Photosynthesis in Mesophyll Protoplasts and Bundle Sheath Cells of Various Types of C4 Plants IV. Enzymes of kespiratory Metabolism and Energy Utilizing Enzymes of Photosynthetic Pathways')

Abstract

Summary A study was made of the distribution of energy utilizing enzymes of carbon assimilation and key enzymes of respiratory metabolism between mesophyll and bundle sheath cells of 3 groups of C4 plants: NADP-malic enzyme species, NAD-malic enzyme species, and phosphoenolpyruvate (PEP) carboxykinase species. A number of enzymes of photosynthetic metabolism including some energy utilizing enzymes of carbon assimilation and photorespiratory enzymes showed preferential distribution between mesophyll and bundle sheath cells. Pyruvate Pi dikinase, NADP-malate dehydrogenase and carbonic anhydrase were predominantly localized in mesophyll preparations of all 3 groups of C4 species. NADP-malic enzyme species, as compared with NAD-malic enzyme and PEP carboxykinase species, showed much higher activity of NADP-malate dehydrogenase in mesophyll cells. Phosphoribulokinase of the regenerative phase of the reductive pentose phosphate pathway was localized in bundle sheath cells of all 3 C4 groups. Enzymes of the reductive phase of the reductive pentose phosphate pathway, glycerate-3-phosphate kinase, NADP)-glyceraldehyde-3-phosphate dehydrogenase, and triose-phosphate isomerase were high in activity in both photosynthetic cell types of the various species examined. In a qualitative evaluation of energy requirements in carbon assimilation in C photosynthesis ATP would be required in mesophyll cells for pyruvate Pi dikinase; in bundle sheath cells for phosphoribulokinase; NADPH would be required in mesophyll cells for NADP-malate dehydrogenase; and ATP and NADPH would be required in both mesophyll and bundle sheath cells for the reductive phase of carbon assimilation. Aspartate- and alanine aminotransferase, suggested to have a role in C4 photosynthesis in some C4 species, were high in activity in both cell types in NAD-malic enzyme and PEP carboxykinase species with relatively low activity in NADP-malic enzyme species. Enzymes apparently associated with dark respiratory metabolism including glucose-6phosphate dehydrogenase and 6-phosphogluconic dehydrogenase of the oxidative pentose phosphate pathway, cytochrome c oxidase of the mitochondria, 3-phosphoglycerate mutase and enolase of glycolysis were relatively active in both mesophyll and bundle sheath cells in species of all 3 groups. However, on a chlorophyll basis the cytochrome c oxidase activity was 2 to 3 fold higher in activity in bundle sheath than in mesophyll cells. In particular with Zea mays the cytochrome c oxidase was about 4 fold more active in bundle sheath than in mesophyll cells. Enzymes which may be involved in photo respiratory metabolism were primarily associated with bundle sheath cells. The results are discussed with respect to sites of energy utilization in C4 metabolism and differences in carbon assimilation in 3 C4 groups.