On a water soluble factor neutralizing antibodies against the primary acceptor in photosynthetic electron transport of chloroplasts.

Rabbits immunized against the thylakoid system of chloroplasts form two different antibodies against the reducing site of photosystem I. One inhibits photosynthetic NADP(+) and ferredoxin dependent cytochrome c reduction, but not photosynthetic anthraquinone reduction by isolated spinach chloroplasts. The other inhibits all three reductions. Both do not inhibit a Hill reaction with ferricyanide. The presumed antigen moiety against these antibodies was isolated as a water soluble factor released from lyophilized chloroplasts after treatment with diethylether. The heat stable, nondialysable factor has absorption peaks at 262 and 315 mμ. It is autoxidizable and has the property of a cytochrome c reducing substance. The factor neutralizes the antibody inhibition of photosynthetic anthraquinone and ferredoxin reduction. It is proposed that the soluble factor is the prosthetic group of the primary acceptor of photosystem I.