CO(2) Fixation, Glutamate Labeling, and the Krebs Cycle in Ribose-grown Hydrogenomonas facilis

Exposure of ribose-grown Hydrogenomonas facilis to (14)CO(2) for 6 to 12 sec during ribose oxidation resulted in labeling of a number of compounds, three of which were glutamate, phosphoglycerate, and pyruvate. Phosphoglycerate and pyruvate were labeled almost exclusively in C(1), suggesting operation of the reductive pentose phosphate cycle. Glutamate was labeled initially to the extent of 90% in C(1) and 10% in C(5), and this was followed by a concentration of radioisotope in C(5). All of the enzymes of the tricarboxylic acid cycle were detectable in ribose-grown cells, and, in general, specific activities were similar to those found in yeast extract-grown cells. Reduced nicotinamide adenine dinucleotide oxidase, aconitase, and the dehydrogenases for pyruvate, alpha-ketoglutarate, and succinate appeared to be of particulate origin. In addition to enzymes of the tricarboxylic acid cycle, an acetyl coenzyme A-stimulated phosphoenolpyruvate carboxylase was found, as was isocitrate lyase. Possible participation of these catalysts in glutamate synthesis is discussed.