264 



GERTRUDE E. GLOCK 



COOH 



I 

 C=0-n 



I 



CHoOH 



+ 



HCOH 



CH2OPO3H2 

 Hydroxypyruvic acid 

 + 

 Glyceralde-3-phosphate 



Carboligase 



Fig. 6. Carboligase-catalyzed formation of pentose phosphate according to Aka- 

 bori et al."^^ 



boxylation of hydroxypyruvic acid resulted in the formation of ribulose-5- 

 phosphate. Since this formation of ribulose-5-phosphate represents a ketol 

 condensation and no free glycolaldehyde is formed, Racker assumes the 

 formation of "active" glycolaldehyde which condenses with the "acceptor 

 aldehyde" to form a keto-sugar. This enzyme has been called "transketo- 

 lase." Thiamine pyrophosphate is necessary for its activity. This is also 

 true of the liver and spinach leaf enzymes.'^^ 



2, Decarboxylation of Uronic Acids 



The first indication that pentose might be derived enzymically from 

 uronic acid was the isolation of D-xylose from a decaying mince incubated 

 with D-glucuronic acid/^ It was subsequently observed in numerous plant 

 gums and mucilages that when a single uronic acid and a single pentose 

 are present together, the two are frequently in a homologous series, for 

 example D-glucuronic acid and D-xylose, and D-galacturonic acid and l- 

 arabinose. These findings led to the hypothesis that the uronic acids are 

 directly decarboxylated to the homologous pentoses. ^^ However, the isola- 

 tion of D-mannuronic acid and the failure to detect D-lyxose in plant prod- 

 ucts, and the widespread occurrence of D-ribose but absence of D-alluronic 

 acid has thrown some doubt on the validity of this hypothesis. This prob- 

 lem is discussed in detail by Hirst.^" 



Cohen decided to test this hypothesis experimentally using uronic acid- 

 's E. Salkowski and C. Neuberg, Z. physiol. Chem. 36, 261, (1902). 

 " J. M. GuUand, /. Chem. Soc. 1944, 208; E. L. Hirst, J. Chem. Soc. 1942, 70. 

 8" E. L. Hirst, J. Chem. Soc. 1949, 522. 



