Regulatory effects of ammonia on carbon metabolism in photosynthesizing Chlorella pyrenoidosa

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Addition of ammonia to Chlorella pyrenoidosa, photosynthesizing under steady-state conditions, causes changes in the metabolism which are due not only to the increased availability of NH 4 + for reductive amination but also to regulation of controlled enzymes. One such effect is an increased rate of the reaction which converts phosphoenolpyruvate to pyruvate in vivo. This regulatory effect was revealed by kinetic tracer studies with 14CO 2, paper chromatography and radioautographic analysis, which showed that upon addition of NH 4 + (1) the levels of both 3-phosphoglycerate and phosphoenolpyruvate drop, with the ratio of 3-phosphoglycerate/phosphoenolpyruvate increasing, (2) the level of labeled pyruvic acid increases and the rate of formation of alanine increases rapidly, while the rate of formation of serine is unaffected, (3) the rate of flow of carbon into the tricarboxylic acid cycle acids, malate and citrate, increases along with the increased rates of formation of glutamate, glutamine and aspartate and (4) the rate of labeling of lipids increases. The increased flow of carbon into amino acids is mostly at the expense of sucrose synthesis; starch synthesis decreases only slightly. The interruption of sucrose synthesis apparently is due to stopping the reaction between UDP-glucose and fructose 6-phosphate. The rate of conversion of fructose 1,6-diphosphate to fructose 6-phosphate is also decreased upon NH 4 + addition.