INTERCONVERSION OF SUGARS 



HgC^O-POg-- HGc-O-POq- 



HCOH 



HCOH 



HOCH O HOGH O 



HCOH 



I 

 HC 



HOCH 

 I 

 HC — 



CH2OH CH2OH 



(H) (HI) 



Glucose- 1 -phosphate Galactose- 1 -phosphate 



position 4 of the hexose. The equiUbrium mixture contains 

 about 25% of the galactose ester. Besides the enzyme (s), the 

 reaction requires a coenzyme, uridine-diphosphate-glucose 

 (UDPG) (I), which was first isolated from yeast and has been 

 found to be present in animal and plant tissues. It has also been 

 prepared by synthesis. An indication of how this coenzyme acts 

 was obtained when analyzable amounts of UDPG were incubated 

 with the yeast enzyme. It was found that about one-fourth of 

 the glucose moiety was transformed into galactose, and it was 

 concluded (13) that the yeast extracts contain an enzyme catalyz- 

 ing the equilibrium 



UDP-galactose , UDP-glucose (3) 



In order to relate this fact with the catalytic action of UDPG 

 in the transformation of galactose- 1 -phosphate, it was assumed 

 that another enzyme would exchange the hexose- 1 -phosphate 

 residues in UDPG as follows : 



gaIactose-1 -phosphate + UDP-glucose ^ 



glucose-1 -phosphate + UDP-galactose (4) 

 The sum of these two reactions gives 



galactose-1 -phosphate ; ^ glucose-1 -phosphate (5) 



Information on the mechanism by which UDPG is trans- 

 formed into the galactose derivative is lacking. Many mecha- 

 nisms which would lead to such a change in the configuration of 

 carbon 4 can be imagined. Any reaction which produces a loss 



591 



