PHOTOCHEMICAL REDUCTION OF CHLOROPLAST GRANA

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Abstract
A study has been made of the reaction described by Warburg and Luttgens [Naturwiss., 32, 161, 304, (1944)] in which benzoquinone is reduced to hydroquinone and water is oxidized to oxygen in the presence of chloroplast grana and light. The reaction is shown to occur not only with grana but with a clear yellow-green soln. containing only minute amts. of chlorophyll, and which is almost 10 times as active as grana, per unit of chlorophyll. The soln. is much more stable than grana. Both materials can be lyophilized and retain activity after hydration. The rate of O2 evolution varies with different quinones (benzoquinone, l,2-naphthoquinone-4-sulfonate, 1,4-naphtho-quinone-2-sulfonate, [beta]-anthraquinone sulfonate) and with the light intensity. At lower light intensities the order of rates is the reverse (with the possible exception of anthra-quinone) of that at the high light intensities. Saturation, or an approach to it, is indicated at high light intensities. Various other substances give small effects benzaldehyde, salicylaldehyde, benzoyl peroxide, fructose, dehydroascorbic acid. Others which might have been expected to be active are not: butadiene monoxide, salicylic acid. Urea peroxide is decomposed in the dark by both the grana and a soln. of pure catalase. The photochemical reduction of quinone is inhibited strongly by [omicron]-phenanthroline, phenylurethane, and thymol. Complete inhibition by phenylurethane does not occur in satd. soln.; the degree of inhibition is not increased with time or with excess material. Thymol is more fat-soluble than phenylurethane and is more effective as an inhibitor at the same molar concentration. A brief discussion of the relation of photochemical reduction with various substances by grana to photosynthesis is given.

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