The mechanism of ribose formation in Escherichia coli.

Metabolic pathways by which ribose may be formed have been studied in this laboratory (1, 2). There is now ample proof of the existence in many organisms of enzyme systems able to form ribose-5-phosphate from glucose-6-phosphate with intermediate stages of 6-phosphogluconate and ribulose-5-phosphate. Recent studies on the metabolism of ribulose-5phosphate have shown that it may be split by a transketolase into triose phosphate, and an active 2-carbon moiety that reappears at the hexose or heptulose level (3-6). This enzyme has been demonstrated in yeast, rat liver, and plant tissue, and in two cases the synthesis in vitro of ribulose-5phosphate has been achieved by the action of this enzyme in the presence of glyceraldehyde-3-phosphate and either L-erythrulose or hydroxypyruvate. Although the true biological donor of the ketol fragment is not yet known, a synthetic action of the transketolase must be considered as a possible source of ribulose. The hypothesis that ribose formation represents a synthesis from smaller molecules was the basis for the demonstrations of enzymatic synthesis of pentose in vitro by aldolases of animal and plant origin from triose phosphate and glycolaldehyde. However, the product of these syntheses was xyloketose-l-phosphate (7), and a direct synthesis of the ribose configuration by an aldolase is not likely. Two studies of the incorporation in viva of small molecules into the ribose of rat liver ribose nucleic acid have been made. The carboxyl carbon atom of glycine has been shown by Low to contribute to the carbon of the pentose (8), but the fact that the specific activity of the isolated ribose was less than that of the mixed polynucleotides suggests that the incorporation of glycine carboxyl into ribose is via a less direct route than its incorporation into the 4 position of the purine bases. The incorporation of doubly