LUIS F. LELOIR 



that is, a substance which combines with toxic products to render 

 them harmless. Glucuronic acid is also present in many poly- 

 mers such as hyaluronic acid, chondroitin sulfate, heparin, bac- 

 terial polysaccharides, and plant gums. 



By the use of tracers it has been proved that mammals 

 transform glucose directly into glucuronic acid — that is, without 

 breaking down the carbon atom chain. A signal advance in 

 these studies has been the isolation of UDP-glucuronic acid. It 

 has been found that the formation of glucuronides with liver 

 enzymes requires the presence of a specific donor, which turned 

 out to be a uridine compound (5). The reaction takes place as 

 follows : 



UDP-glucuronic acid + phenol > phenol-glucuronide + UDP 



(12) 



The identification of this substance has provided a working 

 hypothesis for the study of the oxidation step which leads from 

 glucose to glucuronic acid. Since studies with tracers prove that 

 free glucuronic acid is not an intermediate in the formation of 

 glucuronides, it may be assumed that the oxidation step takes 

 place either on UDPG or on glucose- 1 -phosphate, yielding UDP- 

 glucuronic acid or glucuronic acid- 1 -phosphate, respectively. 



The knowledge acquired in the formation of glucuronic acid 

 will no doubt be applied to the elucidation of the origin of the 

 other uronic acids such as D-galacturonic acid, which is present in 

 pectins, flaxseed mucilage, and some specific polysaccharides of 

 pneumococci, and D-mannuronic acid, which is the sole con- 

 stituent of the alginic acid produced by seaweeds. 



The oxidation of an uronic acid probably leads to the forma- 

 tion of ascorbic acid. One of the many schemes which have been 

 proposed (10) shown on next page. 



An interesting point about this scheme is the mechanism by 

 which the molecule passes from the d to the l series. This is 

 accomplished by reducing the aldehyde group of D-galacturonic 

 to a primary alcohol, and thus forming L-galactonic acid. In 

 order to adhere to the usual convention of writing carbon atom 



598 



