Abstract
In the higher plant Vicia faba, anomalous labeling patterns in the organic acids and related amino acids of the tricarboxylic acid cycle which result from photosynthetic (14)CO(2) fixation (in conjunction with an enzyme localization pattern unique to plant mitochondria) suggest that the tricarboxylic acid cycle functions primarily as a pathway leading to glutamic acid biosynthesis during autotrophic growth. The distribution of isotope in citrate indicates little recycling of oxaloacetate for the resynthesis of citrate. Rather, malate appears to provide both the C(2) and C(4) fragments for the synthesis of citrate, and [(3)H]formate and (14)CO(2)-labeling patterns implicate serine as the ultimate C(3) precursor of malate.