Radioactive Carbon as an Indicator of Carbon Dioxide Utilization

Recent experiments,' carried out with the aid of labeled carbon, have shown beyond any doubt that CO2 enters into the metabolism of a large variety of organisms in the absence of light. As a consequence it has been suggested that CO2 assimilation may be a process of wide-spread occurrence in living systems. The metabolic processes of living organisms are for the most part enzyme catalyzed. The recent studies of Virtanen, Woods, Borsook, Warburg, Cori and co-workers, Hanes and many others2 have clearly demonstrated that many of these processes are reversible, and there is at present no reason for assuming that this is not true in all cases. It is generally believed that the enzymatic decomposition of keto acids accounts for the greater part of the CO2 produced by living organisms. If these latter reactions are reversible, they could be a mode of entry of CO2 into cellular biochemical processes.3 With this point in mind we have investigated the decarboxylation of pyruvic acid using radioactive carbon (C'1) as a tracer.4 The enzyme carboxylase is known to catalyze the decomposition of pyruvic acid with the production of CO2 as one of the end-products. This enzyme is of wide-spread occurrence among living organisms. Furthermore, cell-free preparations which will function in vitro can be easily prepared and its action on pyruvic acid has been carefully studied. It is known that the decarboxylation of pyruvic ac d gives rise to CO2 and acetaldehyde