THE BIOCHEMICAL PROCESSES OF DIGESTION 491 



The importance of the presence of erepsin in the mucous membrane 

 of the intestine is that it serves as a barrier to the passage of any unsplit 

 amino acids from the intestinal contents into the blood. It insures the 

 breaking up of the protein molecule into its ultimate units before absorp- 

 tion. The further fate of the absorbed amino acids will be considered 

 under the subject of protein metabolism. 



THE ACTION OF LIPASE 



Neutral fat is decomposed into fatty acids and glycerine by the lipase 

 present in the pancreatic juice. This enzyme may also be extracted from 

 the glands by means of 60 per cent alcohol. Its action is remarkably 

 accelerated by the presence of bile, and considerably depressed by inor- 

 ganic salts. It is also very dependent on the degree of alkalinity, the 

 optimum being a hydrogen-ion concentration of H x 1O 8 . The favoring 

 action of bile is undoubtedly owing to the bile salts (see page 493), and 

 it is probable that this action is dependent upon the influence which 

 these have in lowering surface tension and therefore bringing about a 

 more intimate contact between fat and water. 



THE ACTION OF AMYLOPSIN 



The action of pancreatic juice on carbohydrates depends on the 

 amylolytic enzyme called amylopsin. In animals having no active ptyalin 

 in the saliva, amylopsin serves as the only diastatic enzyme concerned 

 in the digestive process. In any case, at least for the first stages of the 

 disruption of the starch molecule that is, its conversion into dextrines 

 amylopsin is a more powerful enzyme than ptyalin. It does not appear 

 to be so efficient as ptyalin in the final stages of the hydrolysis, for it 

 does not produce so much reducing sugar as ptyalin does. Indeed ex- 

 tracts of pancreas w r ill sometimes convert starch into soluble starch and 

 dextrine with great speed, but produce scarcely any reducing sugar. 

 On this account it is believed by many investigators that there are at least 

 two distinct and separate enzymes in amylopsin and also perhaps in 

 ptyalin, one a true amylase, which converts starch into dextrine, and 

 the other a dextrinase, which converts dextrine into maltose. In the 

 case of both ptyalin and amylopsin digestion proceeds best in a very 

 weak acid reaction. Amylopsin, as it is secreted in the pancreatic juice, 

 is fully activated; bile, apart from the alkali which it contains, having 

 no influence on its digestive power. 



Besides amylopsin the pancreatic juice also contains maltase, and in 

 the case of young animals or of those that take milk with their food 

 throughout their lives, lactase also. After the suckling animal has dis- 



