220 



KONRAD BlOCH 



of isotopic carbon from all but carbon atoms 2 and 6 immedi- 

 ately restricts the number of possible reaction mechanisms. 

 If we accept the current scheme of glycolysis, then [l-^^C] 

 glucose will be converted to two C3 units, such as triosephos- 

 phate, which are isotopically equilibrated and contain one 

 labelled terminal carbon. "Head to head" condensation of 



A) 



B) 



c 

 I 

 c 



I 



c 

 c 



t 



c 



I 



c 



c c 



^c^ 



c c 



c c 



If 



c 



c 



I 



c 

 I 



c 



Ik 

 c 



I 



c 

 I 

 c 



C) 



c c 



I 



c c 



Fig. 3. Pathways for the transformation of hexose to a benzene 



ring. 



two such units will yield a six-membered ring with ^*C in 

 adjacent positions, while "head to tail" condensation will 

 distribute the label in para positions of the six-membered 

 product. Both possibilities are ruled out by the isotope 

 distribution which was actually observed (Figs. 2 and 3). A 

 mechanism which fits the isotope data, at least qualitatively, 

 and which does not necessitate the postulation of novel 

 metabolic pathways for glucose, would involve the cyclization 



