74 METABOLIC PATHWAYS IN MICROORGANISMS 



terment enzyme, or the "direct" oxidation of glucose. This 

 pathway was discovered independently by Warburg, Lip- 

 mann, and by Dickens; it referred to the conversion of glu- 

 cose-6-P04 to pentose phosphate, using (usually) TPN as co- 

 enzyme. This work was done in 1935-1936 (10-12); yet 

 some fifteen years passed before most of the workers in 

 the field appreciated its significance, although Engelhardt 

 and Barkhash (13), as well as Dickens (14), felt that the 

 new oxidation of glucose might be charted by quite a dif- 

 ferent pathway from the glycolysis-Krebs cycle sequence. In 

 retrospect, it seems that the finding of a mechanism that 

 accounted for the formation of pentose satisfied the work- 

 ers of the day; certainly few people appreciated the possi- 

 bility that the "direct" oxidation of glucose might take on 

 major importance, since the Krebs cycle seemed adequately 

 to account for the terminal oxidation of sugars. 



The findings of Claude's successors merely solidified this 

 view: the discovery that pentose phosphate could give rise 

 to sedoheptulose phosphate and tetrose phosphate was 

 brushed aside, and instead the new findings were empha- 

 sized for their simultaneous formation of triose phosphate. 

 The latter compound seemed important because it gave 

 rise easily to pyruvate, which in turn semed to provide a 

 ready entree to the Krebs cycle; it appeared, in short, as 

 if the excess pentose were simply being shunted into the 

 Krebs cycle via triose phosphate, and the name "hexose 

 monophosphate shunt" became for a time a popular term 

 to alternately describe the "direct" oxidation of glucose. 



The work of Horecker and Racker and their respective 

 colleagues (15-17) changed this concept, for they discovered 

 that the enzymes involved were not of the nature of a shunt 

 mechanism at all, but instead constituted an intact system 

 that might be considered completely separately. Although 

 triose phosphate is formed in the transketolase reaction, 



