LUIS F. LELOIR 



centration is high, a digalactoside will be formed, or with a high 

 lactose concentration the main product will be a trisaccharide 

 containing two galactose molecules and one glucose molecule. 

 In this manner, a number of galactose-containing di- or oligo- 

 saccharides can be formed. All these products disappear if the 

 reaction is allowed to proceed to completion, and have therefore 

 /3 linkages. When water is the acceptor, the product is also 

 the anomer, as was shown many years ago by Armstrong. 



Similar events have been found to take place with other 

 hydrolytic enzymes such as yeast invertase, which can transfer 

 fructose residues. It seems rather doubtful that these reactions 

 have a role in biosynthetic processes, because they usually re- 

 quire high substrate concentrations. 



The fact that hydrolytic enzymes can act as transferring 

 enzymes raises the question of whether enzymes which are con- 

 sidered as purely transferring have some hydrolytic action which 

 is usually attributed to impurities. Perhaps the difference be- 

 tween these two types of enzymes is not quite clear-cut, and all 

 the intermediate types can be found. 



The Synthesis of Polysaccharides 



Several polysaccharides have been already synthesized in 

 vitro. Except for the case of phosphorylase, all the other enzymes 

 use a disaccharide as substrate : maltose in some cases, and su- 

 crose in others. The glucose half of sucrose can be used in making 

 dextrans, while other enzymes use the fructose half for the syn- 

 thesis of levans or fructans (1). 



It is interesting to point out how UDPG could be used by 

 cells to step up the energy of fructose in order to make a fructan. 

 Two known reactions could be combined as follows : 



UDPG + fructose , sucrose + UDP (25) 



sucrose + (fructose) „ ^ glucose + (fructose)„+i (26) 



The result would be the hydrolysis of UDPG to UDP and 

 free glucose, and the introduction of free fructose into the poly- 

 mer. It is not known whether UDP-sugar compounds lead to 



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