164 S. S. COHEN 



Starch, glycogen, or other dextrin is present as a primer. Phosphorylases of 

 different origin possess different primer requirements. The primer must be 

 larger than a disaccharide and provides terminal nonreducing end groups to 

 which glucosyl moieties may be added, permitting the stepwise extension of 

 the chain. These phosphorylases are exceedingly specific for the substrate, 

 a-D-glucose-1 -phosphate. However, the mechanism of activation of the 

 glucosyl moieties is far from clear. Pyridoxal phosphate, of unknown function, 

 has recently been identified in muscle phosphorylase (Cori and lUingworth, 

 1957), an enzyme which also contains bound adenyhc acid. 



However, glycogen is a complex, branched polysaccharide, composed of 

 both straight chaui amylose molecules in which glucosidic linkages are of the 

 maltosidic a 1, 4 t}T3e, sensitive to phosphorylase, and phosphorylase- 

 insensitive branch points in wliich the glucosidic linkage is of the 1, 6 

 type. Purified phosphorylase synthesizes amylose chains alone from a- 

 g]ucose-l -phosphate. In the degradation of glycogen, phosphorylase wiU 

 degrade amylose chains to the branch points. An amylo-1, 6-glucosidase is 

 then required to cleave the 1, 6 bonds; such cleavage then provides an entry 

 for the further phosphorylytic attack of underlying amylose chains. Thus, 

 the polysaccharide contains many layers of amylose chains which are 

 approached by degradation with successive treatments of phosphorylase and 

 glucosidase. With this technique it was found that the peripheral glucose 

 moieties of the glycogen of animals (Stetten et at., 1956) and bacteria (Holme 

 and Palmstierua, 1956) are metabohcaUy more active than mternal glucose. 



In addition to the phosphorylases, two other types of enzymes have been 

 implicated in glycogen synthesis and m polysaccharide syntliesis in general. 

 The first of these are the transglycosidases in which sugar phosphates are not 

 intermediates. The second group involve the intermediation of the coenzymes 

 containing diphosphouridine. 



As w^ill be seen below, glycosyl transfer of hexoses, amino sugars, uronic 

 acids, and pentoses may be effected via the intermediation of nucleoside 

 diphosplio-1 -glycosides. One of the most important of this group of meta- 

 bohtes, which were discovered by Leloir and his collaborators, was uridine 

 diphosphoglucose (UDPG). Leloir and Cardini (1957) have recently described 

 the transfer of glucose from UDPG to glycogen in the presence of a liver 

 enzyme. That the reaction does not involve the intermediary generation of 

 glucose- 1 -phosphate is suggested by the stoichiometric formation of UDP. 

 n UDP glucose -f glycogen ^ glycogen (glucose)„ + n UDP 



The analysis of the action of sucrose phosphorylase revealed the trans- 

 glucosidatic activity of this enzyme (Doudoroff e^ at., 1947). This enzyme, 

 isolated from Pseudomonas saccharophila, was capable not only of exchangmg 

 the phosphate of glucose- 1 -phosphate with inorganic P^^ without releasing 

 glucose, but could also exchange the unlabeled fructose of sucrose with 



