INTERCONVERSION OF SUGARS 



Thus, D-xylose and L-arabinose might originate from glucose 

 and galactose by oxidation and decarboxylation of the free sugars 

 or some derivatives such as the 1 -phosphate. The vertical rela- 

 tionship in the scheme is of interest because each substance in 

 the lower row might be obtained from that in the upper row by 

 inversion at position 4. Such an inversion has been obtained 

 experimentally in the pair UDP-glucose-UDP-galactose, and 

 UDP-glucuronic acid has been isolated, so that workers should 

 be alert to the possibility of discovering UDP-galacturonic acid, 

 and perhaps UDP-xylose and UDP-L-arabinose, as well as the 

 enzymes catalyzing the interconversions. 



The Synthesis of Disaccharides 



When organic chemists wish to prepare a glycoside, they 

 usually start by preparing a derivative in which the OH at posi- 

 tion 1 of the sugar is replaced by a halogen atom. They then 

 react the halogen sugar with the acceptor in the presence of silver 

 or mercury ions. In this manner, the energy of the reactants is 

 stepped up and that of the products is stepped down (by remov- 

 ing the halogen ion as an insoluble salt), so that the equilibrium 

 of the reaction becomes favorable to the synthesis or, in other 

 words, the AF becomes negative. In living cells this stepping up 

 of the reactant may be accomplished by the phosphorylation of 

 glucose to glucose- 1 -phosphate, or it may be stepped up higher 

 by the introduction of the uridine pyrophosphate residue. Both 

 mechanisms can be used for the synthesis of sucrose. 



DoudorofT, Hassid, and co-workers (8) prepared an enzyme 

 from bacteria {Pseudomonas saccharophila) which catalyzes sucrose 

 synthesis according to the following reaction : 



glucose- 1 -phosphate + fructose ^ ' sucrose + inorganic phosphate 



(20) 



The equilibrium constant is 0.053 at pW 6.6 and 30° C. 

 (AF = +1770). The original enthusiasm which led to believe 

 that this reaction explained sucrose synthesis in plants was soon 



603 



