The photoreduction of spinach ferredoxin in the presence of porphyrin and an electron donor.

This is the authors' abstract. We don't add key points for this paper.

It has previously been established that photosynthetic pyridine nucleotide reductase (PPNR)' or spinach ferredoxin (Fd)2 is required for the reduction of pyridine nucleotides by illuminated chloroplasts. The reduction of Fd by illuminated chloroplasts has since been described by various workers.3-5 In addition, it has been shown that Fd can be reduced by hydrogen and hydrogenase,2 hydrosulfite, and hydrogen and platinum asbestos.3 Furthermore, the photoreduction of Fd by illuminated chloroplasts with its subsequent oxidation in a dark reaction leading to the formation of reduced pyridine nucleotide has been demonstrated.4 The NADP-reducing system of spinach chloroplasts is composed of two components, a ferredoxin and a flavoprotein enzyme referred to as ferredoxin-NADP reductase (spinach pyridine nucleotide transhydrogenase or spinach NADPH diaphorase). Evidence that both components are required for the reduction of pyridine nucleotides by illuminated chloroplasts, with Fd being reduced initially by the chloroplast, has been presented.2 6-10 Krasnovsky11 first demonstrated that chlorophyll photosensitized the reduction of a number of dyes in pyridine solution, with ascorbic acid as the electron donor. Thereafter, it was shown12 13 that chlorophylls dissolved in pyridine catalyzed a photoreduction of pyridine nucleotides by ascorbate. Vernon14 reported that various porphyrins and porphyrin derivatives, as well as chlorophyll, catalyzed the photoreduction of NAD and NADP by ascorbate in aqueous media and that the reaction required the presence of partially purified Fd. Later work5, 16 with purified preparations showed that Fd did not catalyze the photoreduction of pyridine nucleotides with either hematoporphyrin or chlorophyllin a as the photocatalyst when Fd was present either alone or together with the NADP reductase. With either photocatalyst, only the NADP reductase was required for NADP photoreduction in this system. The photoreduction of NAD or NADP in chloroplasts required both the presence of Fd and the NADP diaphorase as previously noted. No evidence was presented for the photoreduction of Fd in the porphyrin (or chlorophyll)-ascorbate system. Fd, however, has been noted as the earliest chemically isolatable reductant formed during conversion of electromagnetic energy into chemical energy in photosynthesis.5 None of these previous studies have provided evidence that Fd might interact directly with chlorophyll in the intact chloroplast as the primary electron acceptor. The work to be described in this paper demonstrates that Fd can be photoreduced in the presence of a porphyrin and light with an electron donor.