OF VITAL PHENOMENA 85 



the isoelectric point at the optimal H ion concentration. Hence, 

 it is the undissociated molecules of the invertase that are active. 

 Above the isoelectric point (alkaline side) the invertase disso- 

 ciates as an acid, and the activity curve corresponds to the curve 

 for the undissociated part of an acid. Below the isoelectric 

 point (acid side) the invertase dissociates as a base, but the 

 invertase cations are very unstable and soon become denatured, 

 so that the activity of the enzyme is not restored by return to 

 the isoelectric point. 



In the same way Michaelis concludes that trypsin and erepsin 

 are acids whose anions are proteolytic. Whether the theory of 

 Michaelis is right or wrong, the relation of the H ion concentra- 

 tion to the activity of enzymes is no less important. The follow- 

 ing table gives the optimal H ion concentration for the action 

 of several enzymes (McClendon, 1916 b). 



The optimum PH for enzyme action is of interest in connec- 

 tion with the reaction of the parts of the alimentary tract, where 

 the digestive enzymes act. 



The saliva is generally supposed to be alkaline, but according 

 to Michaelis (1914) it has a PH of about 6.87, which is near 

 the optimum for the action of ptyalin (6.7). 



