EVALUATION OF METABOLIC PATHWAYS 35 



A third possibility involves the operation of the Entner- 

 Doudoroff pathway (2) (Fig. 2.4). Here the yield of COo 

 will be identical from carbons 1 and 4 of glucose, from 

 carbons 2 and 5, and from 3 and 6. Administration of glu- 

 cose labeled specifically in these positions should reveal 

 the presence of this route. 



LABELING OF FERMENTATION PRODUCTS 



Finally, use may be made of labeling of products isolated 

 from fermentation or other degradation of glucose. Thus, 

 Blumenthal, Lewis, and Weinhouse (3) have used the intra- 

 molecular distribution of isotope in acetate derived from 

 labeled substrate as the criterion for identifying the meta- 

 bolic routes that are being followed. Carbon 1 of glucose, 

 for example, would be converted to carbon 2 of acetate by 

 glycolysis, whereas it would not get into acetate at all via 

 the pentose cycle; and so on. A summary of locations 

 of specific glucose carbon atoms in various metabolic end 

 products by different pathways is given in Fig. 2.5. 



Actually, the evaluation of experimental findings is not 

 quite so simple as might be expected from a cursory in- 

 spection of Fig. 2.1. Early workers, in comparing the 

 specific activities of Ci^02 arising from C-1 and C-6 of 

 glucose, often found the value to be several-fold higher 

 from C-1. The conclusion w^as reached that the major 

 portion of glucose metabolism was via phosphogluconate 

 decarboxylation. This, as is now recognized, was in error, 



catalyzing the decomposition of all the others, seems attractive for this 

 reason; however, the data of Beevers (1) as well as those to be pre- 

 sented below indicate that his suggestion (Fig. 2.2) should probably 

 rate as the preferred mechanism. 



