80 PHYSICOCHEMICAL BASIS OP PHYSIOLOGICAL PROCESSES 



explain the cause of the difference. This has been done on the basis 

 either that enzymes are colloids or that the active (catalytic) group of 

 the enzyme is attached to a colloid molecule. Before a substance can 

 be acted on, it must combine with the colloid, which it does by the proc- 

 ess of adsorption (sec page 65). This can occur, however, only when 

 there is a harmony between the adsorbing substance and the substance 

 adsorbed. Instances of the specificity of adsorption have already been 

 given. 



In support of this view it has been found that of the two proteases, 

 a and fS, in the spleen, one is adsorbed but not the other when a solu- 

 tion containing them is shaken with Kieselguhr. Furthermore, when 

 solutions of invertase are shaken with certain inert powders, the in- 

 vertase is adsorbed by some of them but not by others. In strong sup- 

 port of the adsorption hypothesis is also the fact that the same mathe- 

 matical laws as apply in the process of adsorption are obeyed in the 

 ratio which exists between the activity of an enzyme and its concen- 

 tration in the solution. 



To sum up, then, catalysis as exhibited by enzymes involves three 

 processes: (1) contact between the enzyme and the substrate, which will be 

 dependent on their rates of diffusion; (2) adsorption between them, which 

 will depend on their configurations (cf. the lock and key simile) ; and 

 (3) the chemical change which itself probably takes place in two stages. 

 In connection with the third process, it is probable that an initial com- 

 pound of a definite chemical nature is first formed, followed by the 

 hydrolytic or other chemical change, after which the enzyme group 

 becomes free. 



It is very significant in this connection to note that in their solubil- 

 ities there exists a distinct relationship between the ferments and the 

 substrates on which they react. Thus, trypsin is very soluble in water 

 and acts on water-soluble proteins; lipase is soluble in fat solvents. 



Certain Peculiarities of Enzymes 



Notwithstanding the very strong case that is made out for the cata- 

 lytic hypothesis, there are certain facts which many find it difficult to 

 make conform with such a view. One of these is that dextrose can 

 undergo three distinct and separate types of decomposition according 

 to the enzyme allowed to act on it. These are alcoholic fermentation, 

 butyric acid fermentation and lactic acid fermentation. It is difficult 

 to see how simple catalytic action can be responsible for all three results. 

 The enzyme must not only initiate the changes but also direct their 

 course. 



Another peculiarity is thai when certain enzymes — e.g., rennin, pep- 



