Effects of Glycine Hydroxamate, Carbon Dioxide, and Oxygen on Photorespiratory Carbon and Nitrogen Metabolism in Spinach Mesophyll Cells

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Abstract
The effects of added glycine hydroxamate on the photosynthetic incorporation of 14CO2 into metabolites by isolated mesophyll cells of spinach (C. oleracea L.) was investigated under conditions favorable to photorespiratory (PR) metabolism (0.04% CO2 and 20% O2) and under conditions leading to nonphotorespiratory (NPR) metabolism (0.2% CO2 and 2.7% O2). Glycine hydroxamate (GH) is a competitive inhibitor of the photorespiratory conversion of glycine to serine, CO2 and NH4+. During PR fixation, addition of the inhibitor increased glycine and decreased glutamine labeling. Labeling of glycine decreased under NPR conditions. Evidently when the rate of glycolate synthesis is slow, the primary route of glycine synthesis is through serine rather than from glycolate. GH addition increased serine labeling under PR conditions but not under NPR conditions. This increase in serine labeling at a time when glycine to serine conversion is partially blocked by the inhibitor may be due to serine accumulation via the reverse flow of photorespiration from 3-P[phospho]-glycerate to hydroxypyruvate when glycine levels are high. GH increased glyoxylate and decreased glycolate labeling. Possible glyoxylate feedback inhibition of photorespiration is discussed.