120 PHYSIOLOGY OF ALIMENTATION. 



in a way analogous to that described for the action of mal- 

 tase on maltose. 



Acid-proteinase is, like other ferments, very sensitive to 

 temperature. At C. it is scarcely active. From this point 

 to 35 C. the velocity of its action increases progressively, 

 attaining an optimum between 35 and 50 C. Beyond this 

 point its activity diminishes. 1 In a neutral solution the fer- 

 ment is in a few minutes destroyed at 55 C. The presence 

 of hydrochloric acid protects the ferment against destruction 

 by heat, so that when present in the concentration in which 

 the acid is found in the stomach, the ferment is destroyed only 

 slowly until 65 C. is reached. Peptones and certain salts also 

 have a protecting influence. 2 As with other ferments, the 

 concentration of the acid-proteinase itself in a pure solution 

 influences the temperature at which it is most rapidly 

 destroyed. 



When natural or artificial gastric juice (a solution of acid- 

 proteinase in dilute hydrochloric acid) is allowed to act upon 

 a protein, such as fibrin, the following changes are observed. 

 The fibrin begins to swell and its superficial portions become 

 translucent, until, under favorable conditions of temperature, 

 the insoluble protein is rendered soluble. If given time 

 enough, the acid alone will bring about a solution of the fibrin; 

 but if acid-proteinase is present this change occurs much more 

 rapidly. If the enzyme is employed in a neutral solution the 

 protein is not dissolved. Acid and ferment together are 

 therefore necessary to bring about the rapid change from the 

 protein to the soluble product. The soluble product consists, 

 as will be seen presently, of a mixture of a number of chemical 

 substances, which are distinguished from the original protein 

 upon which the acid-proteinase acted by their ready solubility 

 in water and their ready diffusibility through animal and 

 vegetable membranes. These properties, it will be noticed, 



v.WrrriCH: PFLUGER'S Arch., 1869, II, p. 193; ibid., 1870, III, 

 p. 339. 

 BIERNACKI. Zeitechr. fur Biol., 1892, XXVIII, p. 453, 



