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

Abstract

Leaves of 10 randomly selected plants [Brassica juncea, Glycine soya, fatshedera sp., Centaurea cineraria, Tanacetum Vulgare, Rumohra adiantiformis, Brassaia actinophylla, Grevillea robusta, Daucus carota sativa and Nicotiana tabacum] representing 8 dicotyledonous families were exposed to 14CO2 for a 10 min period in the light. Citrate and alamine were isolated, purified isotopically and degraded to obtain the 14C-isotope distribution of corresponding C atoms, i.e., citrate (C-1,2) and alanine (C-2,3). The cited C atoms of alanine were equally labeled as is typical of a 3-C intermediate derived from photosynthetic 3-phosphoglycerate. The C atoms of citrate, equivalent to acetyl-CoA, were unequally labeled. The citrate (C-1,2) isotope ratio ranged from 1.20-1.78 for the various plants compared to a ratio of unity in the uniformly labeled control. 3-Phosphoglycerate produced in the chloroplast apparently is not the singular precursor of mitochondrial citrate.