An Endoplasmic Reticulum Plant Enzyme Has Protoporphyrinogen IX Oxidase Activity

Protoporphyrinogen IX oxidase is inhibited by peroxidizing herbicides, resulting in the accumulation of protoporphyrin IX. The mechanism of protoporphyrin IX formation is unclear. We found a decrease in protoporphyrin IX in intact corn and cucumber etioplasts with increasing herbicide concentrations, which suggests an extraplastidic mechanism may be involved in forming protoporphyrin IX in herbicide-treated plants. Since a microsomal fraction from etiolated corn seedlings showed a substantial protoporphyrinogen-IX oxidizing enzyme activity, the endoplasmic reticulum (ER) from this fraction was purified. ApparentKmandVmaxvalues of the ER enzyme for protoporphyrinogen IX were similar to the values reported for protox from corn thylakoids. The ER enzyme activity, however, was more sensitive to reductants like dithiothreitol than the plastidic enzyme activity and exhibited a higher tolerance toward various peroxidizing herbicides. Accordingly, the ER enzyme may oxidize protoporphyrinogen IX in the presence of herbicide concentrations, which inhibit the plastidic and mitochondrial protoporphyrinogen IX oxidase. Apparently the ER enzyme is instrumental in the phytotoxic accumulation of protoporphyrin IX in herbicide-treated plants.