Crithidia fasciculata: Appearance kinetics of intermediates and regulation of aerobic fermentation

In vivo studies of aerobic fermentation in the protozoan, Crithidia fasciculata, were performed using gas-liquid chromatography, mass spectroscopy, and ion-selective electrodes. Findings are consistent with an hypothesis derived from in vitro data: (1) Fermentation proceeds from the triose via the phosphoenolpyruvate carboxykinase to oxanacetic acid and then by reversal of the tricarboxylic acid cycle to permit the formation of succinic acid; (2) amino acids serve as the principal substrate for early growth; and (3) as amino acid metabolism declines in midlog phase of growth, the organism begins to ferment carbohydrate. Aerobic fermentation of carbohydrate is of secondary importance to this organism and probably also to other hemoflagellates.