ACTIVATORS, KINASES AND HORMONES 185 



process of activation is an instance of chemiccl coordina- 

 tion. The pepsin formed in one kind of gland cell is acti- 

 vated by the acid produced in a different variety of cell. 

 The hydrochloric acid produced in the stomach is carried 

 into the intestin with the flow of chyme and there activates 

 the prosecretin of the intestinal epithelium either directly 

 or indirectly. One tissue, in other words, through its 

 products of metabolism aids another tissue in the perform- 

 ance of its functional duties. 



The term kinase is used at present in animal physiology 

 in connection with two reactions only. In both cases it 

 refers to an activating process similar to those just con- 

 sidered, except that the activator is a colloidal substance 

 of unknown composition. The pancreatic juice poured 

 into the duodenum contains its proteolytic enzyme in the 

 form of a trypsinogen which is activated immediately by 

 trypsin by contact with the duodenal epithelium or with 

 the secretion furnished by this epithelium. The activating 

 substance is designated as enterokinase. It is present 

 normally in the intestinal juice formed in this part of 

 the alimentary canal, or it may be obtained in extracts 

 of the mucous membrane of the duodenum or jejunum. 

 According to Pawlow, however, the intestinal secretion ob- 

 tained by direct mechanical stimulation of the epithelium 

 is lacking in enterokinase. This latter substance is pro- 

 duced in fact only under the influence of some constituent 

 of the pancreatic juice, possibly the trypsinogen itself. In 

 other words, it would seem that the enterokinase must itself 

 be activated before it can fulfil its functions as an activator 

 of the trypsinogen. The chain of inter-related processes 

 occurring at this point in the act of digestion becomes 

 somewhat intricate, as follows : Hydrochloric acid formed 

 in the stomach and brought into the intestin with the 

 chyme stimulates the epithelial cells of the intestin to 

 form secretin and to pass it into the blood. The secretin 

 conveyed by the blood to the pancreas stimulates this organ 

 to secrete pancreatic juice. The pancreatic juice is carried 



