An in vitro toxicological assay utilizing submitochondrial particles (SMP) has been used to evaluate the toxic effects of herbicides belonging to two different chemical classes: phenols and phenoxyalcanoic acids. The SMP assay, based on reverse electron transfer (RET) from succinate to NAD+, is simple and rapid and allows for the quantification of the toxicity of compounds with different mechanisms of action: uncouplers, inhibitors of the enzyme complexes involved in reverse electron transfer and in oxidative phosphorylation and, in addition, chemicals that alter the membrane integrity. Previous studies showed that the response of this assay is comparable to that of standard aquatic toxicity assays. The two groups of herbicides showed different levels of toxicity. For phenol derivatives, EC50 values ranged from 0.16 µM for ioxynil to 6.7 µM for 2,4-dinitrophenol; the phenoxyalcanoic herbicides presented EC50 values between a minimum of 21 µM for 2,4,5-T and a maximum of 110 µM for MCPA. On an average, the toxicity of phenolic compounds was greater than that of phenoxyalcanoic acids by about two orders of magnitude. In order to evaluate the relationships between the physico-chemical and structural properties of the examined compounds and their toxicity, QSARs were developed by correlating EC50 values with various molecular descriptors. The results, together with the order of magnitude of EC50 values, suggest the existence of different mechanisms of action for the two classes of compounds. The findings obtained for phenolic herbicides are consistent with a protonophoric uncoupling action, whereas for phenoxyalcanoic herbicides a non-specific action at membrane level has been hypothesized.
Determination of herbicide toxicity by the submitochondrial particle (SMP) assay and quantitative structure-activity relationships
BETTIOL, Cinzia;MARCHETTO, Davide;ARGESE, Emanuele
2002-01-01
Abstract
An in vitro toxicological assay utilizing submitochondrial particles (SMP) has been used to evaluate the toxic effects of herbicides belonging to two different chemical classes: phenols and phenoxyalcanoic acids. The SMP assay, based on reverse electron transfer (RET) from succinate to NAD+, is simple and rapid and allows for the quantification of the toxicity of compounds with different mechanisms of action: uncouplers, inhibitors of the enzyme complexes involved in reverse electron transfer and in oxidative phosphorylation and, in addition, chemicals that alter the membrane integrity. Previous studies showed that the response of this assay is comparable to that of standard aquatic toxicity assays. The two groups of herbicides showed different levels of toxicity. For phenol derivatives, EC50 values ranged from 0.16 µM for ioxynil to 6.7 µM for 2,4-dinitrophenol; the phenoxyalcanoic herbicides presented EC50 values between a minimum of 21 µM for 2,4,5-T and a maximum of 110 µM for MCPA. On an average, the toxicity of phenolic compounds was greater than that of phenoxyalcanoic acids by about two orders of magnitude. In order to evaluate the relationships between the physico-chemical and structural properties of the examined compounds and their toxicity, QSARs were developed by correlating EC50 values with various molecular descriptors. The results, together with the order of magnitude of EC50 values, suggest the existence of different mechanisms of action for the two classes of compounds. The findings obtained for phenolic herbicides are consistent with a protonophoric uncoupling action, whereas for phenoxyalcanoic herbicides a non-specific action at membrane level has been hypothesized.
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