LUIS F. LELOIR 



Some Unknown Processes 



Whereas ribose phosphate has been obtained in vitro by two 

 different processes, by oxidation of gluconic acid 5-phosphate 

 and by transfer of a two-carbon unit to glyceraldehyde phos- 

 phate, information on the origin of other pentoses is lacking. 

 Nothing is known on the biogenesis of D-xylose, present in plants 

 as the polymer xylan, which is second only to cellulose in abun- 

 dance. Another pentose which is widely distributed in plant 

 products is L-arabinose. It is found in hemicelluloses, pectins, 

 gums, and some bacterial polysaccharides. 



The stereochemical relationships of these two pentoses are 

 made apparent by the following scheme. 



> D-glucuronic acid -^ > D-xylose (18) 



D-glucose 

 D-galactose — 



> D-galacturonic acid > L-arabinose (19) 



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