Metabolism of ribose-5-phosphate by Azotobacter vinelandii.

Because ribose-5-phosphate (R-5-P) and sedoheptulose-7-phosphate (S-7-P) have been found free in cells of the axotobacter, the mechanism of synthesis of these compounds was sought. Previously, Horecker (1) and Wood and Schwerdt (2) demonstrated in yeast and Pseudomonas Jluorestens that S-7-P was produced from R-5-P by an enzyme called transketolase. The R-5-P was first converted by an isomerase to ribulosed-phosphate, which was then split into glyceraldehyde-3-phosphate (G-3-P) and a 2-carbon cocarboxylase complex of the oxidation level of glycolaldehyde. The 2-carbon fragment in turn condensed with R-5-P to produce S-7-P; the over-all reaction was the production of 1 molecule each of S-7-P and G-3-P from 2 molecules of R-5-P. Recently Horecker et al. (3) have shown that the S-7-P in turn is converted into tetrose phosphate and a dihydroxyacetone-enzyme complex that condensed with G-3-P to yield fructose-6-phosphate (F-6-P). That enzyme preparations of the azotobacter also convert R-5-P into S-7-P and G-3-P will be shown in this paper. The finding of R-5-P and S-7-P in cells of the azotobacter indicates that this reaction is significant in its metabolism. Although the conversion of 6-phosphogluconate (6-PG) into ribulosed-phosphate has been demonstrated in other organisms, it has not yet been established in this bacterium; in fact, all evidence suggests that the azotobacter splits almost quantitatively 6-PG into pyruvate and G-3-P (4). A possible source of R-5-P in this organism could be synthesis from G-3-P and acetate, as has been demonstrated in Leuconostoc mesenteroides.’ This would explain the synthesis of some pentose by the azotobatter when incubated with 6-PG (5), since the products are G-3-P and pyruvate; pyruvate is readily converted to acetate by this organism.