On the Mechanism of Carbon Dioxide Replacement in Heterotrophic Metabolism.

Ajl and Werkman (1948) have previously reported on the heterotrophic replacement of carbon dioxide and reviewed the literature on the bacterial requirement of the gas for optimal growth. A number of compounds have been found that in the case of both Escherichia coli and Aerobacter aerogenes substitute for carbon dioxide to give not only normal but, in many instances, enhanced growth. These compounds include the members of the Krebs cycle or their metabolic precursors. This contribution considers the mechanism of the substitution. The evidence points to an orderly manner in which this phenomenon takes place. Oxalacetic and a-ketoglutaric acids appear to function as the cardinal compounds in the replacement of C02 when added as such or in the form of their precursors. Inhibitors prevent the oxidation or the deamination of the various precursors of oxalacetate and a-ketoglutarate. Under such conditions no growth is obtained unless oxalacetate or a-ketoglutarate is present. Cyclohexanol inhibits the deamination of aspartic acid to fumarate; addition of this compound to a C02free medium containing aspartic acid results in no growth. The addition of oxalacetate now provides abundant growth. Similar results have been obtained with the other members of the Krebs cycle.