Dark CO2 Fixation and Its First Product

It is now generally accepted that dark fixation of COj is the key reaction in CAM. Virtually all experiments conducted to date substantiate the hypothesis that malate is the first and primary stable product of CO2 fixation in CAM. Thurlow and Bonner and Bonner and Bonner in 1948 were perhaps the first to show dark C02 fixation into malate as well as other organic acids including citrate, isocitrate, fumarate, and succinate. Yet the equilibration of malate- C with the other organic acids was slow. Subsequently Thomas and Ranson (1954) reported the stoichiometry between CO2 fixation and acid synthesis. 

Vickery s early experiments (1954 a, b) indicated that during the early phase of acidification, there was more malate produced than starch consumed. Hence, he inferred CO2 uptake. Finally, based on data of Beevers and Gibbs (1954) and Stiller (1959), suggesting both glycolytic and pentose phosphate activity in supplying substrate from starch for malate synthesis, Ranson and Thomas (1960) proposed the following hypothesis for malate synthesis  

Saltman et al. (1957), using the eloquent techniques developed by Calvin s research group at Berkeley, showed that after 1 min of dark 2 fixation. 

Subsequently, additional research by Walker (1957) and others suggested that P-enolpyruvate was the main substrate for carboxylation rather than pyruvate, and 

It was immediately recognized that the reaction catalyzed by malate dehydrogenase (Walker, 1957) probably was responsible for the reduction of oxalacetate to malate and aspartate aminotransaminase was responsible for aspartate formation. 

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