On the mechanism of photosynthetic oxygen reduction by isolated chloroplast lamellae.

Ferredoxin-stimulated photosynthetic oxygen reduction and concommitant decarboxylation of glyoxylate by chloroplast lamellar systems is inhibited by ascorbate but ferredoxin dependent NADP+-reduction is not. In the presence of low potential electron acceptors (AQ or MV) this influence of ascorbate on glyoxylate decarboxylation by chloroplast lamellar systems is no longer observed. Using the diaphorase activity of NADP-ferredoxin reductase either bound to the chloroplast lamellar system or as an isolated enzyme fraction glyoxylate decarboxylation in the dark can be observed in the presence of NADPH + H* and autooxidizable electron acceptors (AQ, MY, fer­ redoxin). The influence of ascorbate on this dark reaction, depends on whether the activity of the isolated or the lamellae-bound enzyme is measured: 1. With the isolated enzyme no influence of ascorbate on AQ-, MVor ferredoxin-stimulated glyoxylate decarboxylation is observed. 2. The dark-reaction with NADPH + H* as electron donor, catalyzed by the bound enzyme however is inhibited by ascorbate both in the presence of either ferredoxin or AQ. This and other observations support the view that the site of inhibition by ascorbate of oxygen reduction by chloroplast lamellar systems in the presence of ferredoxin is not identical with either the reducing side of photosystem I, ferredoxin or NADP-ferredoxin reductase. The site of inhibition by ascor­ bate is more likely connected with an additional pathway involved in photosynthetic oxygen re­ duction by chloroplast lamellar systems, which is located at the reducing side of photosystem I, i. e. in the vicinity of the NADP-ferredoxin reductase. By heat treatment of isolated chloroplast lamellar systems a factor is released showing the activity of an ascorbate-sensitive oxygen reductant upon illumination in the presence of chloroplast lamellar systems. A model for photosynthetic oxygen reduction is proposed which includes ferredoxin and a membrane-bound oxygen reductant in series. Oxygen reduction by this pathway is only operating when the available NADP is fully reduced.