NATURE OF THE BLACKMAN REACTION IN PHOTOSYNTHESIS

Abstract

Among those writing on the mechanism of photosynthesis, which include Warburg (7), Willstatter (9), and Franck (3), the opinion is widely held that the Blackman reaction, which limits the rate of assimilation at high light intensities and carbon dioxide concentrations, involves the decomposition of a peroxide, formed in a preceding photochemical reaction, by the enzyme catalase. The best experimental support of this is found in the work of Warburg and Uyesugi (8), and Yabusoe (11), who compared the temperature coefficients and sensitivity to several inhibitors for the Blackman reaction and for the decomposition of added hydrogen peroxide. In these respects they found certain similarities between the two processes. Such differences as they found were readily explainable, they believed, on the ground that while the same enzyme, catalase, played a part in both cases, the substrate was probably different, being very likely some organic peroxide, in the case of the Blackman reaction, rather than hydrogen peroxide. Largely as a result of this work, Warburg (6) gave up his acceptor theory in which the Blackman reaction preceded the photochemical reaction, and in agreement with Willstatter and Stoll (10) regarded the Blackman reaction as involving the decomposition of a peroxide previously formed in the photochemical reaction. The writers have made further comparisons between peroxide decomposition and the Blackman reaction, and believe Warburg's conclusion should be reconsidered, especially in view of the prominent part played by peroxide in the recently proposed chemical mechanisms for photosypthesis. Gaffron and Wohl (5), in a theoretical paper, have objected that the energy available for the assimilatory process is insufficient to permit the formation of hydrogen peroxide as an intermediate product; but this objection seems to us of minor importance in the present discussion, because no definite claim has been made that the peroxide formed must be hydrogen peroxide. Willstatter and Stoll (9) are more inclined to the opinion that the action of catalase in photosynthesis is on some peroxidic compound other than hydrogen peroxide. Gaffron and Wohl also believe the formation of some sort of peroxide is hardly to be avoided, and suggest, on purely theoretical grounds, a peroxide of organic nitrogen, still regarding catalase as responsible for the freeing of the oxygen produced in photosynthesis. 537