Photoreduction of Pyridine Nucleotides by Ascorbate: Catalysis by Chlorophylls and Porphyrins in Aqueous Media.

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Abstract
In the presence of ascorbate and partially purified photosynthetic pyridine nucleotide reductase (PPNR), a photoreduction of pyridine nucleotides was catalyzed by chlorophyll a, chlorophyll b, hematoporphyrin DC, protoporphyrin DC, coproporphyrin III, and to a lesser extent by pheophytin a and pheophytin b. Fe porphyrins and hemeproteins were inactive. Phycocyanin was moderately active. In general, TPN was photoreduced at a faster rate than DPN. With purified chlorophylls, however, the reverse was true. Addition of 2,6-dichlorophenolindophenol or quinacrine resulted in an inhibition of the photoreductions. In the absence of PPNR, hematoporphyrin catalyzed a slow photoreduction of DPN. Under the same conditions with TPN, a photoreduction of the porphyrin was observed.