same or a lower concentration did not exhibit a lag phase and grew at rates equal to control populations. Pre-exposed populations transferred to a higher concentration exhibited a lag phase in growth. This lag phase was shorter for pre-exposed populations than for naive populations. The relative percent of the following fatty acid methyl esters (FAMEs) did not change with 1-octanol exposure: 12:0, 14:0, 15:0, iso15:0, and 17:0. However, an increase was observed for FAMEs 16:0 and 18:0 with exposure to 1-octanol. Conversely, with exposure there was a decrease for the following FAMEs: 16:1, 18:1, and 18:2Δ6,11. The overall decrease in the number of π-bonds is thought to be related to a net decrease in overall fluidity. Additionally, there was an increase in the ratio of 16/18 carbon FAMEs implicating physical accommodation of the compound within the membrane. Molecular changes are directly correlated with the population growth trends.

Structure-toxicity relationships for aminoalkanols: a comparison with alkanols and alkanamines

The relative toxicity (log IGC50(-1)) of 49 selected aliphatic amines and aminoalkanols was evaluated in the static Tetrahymena pyriformis population growth impairment assay. Excess toxicity, indicated by potency greater than predicted for non-polar narcotic alkanols, was associated with both classes of test chemicals. Moreover, the aminoalkanols were found to be more toxic than the corresponding alkanamines. A high quality 1-octanol/water partition coefficient (log K(ow)) dependent quantitative structure-activity relationship (QSAR), logIGC50(-1) = 0.78 (log K(ow)) - 1.42; r2 = 0.934, was developed for alkanamines. This QSAR represented the amine narcosis mechanism of toxic action. No quality QSAR was developed for the aminoalkanols. However, several structure-toxicity features were observed for this class of chemicals. Two-amino-1-hydroxy derivatives being more toxic than the corresponding derivatives, where the amino and hydroxy moieties were separated by methylene groups. Hydrocarbon branching next to the amino moiety resulted in decreased toxicity. Aminoalkanol alters lipid metabolism in T. pyriformis.

Population growth kinetics of Tetrahymena pyriformis exposed to selected pyridines

Summary Alterations in growth kinetics of populations of the freshwater ciliate Tetrahymena pyriformis due to exposure to six specific pyridines were investigated. Selected pyridines varied in mode of toxic action and hydrophobicity (quantified by the logarithm of the 1-octanol-water partition coefficient, log Kow). Three pyridines containing lower hydrophobicity (log Kow ≈ 0.5) and higher hydrophobicity (log Kow ≈ 1.3) were tested. Within each hydrophobicity group were representative pyridines of both non-covalent (neutral narcotics and other narcotics) and covalent modes of toxic action. Growth kinetic trends were compared and contrasted, both between and within modes of toxic action. Pyridine and 3-chloropyridine exposed populations inhibited by 32% or 50%, respectively, exhibited doubling times significantly longer than control populations. The growth kinetic trends were hydrophobicity-dependent, as established by an increased lag phase in the more hydrophobic 3-chloropyridine. The hydroxylated pyridines demonstrated similar growth kinetics independent of hydrophobicity, but distinct from the neutral narcotic pyridines. No lag phase existed and a concentration-dependent decrease in generation time occurred. This distinction suggests that hydroxylated pyridines elicit toxicity via a unique mechanism of toxic action from baseline narcotics within the non-covalent mode of action. T. pyriformis populations exposed to the nitro derivatives exhibited different trends. Two-chloro-3,5-dinitropyridine displayed concentration-dependent mortality of the initial population over the first 4 h of exposure. Remaining cells then commenced growth with generation times similar to control populations. The growth kinetic trends of 2-bromo-5-nitropyridine were a hybrid between those observed for the reactive dinitro derivative and the neutral narcotics. Thus, growth kinetic trends with the covalent mode of action may be mechanism specific as well.

Effect of substituent size and dimensionality on potency of phenolic xenoestrogens

Abstract Previous results show the position, size, and shape of the non-phenolic moiety of xenoestrogen effect potency. Para-substituted compounds were more potent than meta-substituted ones, which in turn were more potent than ortho-substituted compounds. A minimum three-carbon-moiety is required for activity and tertiary-branched or highly substituted congeners were more potent than secondary/normal or less substituted ones. In an effort to quantify the relationship between size/shape of the non-phenolic moiety and estrogenic potency, a series of substituted phenols were evaluated and activity was correlated with a series of bulk and shape parameters. The Saccharomyces cerevisiae-based Lac-Z reporter assay was then used and estrogenicity reported as the logarithm of the inverse of the 50% β-galactosidase activity [log (EC50−1)]. There is a trend of an increase in estrogenicity with an increase in substituent size. Selected size parameters (molecular volume and molar refractivity), Verloops sterimol shape parameters (L and B5) and the fourth order path/cluster molecular connectivity index were found to be correlate poorly (r2 ≈0.65) with estrogenicity. The r2 value of the order of the path, versus log (EC50−1) showed parabolic relationships. The third order path index (r2 > 0.80) provided the best fit with the estrogenicity.