Use of [3,4-^<14>C] Glucose to Assess in vivo Competition for Phosphoenolpyruvate Between Phosphoenolpyruvate Carboxylase and Pyruvate Kinase in Developing Soybean Seeds

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According to the conventional glycolytic sequence [3,4-^<14>C]glucose yields phosphoenolpyruvate (PEP) labeled in position C-1. This yields pyruvate through pyruvate kinase reaction also labeled in C-1. Subsequent metabolism of pyruvate to acetyl CoA releases radioactive carbon dioxide. Alternatively PEP may be converted to oxalacetate by PEP carboxylase and then into organic and amino acids which retain the label. The procedure adopted was to trap carbon dioxide evolved and isolate organic acids produced after feeding [3,4-^<14>C]glucose to developing soybean cotyledons. Under conditions of 27℃ and pH of 7.5 and 8.5 about 60% of the glycolytic carbon was processed by pyruvate kinase and 40% by PEP carboxylase. At lower temperature (15℃) 60% of the carbon was directed through the PEP carboxylase reaction. This may be caused by cold lability of pyruvate kinase which was demonstrated in in vitro assays. Low pH, down to 5.5, reduced organic acid production by inhibition of PEP carboxylase activity. Pyruvate kinase was not affected and carbon dioxide evolution remained constant at varying pH. PEP carboxylase and pyruvate kinase independently feed their products into two separate metabolic pools. Possibly they should jointly be considered as final enzymes in the glycolytic pathway of plants.
日本植物生理学会の論文