74 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



relationship between enzymes and inorganic catalysts, for on the one 

 hand there are several enzymes which do not exhibit this property, and 

 on the other, there are inorganic catalysts which do. For example, 

 lipase, the fat-splitting enzyme of pancreatic juice, decomposes not only 

 fats but to a greater or less degree a number of bodies of the same gen- 

 eral build (esters), and tyrosinase can decompose, not tyrosin alone, 

 but all phenol compounds. Conversely, the hydrogen ion to the pres- 

 ence of which acids owe their catalytic powers can decompose the ordi- 

 nary esters (that is, of acids containing the carboxyl or COOH group) 

 but it has no action on the sulphonic esters. However, enzymes are cer- 

 tainly much more specific in their action than inorganic catalysts. 



2. Temperature does not influence catalysis and enzyme action in the 

 same way. As the temperature is raised in the case of inorganic catalysts, 

 the reaction becomes about doubled in rapidity for each rise of 10 C., 

 whereas in the case of enzymes it becomes increased up to a certain opti- 

 mum temperature, beyond which, as the temperatiire rises, the reaction is 

 first slowed and then disappears altogether. 



This peculiarity of enzymes as compared with inorganic catalysts need 

 not in itself disprove the analogy between the tw r o, because enzymes do 

 not form true, but colloidal solutions. Colloidal solutions, as we have 

 seen, are really fine suspensions of ultramicroscopic particles ; there is no 

 splitting into ions of the dissolved substance, as is the case with true 

 (molecular) solutions, but the colloid is suspended in the water or other 

 solvent to form a heterogeneous system (page 52), on which account 

 the surface area of the menstruum is enormously increased. Eise in 

 temperature alters the extent of the surface area, and thereby intro- 

 duces an influence which progressively opposes catalysis. 



Although inorganic catalysts in molecular solution show no optimum 

 temperature but increase in activity in proportion as the temperature is 

 raised, inorganic colloidal catalysts may show an optimum temperature. 

 Thus, spongy platinum shows an optimum temperature in its action on a 

 mixture of hydrogen and oxygen. It has therefore been suggested that 

 it is because they are colloids that enzymes exhibit this property. 



3. Inorganic catalysts frequently carry the reaction to a further stage 

 than that attained by the action of enzymes. For example, acid breaks 

 down the protein molecule much more completely than do the proteolytic 

 enzymes. This difference is perhaps explained by the fact that enzymes 

 are retarded in their activities when there comes to be a certain accumu- 

 lation of the products of the reaction present. The final stages in the 

 reaction may become so slow as to be almost inappreciable. This de- 

 crease in activity is partly due to a union between the enzyme and the 

 products of its activity. 



