Dark fixation of CO2during gluconeogenesis by the cotyledons ofCucurbita pepoL.

✍ Scribed by R. C. Leegood; T. Rees

• Publisher: Springer-Verlag
• Year: 1978
• Tongue: English
• Weight: 754 KB
• Volume: 140
• Category: Article
• ISSN: 0032-0935
• DOI: 10.1007/bf00390260

✦ Synopsis

We did this work to discover the pathway of CO 2 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 ld-CO 2 in the dark was determined in a series of pulse and chase experiments. After 4s in ~CO 2, 89 % of the 14C fixed was in malate and aspartate. In longer exposures, and in chases in ~2CO2, 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.