The transport and metabolism of glucose in cowpea rhizobia

Abstract

Cell-free extracts of several strains of cowpea rhizobia grown under free-living conditions were surveyed for key enzymes of carbohydrate metabolism. Enzymes of the Entner-Doudoroff (ED) and Embden-Meyerhof-Parnas (EMP) pathways were detected while 6-phosphogluconate dehydrogenase was not detected, indicating the apparent absence of the pentose phosphate (PP) pathway. Transketolase and transaldolase reactions were present, indicating a pathway for the synthesis of pyrimidines and purines from fructose-6-phosphate and glyceraldehyde-3-phosphate. Radiorespirometric analysis with specifically labeled glucose indicated that the ED pathway with the hexose cycle was the primary pathway of glucose dissimilation. The presence of isocitrate and malate dehydrogenases and results of radiorespirometric analysis with pyruvate and succinate demonstrates an operational tricarboxylic acid (TCA) cycle in glucose-grown cowpea rhizobia. The transport of glucose was inhibited by carbonyl cyanide m-chlorphenylhydrazone (CCCP), dinitrophenol and potassium cyanide, indicating that the process was active, probably using an energized membrane state. The transport of glucose was glucose specific. A lower rate of glucose uptake was seen when cells were cultured on hexoses other than glucose.