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Metabolism and Enzymology

Mode of Action Studies on a Chiral Diphenyl Ether Peroxidizing Herbicide

Correlation between Differential Inhibition of Protoporphyrinogen IX Oxidase Activity and Induction of Tetrapyrrole Accumulation by the Enantiomers

Department of Biochemistry, Royal Holloway and Bedford New College, University of London, Egham Hill, Egham, Surrey, TW20 0EX, United Kingdom, Shell Research Ltd., Sittingbourne Research Centre, Sittingbourne, Kent, ME9 8AG, United Kingdom, Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, United Kingdom

The nitrodiphenyl ether herbicide 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitroacetophenone oxime-O-(acetic acid, methyl ester) (DPEI) induced an abnormal accumulation of protoporphyrin IX in darkness in the green alga Chlamydomonas reinhardtii, as determined by high-performance liquid chromatography and spectrofluorimetry. It also inhibited the increase in cell density of the alga in light-grown cultures with an I₅₀ (concentration required to decrease cell density increase to 50% of the noninhibited control value) of 0.16 µM. The relative ability of four peroxidizing diphenyl ether herbicides to cause tetrapyrrole accumulation in C. reinhardtii correlated qualitatively with their ability to inhibit the increase in cell density in light-grown cultures. The purified S(–) enantiomer of the optically active phthalide DPE 5-[2-chloro-4-(trifluoromethyl)phenoxy]-3-methylphthalide (DPEIII), which has greater herbicidal activity than the R(+) isomer, induces a 4- to 5-fold greater tetrapyrrole accumulation than the R(+) isomer. The I₅₀ for inhibition of increase in cell density in light-grown cultures of C. reinhardtii by the S(–) isomer (0.019 µM) is less than 25% of that for the R(+) isomer. DPEIII inhibits protoporphyrinogen IX oxidase activity in pea (Pisum sativum) etioplast lysates, with the S(–) enantiomer showing considerably greater potency than the R(+) isomer and the racemic mixture showing a potency intermediate between the two. The results indicate that the site at which DPEs inhibit protoporphyrinogen IX oxidase shows chiral discrimination and provide further evidence for the link between inhibition of this enzyme, protoporphyrin IX accumulation, and the phytotoxicity of DPE herbicides.

² Present address: Department of Biology, Darwin Building, University College London, Gower Street, London WC1E 6BT, UK.

³ Present address: SmithKline Beecham Research Ltd., The Frythe, Welwyn, Herts. AL6 9AR, UK.