Oxidative Pathways. of Carbohydrate Metabolism 155 



1957). The UDP-glucuronate so formed is believed to yield 

 free glucuronic acid, which can undergo reduction by a TPNH 

 linked enzyme system acting at the aldehydic C-1 end of the 

 glucose molecule, to give L-gulonic acid. The latter is believed 

 to be oxidized by a DPN system to an intermediate (possibly 

 3-ketogulonic acid), which can either form ascorbic acid by 

 enolization and lactonization, or L-xylulose by decarboxyla- 

 tion at C-6 of the original glucose molecule (Touster, Hutche- 

 son and Rice, 1955; Bublitz, Grollman and Lehninger, 1957; 

 Ashwell, 1957; Burns and Kanfer, 1957). It is thought that 

 only the latter reaction occurs in species such as man and 

 guinea pig which are unable to synthesize ascorbic acid. 



In cases of essential pentosuria, L-xylulose is excreted; 

 otherwise it is reduced by a TPNH-coupled enzyme to xylitol, 

 which then becomes oxidized by a DPN enzyme system of 

 liver to D-xylulose (Hollmann and Touster, 1957). The latter 

 can then be phosphorylated by ATP acting with a specific 

 phosphokinase to give d-Xu5P (Hickman and Ashwell, 1957) 

 which can enter the pentose phosphate cycle, described below, 

 and thus be converted to hexose. These reactions explain why 

 xylitol and D-ribose can both be efficiently incorporated as 

 glucose of liver glycogen (McCormick and Touster, 1957), 

 D-ribose being also phosphorylated to the 5-phosphate by a 

 liver kinase (Agranoff and Brady, 1956). 



The pentose phosphate pathway 



The metabolism via pentose phosphates appears to be the 

 main alternative to the glycolytic pathway of carbohydrate 

 metabolism in animal tissues, although quantitatively it is 

 nearly always much less extensive. The broad outline of this 

 route is compared with that of glycolysis in Fig. 1, which 

 shows that there are two points — G6P and F6P — at which 

 these pathways diverge. 



(a) The HMP oxidative pathw^ay 



G6P is the branching point for this metabolic route, which 

 is also known as the phosphogluconate or oxidative pentose 



