Molecular Aspects of Herbicide Action on Protoporphyrinogen Oxidase

Protoporphyrinogen oxidase, the last enzyme o f the com m on tetrapyrrole biosynthetic pathway, is inhibited by several peroxidizing com pounds resulting in accum ulation o f pho to ­ dynamic tetrapyrroles, mainly protoporphyrin IX. The inhibition characteristics o f two chemi­ cally unrelated com pounds were studied using m em brane bound pro toporphyrinogen oxidase from corn etioplasts. As shown by Lineweaver-Burk-analysis, the inhibition o f enzyme activity by the diphenyl ether oxyfluorfen and the cyclic imide MCI 15 are competitive with respect to the substrate. The competitive interaction o f protoporphyrinogen and the two chemically un­ related inhibitors indicate a relative specificity o f the binding site. The reversibility o f oxyfluor­ fen inhibition was evaluated by dilution experiments and was shown to be independent o f the presence o f DTT. The analysis o f structure-activity-relationship on pro toporphyrinogen oxi­ dase inhibition was investigated with para-substituted derivatives o f phenyl-3,4,5,6-tetrahydrophthalim ides. The results obtained by QSAR-calculation yielded a good correlation o f the in­ hibitory activity determ ined by the lipophilicity o f the para-substituent. These data poin t to one binding region o f the inhibitors within a lipophilic environm ent associated with the active cen­ ter o f the enzyme.