The Reducing Side of Photosystem I
This chapter reviews the reducing side of photosystem I. Yocum and San Pietro found that vigorous sonication of osmotically shocked chloroplasts produced chloroplast fragments with low rates of NADP reduction. These fragments, fractionated by density-gradient centrifugation or gel filtration, showed higher rates of NADP reduction when supplemented with a factor termed “ferredoxin reducing substance” (FRS). A rate vs. concentration plot of FRS gave rise to saturation kinetics, suggesting that FRS was acting catalytically in the assay system. Under anaerobic conditions, substrate amounts of photo-reduced FRS were capable of reducing ferredoxin added in a succeeding dark step. Photoreduction of methyl viologen was not observed with FRS-depleted subchloroplast fragments, but this activity was restored by adding FRS to the fragments. The principal difficulty encountered in studies of FRS has been the method of extraction, namely sonication. While chloroplast fragments prepared by this procedure retain normal physiological properties, that is, requirements for plastocyanin, ferredoxin, and ferredoxin-NADP reductase, basal rates of NADP reduction from an ascorbate-DPIP donor system have been extremely variable. Sonication is not a reproducible method for extraction of FRS. © 1973, ACADEMIC PRESS, INC.
Siedow, J; Yocum, CF; Pietro, AS
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