Metabolism of ribose-1-C14 by cell-free extracts of yeast.

The fermentation of ribose-5-phosphate by cell-free extracts of yeast is characterized by the equimolar formation of COZ, ethyl alcohol, and inorganic phosphate (1). Dickens (1) and Sable (2) suggested from their studies of pentose phosphate metabolism by cell-free yeast preparations that fermentation proceeded in the following manner: (1) cleavage of ribosed-phosphate to a 2-carbon fragment and triose phosphate and (2) metabolism of the latter via classical glycolysis to CO*, ethanol, and inorganic phosphate. The fate of the 2-carbon component was undetermined. Recently another pathway of pentose phosphate metabolism has been elucidated in liver and in higher plants by Horecker and coworkers (3, 4). In this pathway, ribose-5-phosphate is converted via ribulose-5-phosphate and sedoheptulose-7-phosphate to fructose-6-phosphate. The fructose-6phosphate would be further metabolized by the classical pathway. In this pathway carbon atoms 1 and 2 of the pentose are involved in COZ and ethanol formation. To determine the pattern of pentose fermentation by yeast extracts, ribose-l-Cl4 was used. Unlabeled ethanol and CO2 would suggest the pattern described by Dickens and Sable, since in their scheme the endproducts are derived from non-isotopic carbon atoms 3, 4, and 5 of the pentose. However, the appearance of tracer in the end-products would favor the scheme involving sedoheptulose, since carbon atom 1 enters the fructose-6-phosphate, which in turn gives rise to the alcohol and CO,. The results of the fermentation of ribose-l-Cl4 with yeast extracts are the subject of the present paper.