52 METABOLISM 



as volatile acids (34-8 per cent, and 37-8 per cent.) but formed little if any lactic 

 acid or butylene glycol. 



Many hypotheses have been put forward to explain the precise mechanism 

 of the dissimilation of the relatively simple carbohydrates. It is impracticable 

 to deal with them fully in this chapter, and the student is referred to the mono- 

 graph of Stephenson (1939) and the review of Werkman (1939) for a detailed 

 consideration of the subj ect. As an illustration of the complexity of the mechanisms 

 so far elucidated and the multiplicity of enzymes and carriers that may take part 

 we may cite the Embden-Meyerhof-Parnas scheme for the anaerobic dissimilation 

 of glucose by animal tissues and yeast cells. 



According to this scheme the hexose first reacts with adenosine triphosphate in the 

 presence of a phosphorylase to produce a hexose diphosphate and adenosine monophosphate. 

 The hexose diphosphate then breaks down into two molecules of triosephosphate under 

 the influence of an aldolase. The triosephosphate combines with acetaldehyde in the 

 presence of a mutase and co-enzyme I to form one end-product of the breakdown, ethyl 

 alcohol, and 3-phosphnglyceric acid. An enolase changes the 3-phosphoglyceric acid 

 into phosphopyruvate bj' the removal of a molecule of water, and the phosphopyruvate 

 is dephosphorylated in the presence of the adenosine monophosphate (left from the 

 phosphorylation of the hexose) to form pjTuvic acid and adenosine triphosphate. The 

 adenosine monophosphate (adenylic acid) thus acts as a co-enzyme in the transfer of 

 phosphate from phosphopyruvic acid to glucose. The pyruvic acid, according to the 

 enzymic condition of the cells concerned, may react with more triosephosphate to yield 

 lactic acid and phosphoglycerate, or be acted upon by a carboxylase in the presence of 

 co-carboxylase to give carbon dioxide and acetaldehj^de. The carbon dioxide is another 

 end-product, and the acetaldeliyde is available for the reaction with triosephosphate to 

 form alcohol. 



The co-enzyme I acts as a hydrogen carrier in the third system, the fuU reaction 

 being 



Acetaldehyde and reduced co-enzyme — >■ ethyl alcohol -f oxidized co-enzyme. 

 Triosephosphate + oxidized co-enzyme — > phosphoglyceric acid -f reduced co-enzyme. 



The cycle, which is one of many possible cycles, is most easily demonstrated in animal 

 tissues, and in extracts of yeast cells. For technical reasons active extracts of bacterial 

 cells are difficult to prepare, but evidence is accumulating that the Embden-Meyerhof- 

 Parnas scheme is applicable to anaerobic glycolysis of some bacteria. The evidence is 

 of four kinds. 



Firstly, the presence of the enzymes concerned may be inferred by comparing the 

 action of relatively specific enzyme poisons on the dissimilation by the cells under study, 

 with its action on defined enzyme systems. 



Secondly, the presence of the various oxidative, hydrolytic and phosphorylating 

 systems may be demonstrated by adding the hypothetical intermediaries to a suspension 

 of the cells, and measuring its power to deal with them ; but, as Stephenson (1939) points 

 out, though this demonstrates that a particular metabolic path may be followed in experi- 

 mental dissimilation, it is not necessarily followed in natural fermentation. 



Thirdly, the unlvnown dialysable carriers may be removed from the enzyme prepara- 

 tion, and the need for them demonstrated by adding known carriers from other sources. 



And lastly, the intermediate products postulat^ed may be isolated from the fermenta- 

 tion system, particularly aftx?r the dissimilation has been interrupted by various enzyme 

 j)oisons. 



Virtanen (1924, 1925) and Vulanen and Karstrom (1931) demonstrated that phos- 

 phorylated hexoses occur in bacterial metabolism. A key intermediate, phosphoglyceric 

 acid, has been isolated from a large number of bacteria by Werkman and his colleagues 



