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Articles

Effects of Carbon Dioxide and Oxygen on the Regulation of Photosynthetic Carbon Metabolism by Ammonia in Spinach Mesophyll Cells ¹

Laboratory of Chemical Biodynamics, University of California, Berkeley, California 94720, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720

Photosynthetic carbon metabolism of isolated spinach mesophyll cells was characterized under conditions favoring photorespiratory (PR; 0.04% CO₂ and 20% O₂) and nonphotorespiratory (NPR; 0.2% CO₂ and 2% O₂) metabolism, as well as intermediate conditions. Comparisons were made between the metabolic effects of extracellularly supplied NH₄⁺ and intracellular NH₄⁺, produced primarily via PR metabolism. The metabolic effects of ¹⁴CO₂ fixation under PR conditions were similar to perturbations of photosynthetic metabolism brought about by externally supplied NH₄⁺; both increased labeling and intracellular concentrations of glutamine at the expense of glutamate and increased anaplerotic synthesis through -ketoglutarate. The metabolic effects of added NH₄⁺ during NPR fixation were greater than those during PR fixation, presumably due to lower initial NH₄⁺ levels during NPR fixation. During PR fixation, addition of ammonia caused decreased pools and labeling of glutamate and serine and increased glycolate, glyoxylate, and glycine labeling. The glycolate pathway was thus affected by increased rates of carbon flow and decreased glutamate availability for glyoxylate transamination, resulting in increased usage of serine for transamination. Sucrose labeling decreased with NH₄⁺ addition only during PR fixation, suggesting that higher photosynthetic rates under NPR conditions can accommodate the increased drain of carbon toward amino acid synthesis while maintaining sucrose synthesis.

³ Present address: Chemistry Department, Royal Veterinary and Agricultural University, 40 Thorvaldsensvej, DK-1871, Copenhagen V, Denmark.

¹ Supported by the Division of Biological Energy Conversion and Conservation, Office of Basic Energy Sciences, United States Department of Energy under Contract W-7405-ENG-48, by a Rockefeller Foundation Postdoctoral Fellowship to A. L. L., and by the Danish National Science Research Council 511-15546 to P. O. L.