in chloroplasts, could account for virtually all the monosac- 

 charide phosphates found to be significantly labeled with C^"* 

 following a period of photosynthesis with C^*02 for several 

 minutes in algae. Presumably there is present also another 

 phosphohexose isomerase which catalyzes the conversion of 

 fructose-6-phosphate to mannose-6-phosphate. 



Among the enzyme systems listed in Table 3 are several 

 that utilize sugar nucleotides in the biosynthetic conversion 

 of sugars. Such systems have been widely studied and have 

 been discussed and reviewed elsewhere (88-90). Hassid and 

 co-workers have widely studied the interconversions of sugars 

 by these systems in higher plants and have summarized the 

 interrelations of many of these systems in plants (91). Certain 

 of these systems, which appear in Table 3, are particularly 

 active in the early labeling of sugars in plants photosynthe- 

 sizing with C^*02 and must be mentioned here, if only briefly. 



Buchanan et al. (15) identified uridine diphosphate glu- 

 cose (UDPG) and uridine diphosphate galactose (UDPGal) 

 in algae and found that the hexose moieties of these com- 

 pounds were labeled with C^^ during short periods of 0^^*02 

 photosynthesis even before sucrose. Thus the galactose found 

 to be labeled in some experiments may be formed by the 

 UDPG-UDPGal system. 



Disaccharides and polysaccharides 



As already indicated, when Chlorella pyrenoidosa photo- 

 synthesizes in the presence of C^*02, sucrose is the first free 

 sugar to be labeled to any extent. Benson (92) found that the 

 radiocarbon in the fructose moiety of the sucrose, following 

 photosynthesis of C'^02 by Chlorella, Scenedesmus, and soy- 

 bean leaves, was greater than the radioactivity in the glucose 

 moiety. Moreover, the difference between fructose and glu- 

 cose became greater as the time of photosynthesis was de- 

 creased. 1 he prior labeling of the fructose indicated that the 

 glucose phosphate used in the synthesis of sucrose is formed 

 from fructose phosphate. 



53 



