Protoporphyrinogen Oxidase of Mouse and Maize: Target Site Selectivity and Thiol Effects on Peroxidizing Herbicide Action

Abstract

The action of light-dependent peroxidizing herbicides involves inhibition of protoporphyrinogen oxidase (protox) at a high-affinity specific binding site readily assayed with our newN-aryltetrahydrophthalimide radioligand ([3H]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 [3H]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 ∼7-fold by GSH and ∼200-fold by DTT relative to their potency in [3H]THP 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 aN-aryl-cis-hexahydrophthalimide from incubation of flumipropyn with DTT.