84 PHYSICAL CHEMISTRY 



in the first phase toward the second. This seems to be the case 

 with many reactions catalyzed by finely divided platinum and 

 is supposed to be a factor in enzyme reactions, since enzymes 

 seem to be of colloidal nature. 



Catalysis with colloidal solutions of precious metals resembles 

 the action of enzymes in many ways. Both have a temperature 

 optimum, that is, both are rendered inactive by boiling and even 

 lower temperatures. According to Ernst (1901) the tempera- 

 ture optimum for the oxidation of hydrogen under the influence of 

 platinum sol lies between 65 ° and 85 °. Both are affected by 

 the reaction of the medium. The decomposition of H 2 2 , either 

 by platinum sol or by the enzyme, catalase, is favored by a slightly 

 alkaline reaction. Both the catalase and oxidase action of 

 enzymes or colloidal precious metals is suspended by HCN and 

 returns when the HCN is removed, if just enough were used 

 to suspend action. The same is true of the action of H 2 S. On 

 the sols of Cu and Fe, however, HCN increases catalytic activity, 

 according to Kastle and Loevenhart (1903). Finally, Meyerhof 

 (1914 b) has shown that the action of platinum sol on H 2 2 is 

 reduced by the presence of anesthetics, in the same way that 

 enzyme actions are reduced by anesthetics. The anesthetic forms 

 a protective film on the platinum particles, due to adsorption. 



The importance of the reaction of the medium, that is, the 

 H ion concentration, on the activity of enzymes has long been 

 known. Michaelis (1914) attempts to explain this action by 

 assuming the enzymes to be weak electrolytes, i. e., acids, bases 

 or ampholytes, whose dissociation is affected by the H ions. 

 At the optimal H ion concentration the activity of the enzyme 

 solution is proportional to the concentration of the enzyme and 

 all of the enzyme is in the active form. At any other H ion 

 concentration some fraction of the enzyme is in an inactive form, 

 and therefore the total activity is reduced. 



According to Sorensen (1909) and Michaelis and Davidsohn 

 (1911) the optimal PH for the invertase of yeast is about 4.3. 

 Above the optimum the activity of the enzyme decreases to zero 

 at about 9, whereas below it the activity decreases (because the 

 enzyme is destroyed?) so that little action is obtained below 3 

 or 2, depending on the duration of the experiment. Michaelis 

 interprets this as follows: The enzyme is an ampholyte with 



