126 E. S. GUZMAN BARRON 



Regulatory Mechanisms 



From this brief review of some enzyme reactions it can be seen that 

 what distinguishes biochemical reactions from chemical reactions in 

 general is the means provided for the rapid attainment of equilibrium, 

 so strikingly demonstrated by the enzymatic reversible oxidation-reduc- 

 tion systems. Furthermore, although on a thermodynamic basis a very 

 large number of reactions are theoretically possible in a given biologic 

 system, the presence of enzymes will result in the selective catalysis of 

 only a limited number of these reactions. But the isolation, purification, 

 and study of the catalytic action of individual enzymes is only the begin- 

 ning of the study of the biochemical reactions which occur in the cell, 

 whether it be in the state of "dynamic equilibrium" or in the state of 

 growth. Living matter is the result of the coordinated interplay of 

 substance and structure, as Pauling (71 ) has remarked, and the advance 

 of biology requires equal devotion to the work of isolation and identi- 

 fication of active chemical substances, to the study of the macromolecular 

 stromatic structures present in the living cell, and to the investigation 

 of the multiple regulating mechanisms which contribute to the cellular 

 dynamic equilibrium. 



The rate of reaction of isolated enzyme systems is extremely high 

 when compared with the rate of reaction in the living cell. Enzyme 

 reactions are controlled in the living cell by a variety of regulating 

 mechanisms. To the usual physico-chemical factors which govern the 

 rate of reactions in solutions there must be added the effect of macro- 

 molecular structures which constitute the framework of the cell, such 

 as the genes, and which can orient or fix the position of the intracellular 

 enzymes by means of loose unions. As the cell increases in efficiency and 

 becomes more differentiated, the activity of the enzymes becomes more 

 and more controlled by specific regulating mechanisms, which in multi- 

 cellular organisms freely circulate through the intercellular fluid. In 

 vertebrates these regulatory mechanisms increase in number and speci- 

 ficity, and they control and orient the rate of enzymatic reactions by 

 interaction among each other. These regulatory mechanisms (hormones, 

 nucleoproteins, genes, electrolytes, certain oxidation-reduction systems) 

 do not act by forming part of enzyme components ; they act by control- 

 ling the rate of enzyme reactions or by changing the orientation of 

 these reactions. They act as the traffic policemen in the crowded streets 

 of a city. Thus the steady state of the adult cell is maintained, and the 

 synthetic processes required in growing cells are brought forth. An 



