The role of C-type cytochrome in the Hill reaction with Euglena chloroplasts

Euglena chloroplasts catalyze the Hill reaction with ferricyanide or dichlorophenol indophenol (DPIP) but not with nicotinamide adenine dinucleotide phosphate (NADP), methyl viologen or horse-heart cytochrome c. The latter compounds can serve as Hill oxidants with Euglena chloroplasts provided Euglena cytochrome-552 is included in the reaction mixture. This c-type cytochrome is solubilized during preparation of the algal chloroplasts. It acts in a catalytic fashion and appears to function in the electron transport chain which interconnects the two photosystems of photosynthesis.Euglena cytochrome-552 is most effective in restoration of NADP photoreduction activity; some restoration is observed with another algal c-type cytochrome, Porphyra tenera cytochrome-553. In contrast, restoration was not observed with either Euglena cytochrome-556 or horse-heart cytochrome c.