96 PHOTOSYNTHESIS 



in Chapter 8. The emphasis here is that a mechanism for 

 the Wood-Werkman reaction can be proposed on the basis 

 of the plant enzyme system described by Bandurski and 

 Greiner. 



The oxidative decarboxylation of phosphogluconic acid. The 

 formation of pentose phosphate from hexose monophos- 

 phate was described by Dickens (1938). More recently an 

 enzyme has been purified from yeast by Horecker and 

 Smyrniotis (1951) and which catalyses the reaction: 



COOH 



The system resembles the malic and isocitric enzymes in 

 that no intermediate keto acid can be detected and that the 

 reaction is reversible. In presence of a pentose phosphate 

 isomerase this product is converted into an equilibrium 

 mixture containing about 75% in the form of ribose-5- 

 phosphate. The isomerase has been purified from a plant 

 source by Axelrod and Jang (1953). Glock (1952) and others 

 have shown that pentose phosphate was converted by liver 

 extracts into hexose monophosphate to give a higher yield 

 than could be accounted for by the three carbon residue of 

 the pentose alone. This supported the now accepted con- 

 clusion that in the oxidation of hexose monophosphate via 

 6-phosphogluconic acid a cyclic process was involved. Re- 

 cent work by Horecker, Racker, and others summarized by 

 Horecker (1953) has shown that the cycle involves both 

 pentose and sedoheptulose phosphates. It forms an impor- 

 tant, sometimes the major, pathway for the oxidation of 



