Chloroplast Respiration : A MEANS OF SUPPLYING OXIDIZED PYRIDINE NUCLEOTIDE FOR DARK CHLOROPLASTIC METABOLISM

A spinach (Spinacia oleracia var. America) chloroplast particle fortified with ferredoxin, fructose-1,6-bisphosphate, or ribose-5-phosphate and NADP has been shown to generate NADPH by the oxidation of glyceraldehyde-3 phosphate to glycerate-3-phosphate (PGA) and to reduce ferredoxin with the NADPH. The resulting reduced ferredoxin can reduce O₂ to H₂O₂, nitrite to ammonia, or protons to H₂. Hydrogen production was the result of adding hydrogenase from Chlamydomonas reinhardii to the chloroplast preparation. The predicted stoichiometry of 1 PGA:1 O₂ in the absence of and 2 PGA:1 O₂ in the presence of catalase was observed indicating H₂O₂ as the end product of O₂ reduction. The predicted stoichiometry of 3 PGA:1 nitrite:1 ammonia was also observed. A scheme is presented to account for a sustained generation of NADP and ATP necessary for the dissimilation of starch in the darkened chloroplast. The unifying term chloroplast respiration is introduced to account for those reactions in which reduced ferredoxin interacts with physiological acceptors other than NADP or nitrite, hydrogen, or O₂ respiration when nitrite, protons, or O₂ is the ultimate electron acceptor.