Counteraction of Paraquat Toxicity at the Chloroplast Level*

Six pyridyl derivatives [benzylviologen, 2-anilinopyridine, l,2-bis(4-pyridyl)ethane, l,2-b is(4pyridyl)ethylene, 2-benzoylpyridine, and 2-benzylaminopyridine] and five heme-iron derivatives [hemoglobin, hemin, hem atin, ferritin, and ferrocene] were screened for their potential to coun­ teract paraquat (l,l'-dim ethyl-4,4'-bipyridinium ion) toxicity on pea (Pisum sativum L.) isolated chloroplasts. The H :0 —» m ethylviologen(M V )/02 and H 20 —> ferredoxin(Fd)/N ADP+ were two Hill reactions assayed with these compounds. Antagonists of paraquat toxicity should inhibit the first Hill reaction but not the latter. All pyridyl derivatives examined did not inhibit the reaction H 20 —» M V /0 2. Ferritin and ferrocene were also ineffective as inhibitors of this reaction. H em o­ globin inhibited the reaction H 20 —> M V/O z without inhibiting the reaction H 20 —> F d/N A D P +, providing protection to pea chloroplasts against paraquat. Hemin and hematin inhibited both Hill reactions examined. They also inhibited H 20 —* d iam inodurene(D A D )ox and durohydroquinone —> M V /0 2 Hill reactions but not the dichlorophenol indophenolred —> M V /0 2 and D A D red —> M V /0 2 Hill reactions. These results suggest that hemin and hematin are inhibiting the photo­ synthetic electron transport in the plastoquinone-pool region.