86 PHYSICAL CHEMISTRY 



The optimum PH for pepsin is, according to Sorensen (1909), 

 1.5-2, and the results of Michaelis and Davidsohn fall between 

 these limits. Sorensen observed considerable digestion at 

 PH = 0.7 near the maximum acidity and PH = 5 near the mini- 

 mum acidity. From curves showing the rise in acidity of the 

 stomach measured with the hydrogen electrode (McClendon, 

 1915 e) it may be observed that the acidity becomes high enough 

 for digestion to commence during the first hour and the optimum 

 is reached by about the end of the second hour if at all. Men- 

 ten (1915) found the PH of pure human gastric juice to vary 

 from 0.92 to 1.58, corresponding to an average of .36 per cent 

 HC1. Apparently the reaction of the stomach is determined by 

 the amount of food in it and the rate of flow of the juice. The 

 juice secreted into an empty stomach would have a very high 

 acidity, but Boldyreff (1915) has shown that it is lowered by 

 regurgitation of pancreatic juice. The PH in hyperacidity, from 

 the figures given by Michaelis (1914), indicates not nearly so 

 high an acidity as the pure gastric juice. It seems probable that 

 hyperacidity is brought about by closure (spasm) of the pylorus, 

 preventing removal or neutralization of the acid. 



The PH of the pure gastric juice of the infant is about 2.3 

 (McClendon, 191 5 e), being nearly the optimum for the action 

 of pepsin, which it contains, but the juice is apparently secreted 

 so slowly that the PH is 5.3, while the stomach is full of milk, 

 which is about the optimum for the action of rennin (pepsin?) 

 in coagulating milk. 



The PH of the adult duodenum is about J.J or the optimum 

 for the action of trypsin and erepsin on peptones. 



In the infant a very peculiar state of affairs exists. As the 

 milk leaves the stomach the acidity rises until it equals that of 

 the pure gastric juice about four hours after nursing. The neu- 

 tralizing power of the pancreatic juice is very low, so that the 

 duodenum has about the same reaction as the pylorus. The PH 

 of the duodenal content is usually about 3.1 and pepsin is pres- 

 ent, so that peptic digestion must take place in it. 



It has been shown by a number of observers that the rate of 

 enzyme action does not always vary in direct ratio with the con- 

 centration of the substance to be transformed, substrate, but that 

 high concentrations of the substrate are relatively detrimental. 



