VI. CELLULAR METABOLISM 

 AND GROWTH 



BY E. S. GUZMAN BARRON^ 



IT IS generally recognized that the energy necessary for cellular 

 activities, whether it be used to maintain the steady state of adult 

 life, the "dynamic equilibrium" of Schoenheimer, or the state of 

 growth and synthesis which predominates in dividing and growing cells, 

 is provided by enzymatic reactions largely of the oxidation-reduction 

 type. This has been amply demonstrated by the increased O2 utilization 

 during growth and by the necessity of aerobic processes for the syn- 

 thesis of carbohydrates, fats, and proteins which together make the bulk 

 of the cells. 



It will be therefore necessary before discussing the role of enzyme 

 reactions in cellular growth to review some fundamental aspects of the 

 properties of the oxidation-reduction enzymes. 



General Properties of Enzymes 



Enzymatic oxidation-reduction reactions are distinguished from 

 chemical reactions in general by the facts that they are reversible, that 

 electron exchange occurs in steps, and that they have a great tendency 

 to form coupled reactions. 



The reversibility of enzyme reactions can be accepted as a general 

 property, since more and more of those reactions hitherto considered 

 irreversible are being shown to be reversible. Witness the enzymatic 

 synthesis of phosphopyruvic acid from pyruvic acid (48), a reversal of 

 the dephosphorylation of phosphopyruvic acid, which had been reported 

 as an irreversible process (58). In many instances reversibility of oxida- 

 tion-reduction reactions is made easy by the entrance of P into the re- 

 actions and generation of energy-rich phosphate bonds, as was shown 

 by Warburg and Christian (83) in the compulsory coupling between 

 the oxidation of phosphoglyceraldehyde and the phosphorylation of 

 adenylic acid, a finding extensively generalized to other reactions by 

 Lipmann (53). 



The first step in an enzymatic reaction is the formation of the enzyme- 

 substrate complex. Evidence in favor of such a concept, introduced by 

 Michaelis and Menten (61), is abundant. Knowledge of the nature of 



1 The Chemical Division, Department of Medicine of the University of Chicago. 



