Effects of chlorophenoxy herbicides and their main transformation products on DNA damage and acetylcholinesterase activity

Abstract

Persistent pesticide transformation products (TPs) are increasingly being detected among different environmental compartments, including groundwater and surface water. However, there is no sufficient experimental data on their toxicological potential to assess the risk associated with TPs, even if their occurrence is known. In this study, the interaction of chlorophenoxy herbicides (MCPA, mecoprop, 2,4-D and dichlorprop) and their main transformation products with calf thymus DNA by UV-visible absorption spectroscopy has been assessed. Additionally, the toxicity of the chlorophenoxy herbicides and TPs was also assessed evaluating the inhibition of acetylcholinesterase activity. On the basis of the results found, it seems that AChE is not the main target of chlorophenoxy herbicides and their TPs. However, the results found showed that the transformation products displayed a higher inhibitory activity when compared with the parent herbicides. The results obtained in the DNA interaction studies showed, in general, a slight effect on the stability of the double helix. However, the data found for 4-chloro-2-methyl-6-nitrophenol suggest that this transformation product can interact with DNA through a noncovalent mode.

Figure 1

Chemical structures of the chlorophenoxy herbicides and their main environmental transformation products: (a) MCPA and mecoprop, (b) 2,4-D and dichlorprop, (c) 4-chloro-2-methylphenol and 2,4-dichlorophenol, and (d) 4-chloro-2-methyl-6-nitrophenol and 2,4-dichloro-6-nitrophenol.

Figure 2

UV-Vis absorption spectra of (a) 5.0 × 10⁻⁵mol dm⁻³ and (b) 1.0 × 10⁻⁴mol dm⁻³ of chlorophenoxy herbicides and TPs standard solutions in pH 7.2 phosphate buffer electrolyte.

Figure 3

UV-Vis absorption spectra of 7.7 × 10⁻⁵ mol dm⁻³ of DNA in the absence and presence of 1.0 × 10⁻⁴ mol dm⁻³ of (a) mecoprop and MCPA, (b) 2,4-D and dichlorprop, (c) 4-chloro-2-methylphenol and 2,4-dichlorophenol, and (d) 4-chloro-2-methyl-6-nitrophenol and 2,4-dichloro-6-nitrophenol in pH 7.2 phosphate buffer electrolyte.

Figure 4

UV-Vis absorption spectra of 7.7 × 10⁻⁵mol dm⁻³ of DNA in the absence and presence of 1.0 × 10⁻⁴mol dm⁻³ of (a) 2,4-dichlorophenol and (b) 4-chloro-2-methyl-6-nitrophenol in pH 7.2 phosphate buffer electrolyte.

Figure 5

Corrected absorbance at 260 nm versus temperature for 7.7 × 10⁻⁵mol dm⁻³ of DNA in the absence and presence of 1.0 × 10⁻⁴mol dm⁻³ of (a) mecoprop and MCPA, (b) 2,4-D and dichlorprop, (c) 4-chloro-2-methylphenol and 2,4-dichlorophenol, and (d) 4-chloro-2-methyl-6-nitrophenol and 2,4-dichloro-6-nitrophenol in pH 7.2 phosphate buffer electrolyte.

Figure 6

Molar fraction of denatured DNA versus temperature for 7.7 × 10⁻⁵ mol dm⁻³ of DNA in the absence and presence of 5.0 × 10⁻⁵ mol dm⁻³ of TPs in pH 7.2 phosphate buffer electrolyte.