Activities of Chloroplast Fragments I . HILL REACTION

Hill activity with ferricyanide and nicotinamide adenine dinucleotide phosphate and photoreduction of NADP and methyl red supported by ascorbate and 2,6-dichloroindophenol were studied as a function of the size of chloroplast fragments. Sonic treatment of chloroplasts resulted in a decrease in Hill activity with NADP and ferricyanide. The decrease was accompanied by a change in the dependency of the reduction rate upon ferricyanide concentration and light intensity. With either electron acceptor, the Hill activity was highest with fragments of medium size and decreased with decreasing size of the fragments. The photoreduction of NADP and methyl red with ascorbate and 2, 6-dichloroindophenol increased after sonic treatment. The larger fragments were less active than the sonically disrupted chloroplasts, whereas the small fragments showed a very high activity; that is, they were several times more active than the larger fragments or sonically treated chloroplasts. Evidence is presented to indicate that the increase in the activity induced by sonic treatment was due to an increased accessibility of the electron-donating reaction.