Differential activation and inhibition of different forms of rat liver glutathione S-transferase by the herbicides 2,4-dichlorophenoxyacetate (2,4-D) and 2,4,5-trichlorophenoxyacetate (2,4,5-T)

Abstract

The predominant forms of the dimeric enzyme glutathione S-transferase were purified from rat liver. Forms Y b Y′ b and Y b Y b (also known as forms C and A, respectively) could be almost completely inhibited by 2,4-dichlorophenoxyacetate (2,4-D). Half-maximal inhibition was obtained at 0.5 m m 2,4-D. Inhibition was seen even at extrapolated infinite concentrations of both substrates for Y b Y b but not Y b Y′ b . These same forms could also be inhibited 70 to 80% by 2,4,5-trichlorophenoxyacetate (2,4,5-T) with half maximal inhibition occurring at 0.2 m m. Glutathione S-transferase from Y a Y a was maximally inhibited by 72 and 30%, respectively, by 2,4-D and 2,4,5-T. The 30% inhibition of Y a Y a caused by 2,4,5-T was shown to reduce the nearly complete inhibition caused by a previously characterized inhibitor, namely bile acids. This suggests competition for a common binding site on the enzyme. In contrast to the above results, it was found that form Y c Y c (also termed AA) was activated 2.7-fold by 2,4,5-T and 1.4-fold by 2,4-D. This activation could be blocked by chenodeoxycholate which, by itself, did not affect the activity of the enzyme. The effects of 2,4,5-T and 2,4-D on the heterodimer Y a Y c (also termed form B) were intermediate between their effects on Y a Y a and Y c Y c , suggesting that each subunit contributes its unique property to the heterodimer. The microsomal membrane-bound form of glutathione S-transferase was also examined and found to be inhibited by both 2,4-D and 2,4,5-T. However, unlike the inhibitions of soluble forms, 2,4,5-T caused more extensive inhibition than 2,4-D. It is concluded that exposure to 2,4-D and 2,4,5-T can limit the ability of glutathione S-transferase forms Y b Y b and Y b Y′ b to metabolize electrophilic toxins. This capacity to potentiate the toxicity of certain electrophiles indicates a need to study the effect of herbicides on glutathione S-transferases from human tissues.