The effect of carbon dioxide on the Hill reaction.

Isolated chloroplasts which are incapable of causing a net reduction of COZ accompanied by 02 evolution can still cause a Hill reaction, that is, a photoreduction of an added oxidant with concomitant evolution of a stoichiometric amount of OZ. The occurrence of the Hill reaction has long been regarded as part of the evidence for the conclusion that the action of light in photosynthesis leads first to an oxidation-reduction dismutation which is quite independent of COz fixation. The COZ is thought to be fixed in an enzymatic dark reaction, and the product of CO* fixation is thought to be reduced by the reducing agent generated in the light. This widely accepted view has been brought into question by Warburg’s and Krippahl’s demonstration (1, 2) that Hill reactions require CO*; that is, they are inhibited by COZ removal and strongly stimulated by addition of CO2 at low tensions, although there is no net reduction of COz occurring simultaneously. Warburg et al. have suggested that the CO2 requirement reflects a catalytic function for CO2 in the Hill reaction mechanism (1, 3). Specifically, they propose that a phosphorylated peroxide of carbonate is the precursor of the 02 evolved. Thus, the elimination of O2 would yield a carbon moiety at the oxidationreduction level of formate as the first reducing agent formed by the light. The isolated chloroplasts cannot retain the reduced COZ, as in normal photosynthesis, however; and all of the reduced CO2 is reoxidized by the Hill reagent, which also serves as the oxidant to convert CO% to its peroxide. In preliminary reports (4, 5) we have presented data to confirm the fundamental observation of Warburg and Krippahl that CO2 is required for the Hill reaction. The present paper represents a more detailed report of our findings. Abeles, Brown, and Mayne have also confirmed the Dahlem discovery (6).