Six pyridyl derivatives [benzylviologen, 2-anilinopyridine, 1,2-bis(4-pyridyl)ethane, 1,2-bis(4-pyridyl)ethylene, 2-benzoylpyridine, and 2-benzylaminopyridine] and five heme-iron derivatives [hemoglobin, hemin, hematin, ferritin, and ferrocene] were screened for their potential to counteract paraquat (1,1′-dimethyl-4.4′-bipyridinium ion) toxicity on pea (Pisum sativum L.) isolated chloroplasts. The H2O → methylviologen(MV)/O2 and H2O → ferredoxin(Fd)/NADP+ were two Hill reactions assayed with these compounds. Antagonists of paraquat toxicity should inhibit the first Hill reaction but not the latter. AU pyridyl derivatives examined did not inhibit the reaction H2O → MV/O2. Ferritin and ferrocene were also ineffective as inhibitors of this reaction. Hemoglobin inhibited the reaction H2O → MV/O2 without inhibiting the reaction H2O → Fd/NADP+, providing protection to pea chloroplasts against paraquat. Hemin and hematin inhibited both Hill reactions examined. They also inhibited H2O → diaminodurene(DAD)ox and durohydro- quinone → MV/O2 Hill reactions but not the dichlorophenol indophenolred → MV/O2 and DADred → MV/O2 Hill reactions. These results suggest that hemin and hematin are inhibiting the photo- synthetic electron transport in the plastoquinone-pool region.
