Effect of pyrophosphate on photosynthetic electron transport reactions.

Inorganic pyrophosphate is found to inhibit the ferredoxin-dependent photoreduction of NADP by isolated chloroplasts. The reduction of ferricyanide is not inhibited, nor is the activity of photosystem 1 as measured with methyl viologen as the electron acceptor. All other ferredoxin-depended reactions are inhibited, such as cytochrome c photoreduction and the reaction sequence: NADPH -->Flavoprotein-->ferredoxin-->cytochrome c. The inhibition by pyrophosphate is, in all cases, competitive with ferredoxin and independent of NADP concentration. Pyrophosphate inhibition of the formation of the ferredoxin-flavoprotein complex is demonstrated spectrophotometrically.