In vitro studies on the chemical reactivity of 2,4-dichlorophenoxyacetyl-S-acyl-CoA thioester.

2,4-Dichlorophenoxyacetic acid (2,4-D) is a widely used broadleaf herbicide that has been associated with acute liver toxicity in exposed humans or animals. Chemically reactive metabolites of 2,4-D are proposed as mediators of 2,4-D-induced hepatotoxicity. The aim of the present study was to investigate a novel reactive metabolite of 2,4-D, namely 2,4-dichlorophenoxyacetyl-S-acyl-CoA (2,4-D-CoA), and to determine its involvement in 2,4-D covalent adduct formation. Thus, incubations of synthetic 2,4-D-CoA (106 microM) with GSH (1 mM) in phosphate buffer (pH 7.4) showed 2,4-D-CoA to be able to transacylate the cysteine sulfhydryl of GSH, resulting in the formation of 2,4-D-S-acyl-glutathione (2,4-D-SG) thioester and reaching a concentration of 65 microM after 1 h of incubation. Under similar conditions, 2,4-D-CoA was shown to covalently bind to nucleophilic groups on human serum albumin (HSA, 30 mg/ml), resulting in time-dependent 2,4-D-HSA covalent adduct formation that reached a maximum of 440 pmol/mg HSA after 1 h of incubation. In addition to these studies, incubations of [1-(14)C]2,4-D (1 mM) with rat hepatocytes showed a time-dependent covalent binding of 2,4-D to hepatocyte protein. Inhibition of acyl-CoA formation by trimethylacetic acid (2 mM) decreased the amount of covalent binding to protein in rat hepatocytes by 50%. These results indicate that 2,4-D-CoA thioester is a reactive metabolite of 2,4-D that may contribute to 2,4-D-protein adduct formation in vivo and therefore the associated hepatotoxicity.