Effect of oxygen on the regulation of intermediate metabolism in Tetrahymena

Tetrahymena grown in poorly aerated cultures have a greater capacity to utilize oxygen than cells grown in relatively well aerated cultures. Paradoxically, the oxidation of (1-14C)-glucose was inhibited, while oxidation of (2-14C)pyruvate and (2-14C)glyoxylate was enhanced in cells grown under relatively anaerobic conditions. Total glycogen content measured after 17 hours of growth was increased 30 to 60% in cells grown partially anaerobically. In 1-hour incubations at the end of this time, the capacity to incorporate label into glycogen from (1-14C)pyruvate and (2-14C)glyoxylate was increased several fold. The ratio of adenosine di- and triphosphates was nearly identical in the cells grown under different conditions of oxygenation, indicating that this ratio may not play a major role in regulating these changes. After 17 hours of growth in cultures of different depths, cells were also incubated with a mixture of acetate, pyruvate, and octanoate, with one substrate labeled at a time in such a way that (1-14C)-acetyl-CoA is generated at the initial step in the metabolism of each. These results were interpreted in terms of a previously developed three-compartment model of acetyl-CoA metabolism. Glyconeogenesis from peroxisomal and mitochondrial precursors was increased in cells grown in low oxygen tension, with the greater contribution coming from the peroxisomes. Oxidation of acetate and pyruvate was increased under these conditions, but appearance of (1-14C)-acetate label in glutamate was decreased. Lipogenesis from labeled peroxisomal precursors was also increased in cells grown under relatively low oxygen tension. After a shift down in O2 tension there is a rapid rise in glyconeogenesis from the peroxisomes which levels off after about 4 hours, whereas the rate of oxidation in the Krebs cycle increases steadily for at least 8 hours following the transition to relatively anaerobic conditions. In response to a shift up in O2 tension there is a decline in peroxisomal glyconeogenesis which continues for 8 hours, whereas the rate of oxidation in the Krebs cycle does not begin decreasing until about 4 hours after the increase in O2 tension. Thus the flux of (1-14C)acetyl-CoA changes according to a different temporal pattern in mitochondria as compared to peroxisomes, and in each compartment the sequence of changes in response to a shift up in O2 tension is not the mirror image of the sequence in response to a shift down.