126 BIOLOGICAL CHEMISTRY 



Liver 



Portal Blood 



Intestine 



The bile is formed continuously with an increase in rate of 

 formation during digestion. The increase in rate is associated 

 with an increased blood flow from the intestine containing 

 products of digestion and the bile acids. These conditions 

 may account for the increased secretion of bile or the increase 

 may be due to secretion (see later). During the periods 

 between digestion the bile is stored in the gall-bladder and 

 during digestion it is forced into the intestine by the con- 

 traction of the muscular walls of the gall-bladder. The 

 outpouring is regulated by a local nervous connection between 

 the stomach and the gall-bladder. 



Pancreatic Juice. Digestion in the duodenum is brought 

 about by three enzymes furnished by the pancreatic juice, 

 trypsin, amylopsin (pancreatic amylase) and lipase. 



The first of these is a proteoclastic enzyme which presents 

 certain differences from pepsin. These differences are that it 

 acts in an alkaline instead of an acid medium and that the 

 hydrolysis proceeds further, giving rise more rapidly than does 

 pepsin, to polypeptides and amino acids. The second of these 

 differences may be due to the presence of a second enzyme, 

 erepsin. The scheme given on p. 120 shows the steps of the 

 hydrolysis, the metaprotein being the form that exists in 

 alkaline solution and the two last stages are more abundant 

 than with pepsin. The relation of the alkali to the activity 

 of trypsin is described on p. 63. 



Trypsin is not found as such in the pancreatic juice. It 

 exists in an inactive form called trypsinogen. Whenever the 

 pancreatic juice comes into contact with the mucous membrane 

 of the duodenum the trypsinogen is converted into trypsin. 

 The activation of trypsinogen is brought about by enterokinase 

 which is an enzyme* formed by the mucous membrane of the 

 duodenum. 



Trypsinogen can be converted into trypsin by calcium salts. f 

 This second method of activation is stated to be due to traces 



* W. M. Bayliss and E. H. Starling, Journ. PhysioL, 1905, vol. 32, 

 p. 129. 



f C. Delezenne, Compt. Rend., 1905, vol. 141, p. 781. 



