Carbon Dioxide & the Hill Reaction

Recently a number of investigators have reported that the Hill reaction is more rapid in the presence of CO., than in its absence. However, the phenomenon of CO., dependence can only be demonstrated under certain conditions and it is therefore regrettable that there has been no agreement as to the nature of these conditions. Warburg and Krippahl (9) have stressed the importance of preilluminating the chloroplasts, whlereas Abeles et al. (1) found the effect primarily in unbuffered media. Stern and Vennesland, on the other hand, observed large CO, effects in buffered media and without preillumination; in their studies the composition of the buffer was important (7). We have confirmed and extended the observations of the last-named authors. We have also investigated the relation of CO. dependence to light intensity, the state of uncoupling of the phosphorylation system, and the nature of the Hill reaction oxidant. The new information presented here provides little support for Warburg's contention that CO.. plays a role in the Hill reaction akin to its role in plhotosynthesis. In fact it is inconsistent with somle of the details of Warburg's postulated mechanismil. However, the crucial question remains unanswered: Is CO, a metabolite in the Hill reaction or are we dealing with indirect and possibly unspecific effects of the bicarbonate ion?