Carbon dioxide abolishes the reverse Pasteur effect in Leishmania major promastigotes

The products released by Leishmania major promastigotes incubated with [1-¹³C]glucose as sole exogenous carbon source were identified using nuclear magnetic resonance (NMR). Under aerobic (95% O₂/5% CO₂) conditions, acetate, succinate, and small amounts of pyruvate, d-lactate, and glycerol were released in addition to CO₂. Under anaerobic (95% N₂/5% CO₂) conditions, the relative amounts of products formed changed and alanine was also released. The changes in the rates of glucose consumption and product formation during the aerobic to anaerobic transition were measured. Under hypoxic conditions (O₂ <0.2%), glucose consumption was decreased by about 50%. Under completely anaerobic conditions (100% N₂), glucose consumption almost ceased (a total reverse Pasteur effect). The inclusion of 5% CO₂ in the gas phase restored hypoxic and anaerobic glucose consumption to the aerobic rate, and increased production of succinate, pyruvate, and d-lactate. Thus, CO₂ and very low concentrations of O₂ have strong regulatory effects on L. major glucose metabolism. A quantitative carbon balance showed that the NMR-identified products accounted for only about 25% of the glucose carbons consumed under aerobic conditions. CO₂, measured as the release of ¹⁴CO₂ from [U-¹⁴C]glucose, accounted for an additional 25% of the glucose consumed. About 11% of the glucose carbon was incorporated into trichloroacetic acid-insoluble products, mostly lipid. Large amounts of label from [U-¹⁴C]glucose were incorporated into the intracellular pools of alanine, glutamate, glutamine, and aspartate, indicating that CO₂ from unlabeled amino acids contributed to the carbon balance. Under anaerobic conditions, all the glucose carbons consumed could be accounted for solely by the NMR-identified products.