DEVELOPMENT OF ACETATE AND PYRUVATE METABOLIC ENZYME-ACTIVITIES IN SOYBEAN NODULES

Acetate and pyruvate play central roles in the metabolism of many eukaryotic and prokaryotic organisms. Results from several sources indicate that acetate and pyruvate metabolism may be important for the Bradyrhizobium japonicum-soybean symbiosis. In determinant symbioses, an assessment of the requirement for a particular enzyme for the support of symbiotic nitrogen fixation can be inferred by the correlation of that enzyme activity relative to the development of nitrogenase activity. The activity of several enzymes of acetate and pyruvate metabolism in the Bradyrhizobium japonicum-soybean symbiosis were determined and compared to the development of nitrogen fixation activity as measured by the acetylene reduction method. Plant PEPC, bacteroid PEPC, PK, PEPCK, AK, ACS and PTA appeared to correlate with the development of nitrogen fixation activity. The notable exceptions were the lactate dehydrogenases from both the plant nodule cytosol and the bacteroid which showed no apparent correlation with nitrogen fixation activity. Of those enzymes that correlated with development of nitrogen fixation activity, bacteroid PK and PEPCK rose to their maximal activity prior to that of whole nodule nitrogen fixation activity. These two enzymes may provide a role in the preparation of the bacteroid for the subsequent period of nitrogen fixation. The other enzymes that more closely followed the rise in nitrogen fixation activity may provide support for the actual nitrogen fixation process.