Oxidative Pathways of Carbohydrate Metabolism 175 



from [2-^*C]glucose into either visceral RNA ribose or into 

 ribose isolated from the riboside excreted after administration 

 of imidazole acetic acid. Ribose synthesized via the oxidative 

 loss of C-1 of [2-i^C]glucose would be labelled in its first carbon 

 atom while that produced by non-oxidative transfer of the 

 first two carbon atoms of hexose phosphate to a triose phos- 

 phate acceptor, as catalysed by TK, would be labelled in C-2. 

 The C-1 /C-2 quotient gives an indication of the relative con- 

 tribution of the two pathways to ribose synthesis. A low 

 C-1 /C-2 ratio is consistent with the operation of the non- 

 oxidative TK pathway whereas a high ratio indicates the 

 predominance of oxidative decarboxylation. Some results 

 obtained by the use of these methods (Hiatt, 1957, 1958; 

 Feigelson and Marks, 1957) are shown in Table IV. 



The C-l/C-2 quotient is unaltered in hyperthyroidism, in- 

 creased somewhat in rats bearing ascites tumour cells and 

 markedly increased in thiamine-deficient rats, this latter 

 effect probably being attributable to a lower TK activity in 

 the absence of its essential cofactor, thiamine pyrophosphate. 



Levels of coenzymes 



Total tissue levels of DPN (DPN+ + DPNH) and TPN 

 (TPN+ + TPNH) as well as the relative proportion of oxidized 



