192 UNITY AND DIVERSITY IN BIOCHEMISTRY 



chain constitutes a cycle resulting from the attachment of a shunt, one end 

 on G — 6 — P and the other on F — 6 — P and the triosephosphates. The 

 multi-enzyme system of the hexosemonophosphate shunt (HMS) is some- 

 times called the pentose cycle because it contains mechanisms for the 

 formation of pentoses either by decarboxylation of hexoses or from 

 phosphoglyceraldehyde. The cycle is quite widely found in the biosphere, 

 but its relative importance compared to glycolysis is extremely variable. 

 The different tissues of an organism differ in this respect. In mammals, for 

 example, glycolysis is predominant in the muscles and the hexosemono- 

 phosphate shunt in the liver. 



Knowledge of the different reactions which have been carried out in 

 vitro with purified enzymes, reactions which are collected together in the 

 scheme shown in Fig. 38, give experimental confirmation to the ideas 

 summarized in this scheme. It does not exclude the existence of other, as 

 yet unknown, pathways. 



The cycle contains two oxidations, each coupled with TPN (and not 

 DPN which in general is the coenzyme required in glycolysis). Glycolysis 

 is inhibited by fluoride and iodoacetate or bromacetate, the first affecting 

 enolase and the second triosephosphate-dehydrogenase. 



In 1936, Lipmann found that an extract of yeast continued to respire in 

 the presence of bromacetate, although fermentation is blocked by this 

 substance. This contradicted the notion that respiration is necessarily an 

 appendix attached to a preceding anaerobic glycolysis leading to pyruvate. 

 The year before, Warburg (1935) had described, at the time he discovered 

 TPN, the oxidative transformation of G — 6 — P into 6-phosphogluconic 

 acid in yeast and erythrocytes, the dehydrogenase being named by him 

 Zwischenferment. The study of these phenomena by Warburg and by 

 Dickens showed that an oxidative decarboxylation with formation of a 

 pentose phosphate was involved. From 1950 onwards, the researches of 

 S. Cohen and Scott on the one hand, and of Horecker and Smyrniotis on 

 the other, provided new information leading to the identification of the 

 pentoses formed. Ribulose-5 -phosphate is first formed in the oxidation 

 followed by decarboxylation of the phosphogluconate brought about by 

 purified preparations of the dehydrogenase. Then the ribulose-5 -phosphate 

 is converted by phosphopentose-isomerase into an equilibrium mixture of 

 two pentose phosphates : ribose-P and xylulose-P. 



The chain leading from the hexoses to the pentoses is theoretically rever- 

 sible, but this reversal is probably only of biological importance under very 

 special circumstances. 



It is nonetheless true that in many cases a pentose phosphate can give a 

 hexose. But it is not by a reversal of the hexosemonophosphate oxidative 

 chain. The action in question is a non-oxidative action by transketolase and 

 transaldolase on the pentose phosphates. The demonstration of this 



