title: 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 YbY'b and YbYb (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 mM 2,4-D. Inhibition was seen even at extrapolated infinite concentrations of both substrates for YbYb but not YbY'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 mM. Glutathione S-transferase form YaYa was maximally inhibited by 72 and 30%, respectively, by 2,4-D and 2,4,5-T. The 30% inhibition of YaYa 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 YcYc (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 YaYc (also termed form B) were intermediate between their effects on YaYa and YcYc, 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.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D015085:2,4,5-Trichlorophenoxyacetic Acid / Q000633:toxicity*; D015084:2,4-Dichlorophenoxyacetic Acid / Q000633:toxicity*; D000818:Animals; D004789:Enzyme Activation / Q000187:drug effects; D005982:Glutathione Transferase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000201:enzymology*; D051381:Rats
chemical_list: D015084:2,4-Dichlorophenoxyacetic Acid; D015085:2,4,5-Trichlorophenoxyacetic Acid; D005982:Glutathione Transferase