166 S. S. COHEN 



sugars, which then may also become components of ohgosaccharides (Srmi- 

 vasan and Quastel, 1958). The enzyme, amylomaltase, catalyzes the followmg 

 reaction, which may build very long amylose chains: 



n maltose (1, 4-glucosidoglucose) ^ n glucose + amylose (maltosidoglucose)„. 



Transglycosidases operating on polysaccharides mclude both transgluco- 

 sidases and transfructosidases. In the former series, enzymes probably exist 

 to transfer glucose to 1, 3, 1, 4, 1, 5 and 1, 6 linkages from a- or ^-1, 4 linkages 

 and from 1, 2-fructosides. 



Among the transfructosidases, transfer of fructosyl moieties are known 

 to occur from the 1, 2 to 2, 6 linkages as m the reaction catalyzed by levan 

 sucrase. 



n 1, 2-glucosidofructose ^ 2, 6-fructosido (fructose)^ + n glucose. 



The corresponding transglucosidation is catalyzed by dextran-sucrase. 



n 1, 2-glucosidofructose ^ 1, 6-glucosido (glucose)^ + n fructose. 



In the latter case, many kmds of glucosyl acceptors can initiate the forma- 

 tion of alternative chains. In the case of the hydrolases, water acting as an 

 acceptor can block the form^ation of chains. 



iiivcrtase 



sucrose + 11,0 -^ glucose + fructose 



1 , 6 glucosidase 

 1, 6-dextrins + HjO > 2 n glucose. 



The counterpart of the purme and pyrimidine nucleoside phosphorylases 

 exists in the ^ra>?s-iV-glycosidases (1) and hydrolases (2) for some nucleo- 

 sides. These are not too widely distributed, existing only in a few micro- 

 organisms, and catalyzmg reactions of the following types: 



Basel iV-deoxyriboside + Base 2 ^ Base^ + Bases -A^-deoxyriboside (1) 



Base-iV^-riboside + H^O > Base + ribose (2) 



Reactions of both types are also known in the cleavage of DPN at the 

 nicotinamide-iV -riboside hnkage. 



3. Systems Utilizing Nucleoside Diphosfhosugars 



As mentioned above, Leloir discovered the coenzyme uridine diphospho- 

 glucose (UDP glucose) in studying the mterconversion of glucose and 

 galactose (see summary by Cohen, 1954). Galactose was phosphorylated to 

 galactose-1 -phosphate and reacted with UDPG as follows: 



galactose-1-P -|- UDP-glucose ^ glucose-1-P + UDP-galactose 



An isomerase then converted UDP-galactose to UDP-glucose, regenerating 



