The metabolism of hexose and pentose phosphates in higher plants.

The pathway of carbohydrate degradation, which begins with the action of G-6-P dehydrogenase’ on G-6-P, has been studied by a number of workers (l-9). Their results have shown the operation of this scheme in a variety of tissues (yeast, red blood cells, and mammalian liver). The existence of G-6-P dehydrogenase in various seeds (10) suggests the occurrence of this system in higher plants. The present paper reports the results of an investigation of this system in extracts of spinach and pea leaves. The sequence of reactions beginning with G-6-P is as follows: The hexose phosphate is dehydrogenated by an apparently conventional TPN enzyme to 6-PG and then oxidatively decarboxylated to pentose phosphate by a TPN 6-PG dehydrogenase. The resulting pentose phosphate, a mixture of ribose and ribulose phosphate, is then converted, in part, to sedoheptulose phosphate. Balance experiments carried out with R-5-P indicate that 2 moles of R-5-P give rise to 1 mole of triose phosphate and 1 mole of heptulose phosphate. Time lags in attaining the proper ratio indicate the transient formation of a non-ribose, non-heptulose compound that may be a precursor to the heptulose phosphate; however, efforts to show the existence of glycolaldehyde were negative. The heptulose phosphate itself is in large part transformed to a mixture of G-6-P and F-6-P. In the presence of TPN (DPN is considerably less effective) the triose phosphate is transformed to pyruvic acid and inorganic phosphate. Unlike its better known counterpart in the Embden-Meyerhof scheme, this reaction does not require ADP, nor is its action accelerated by arsenate in the absence of ADP.