Photoreduction of Methyl Red and Tetrazolium Blue by Spinach Chloroplasts and Chromatophores of Rhodospirillum rubrum.

Abstract

Chloroplasts prepared from spinach leaves have been shown to catalyze a photoreduction of both methyl red and tetrazolium blue in a regular Hill reaction. Addition of 3-(3,4-dichlorophenol)-l,l-dimethylurea (DCMU) at 10-5 [image] inhibited both reactions, but the further addition of ascorbate and 2,6-dichlorophenolindo-phenol (DPIP) restored the activity of the chloroplasts in the photo -reductions. Chromatophores of R. rubrum also catalyzed a photoreduction of both dyes in the presence of DPIP and ascorbate. Addition of quinacrine markedly stimulated both reactions, with half-maximal stimulation occurring at 20 [mu][image] quinacrine. Experiments performed in red light showed the quinacrine did not increase the reaction rate by itself acting as a photocatalyst in the reactions. Addition of quinacrine inhibited the succinate-supported photoreduction of DPN by the chromatophores. The respiratory inhibitors antimycin A and 2-heptyl-4-hydroxyquinoline-N-oxide both failed to stimulate the photoreduction of tetrazolium blue and methyl red by R. rubrum chromatophores, but instead were slightly inhibitory. These inhibitors strongly inhibited the succinate-supported photoreduction of DPN. Thus in terms of inhibition characteristics with chromatophores, quinacrine did not resemble antimycin A and HQNO. The photoreductions catalyzed by chloroplasts were not affected by added quinacrine, which is a distinguishing feature of the 2 photosynthetic systems.