On the Metabolic Relationship between the Calvin Cycle and the Tricarboxylic Acid Cycle

Abstract

Leaves of 10 randomly selected plants representing eight dicotyledonous families were exposed to (14)CO(2) for a 10-minute period in the light. Citrate and alanine were isolated, purified isotopically, and degraded to obtain the (14)C-isotope distribution of corresponding carbon atoms, i.e. citrate (C-1,2) and alanine (C-2,3). The cited carbon atoms of alanine were equally labeled as is typical of a 3-carbon intermediate derived from photosynthetic 3-phosphoglycerate. The carbon atoms of citrate, equivalent to acetyl-CoA, were unequally labeled. The citrate (C-1,2) isotope ratio ranged from 1.20 to 1.78 for the various plants compared to a ratio of unity in the uniformly labeled control. The results infer that 3-phosphoglycerate produced in the chloroplast is not the singular precursor of mitochondrial citrate.