Dark fixation of CO₂ during gluconeogenesis by the cotyledons of Cucurbita pepo L.

Abstract

We did this work to discover the pathway of CO₂ fixation into sugars in the dark during gluconeogenesis by the cotyledons of 5-day-old seedlings of Cucurbita pepo L. We paid particular attention to the possibility of a contribution from ribulosebisphosphate carboxylase. The detailed distribution of ¹⁴C after exposure of excised cotyledons to ¹⁴CO₂ in the dark was determined in a series of pulse and chase experiments. After 4s in ¹⁴CO₂, 89% of the ¹⁴C fixed was in malate and aspartate. In longer exposures, and in chases in ¹²CO₂, label appeared in alanine, phosphoenolpyruvate, 3-phosphoglycerate and sugar phosphates, and accumulated in sugars. The transfer of label from C-4 acids to sugars was restricted by inhibition of phosphoenolpyruvate carboxykinase in vivo by 3-mercaptopicolinic acid. We conclude as follows. Initial fixation of CO₂ in the dark is almost entirely into phosphoenolpyruvate, probably via phosphoenolpyruvate carboxylase (EC 4.1.1.31) which we showed to be present in appreciable amounts. Incorporation into sugars occurs chiefly, if not completely, as a result of randomization of the carboxyl groups of the C-4 acids and subsequent conversion of the oxaloacetate to sugars via the accepted sequence for gluconeogenesis. Ribulosebisphosphate carboxylase appears to make very little contribution to sugar synthesis from fat.