Protoporphyrinogen oxidase of mouse and maize: Target site selectivity and thiol effects on peroxidizing herbicide action

The action of light-dependent peroxidizing herbicides involves inhibition of protoporphyrinogen oxidase (protox) at a high-affinity specific binding site readily assayed with our new N-aryltetrahydrophthalimide radioligand ([H-3]THP), the desmethyl analog of flumipropyn. Protox of mouse liver mitochondria and maize etioplasts is similar in sensitivity to most of the 14 herbicides and analogs examined as inhibitors of [3H]THP binding, indicating that target site specificity is not a major factor in selective toxicity between mammals and plants. In assays using mouse protox, the 14 compounds fall into two groups upon correlating their ability to inhibit [H-3]THP binding (without added thiol) and enzymatic activity [with added glutathione (GSH) or dithiothreitol (DTT) as an antioxidant for the substrate]. The inhibitory potency of the THPs in protox activity assays is reduced similar to 7-fold by GSH and similar to 200-fold by DTT relative to their potency in [H-3]TKP binding assays without added thiol. This ''thiol effect'' is only 2- to 4-fold with diphenyl ethers and oxadiazon. The reduction of THP potency by these thiols may be due to derivatization based on the identification of a N-aryl-cis-hexahydrophthalimide from incubation of flumipropyn with DTT. (C) 1997 Academic Press.