Photoreduction of Methyl Red and Tetrazolium Blue by Ascorbate: Catalysis by Chlorophylls and Porphyrins in Aqueous Media.

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Abstract
A photoreduction of methyl red and tetrazolium blue by ascorbate was observed in the presence of pure chlorophyll a, chlorophyll b, pheophytin a, pheophytin b, protoporphyrin DC, hematoporphyrin IX and coproporphyrin III in aqueous solutions under anaerobic conditions. The chlorophylls and pheophytins, which were added to the aqueous reaction system absorbed on sucrose, gave "solutions" which were visually clear, but were apparently suspensions of small aggregates of the pigment molecules. The photoreductions of both methyl red and tetrazolium blue were stimulated (with chlorophylls or porphyrins as catalysts) if 2,6-dichlorophenolindophenol (DPIP) was also present. This indicates a role of intermediate electron carrier for DPIP, serving to increase the flow of electrons from ascorbate to the photo-catalyst under the influence of light. Addition of quinacrine had no effect upon tetrazolium blue photoreduction, but showed slight inhibition in the case of methyl red photoreduction. A comparison of a number of porphyrins and porphyrin derivatives as catalysts for these reactions was made. Activity was observed for the chlorophylls, pheophytins and porphyrins. Fe porphyrins, hemeproteins and FMN were inactive. During photoreduction of methyl red a simultaneous oxidation of ascorbate was observed. Glutathione could substitute for ascorbate but gave a slower rate of reaction. In the presence of air, the chlorophylls and protoporphyrin catalyzed a photooxidation of DPNH, reduced cytochrome c, reduced DPIP and ascorbate. Comparative rates are given. These results are discussed in relation to the same photoreduction reactions catalyzed by plant chloroplasts and bacterial chromatophores.