Malic enzyme and aminotransferases in relation to 3-phosphoglycerate formation in plants with the C4-dicarboxylic acid pathway of photosynthesis

In plants with the C4-dicarboxylic acid pathway an inverse relationship was found between the content of malic enzyme in leaves and that of aspartate and alanine aminotransferases. Some species contained adequate malic enzyme to support the decarboxylation of malate in the bundle sheath chloroplasts as part of the major route of carbon flow. Other species with low malic enzyme activity contained 5-10 times more aspartate aminotransferase and about 50 times more alanine aminotransferase. These aminotransferases were about equally distributed between the mesophyll and the bundle sheath cells. We propose that in the latter species aspartate rather than malate is transported to the bundle sheath chloroplasts. The results are most compatible with aspartate being converted to oxaloacetate in the bundle sheath chloroplasts and the latter compound being decarboxylated to provide CO2 for 3-phosphoglycerate formation.