Jakuakt 21, 1910] 



SCIENCE 



95 



into existence by investigators to explain 

 or to express special reactions connected 

 with metabolism and particularly with the 

 action of ferments. Their precise meaning 

 must be determined by further knowledge 

 of the facts they are intended to describe, 

 but something may be gained by attempt- 

 ing to define them as they are used in 

 physiology at present. The word activa- 

 tor has reference to the fact long known 

 that the ferments, or some of them at least, 

 are secreted in an inactive form, a profer- 

 ment, which is activated or converted to an 

 active form by a reaction with some 

 definite substance produced elsewhere in 

 the body. Pepsin, for example, is secreted as 

 pepsinogen and is activated to pepsin by the 

 hydrochloric acid formed by other gland 

 cells. Calcium salts are necessary for the 

 activation of the prothrombin, and entero- 

 kinase or calcium plays a similar role with 

 reference to the trypsinogen. It is to be 

 noted that reactions of this kind are not 

 confined to the ferments. The typical hor- 

 mone, secretin, exists in the form of an 

 insoluble prosecretin which may be acti- 

 vated by acids, and, according to Dele- 

 zenne, calcium takes an essential part in 

 the activation of enterokinase, in somewhat 

 the same way as occurs with thrombin. 

 The nature of these activating reactions is 

 not known. The view has been proposed 

 that the inorganic constituents involved, 

 the hydrochloric acid and the calcium for 

 example, act as catalyzers which accelerate 

 a reaction that would occur without their 

 assistance. There is, however, no evidence 

 to show that thrombin is formed in any 

 amount in the absence of calcium salts, 

 nor that pepsinogen yields pepsin without 

 the presence of acids. As Bayliss has 

 pointed out, these reactions belong to the 

 irreversible group, and it is possible that 

 the activator or one of its constituents is 

 represented in the composition of the 



active substance that is formed. However 

 that may be, it is to be noted that the 

 process of activation is an instance of 

 chemical coordination. The pepsin formed 

 in one kind of gland cell is activated by 

 the acid produced in a different variety of 

 cell. The hydrochloric acid produced in 

 the stomach is carried into the intestine 

 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 per- 

 formance 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 

 considered, except that the activator is a 

 colloidal substance of unknown composi- 

 tion. The pancreatic juice poured into the 

 duodenum contains its proteolytic enzyme 

 in the form of a trypsinogen which is 

 activated immediately to trypsin by con- 

 tact 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 

 obtained by direct mechanical stimulation 

 of the epithelium is lacking in entero- 

 kinase. This latter substance is produced 

 in fact only under the influence of some 

 constituent of the pancreatic juice, possi- 

 bly the trypsinogen itself. In other words 

 it would seem that the enterokinase must 

 itself be activated before it can fulfill its 

 functions as an activator of the trypsino- 

 gen. The chain of inter-related processes 

 occurring at this point in the act of diges- 

 tion becomes somewhat intricate, as fol- 



