444 DIGESTION. 



only by the degree of acidity, but by the duration of heating and also by 

 the amount of other bodies in the solution. If an acid (0.2 per cent HC1) 

 infusion of the calf's stomach be warmed for several days to about 40 

 or 45 C., a part of the pepsin is destroyed, but we obtain in this manner 

 an infusion which still dissolves proteins but has no rennin action (HAM- 

 MARSTEN x ) . The pepsin from different animals acts differently in this regard 

 and the pepsin of the pike stomach is very quickly destroyed at 37-40 C. 



Pepsin is extraordinarily sensitive to the action of alkalies, not only 

 caustic, and carbonated, but also against the hydroxides of the alka- 

 line earths. It is easily made inactive by these substances. If the 

 action of the alkali is not too strong then, as shown by PAWLOW and 

 TiCHOMiRow, 2 the enzyme can in part be reactivated by the addition of 

 acid if the greater part (about four-fifths) , of the alkalinity be neutralized 

 by the addition of acid and then after some hours more acid be added. If 

 the entire quantity of acid be added at one time the reaction does not 

 take place. 



The only property which is characteristic of pepsin is that it dissolves 

 protein bodies in acid but not in neutral or alkaline solutions, with the 

 formation of proteoses, peptones, and other products. 



The methods for the preparation of relatively pure pepsin depend 9 

 as a rule, upon its property of being thrown down with finely divided 

 precipitates of other bodies, such as calcium phosphate or cholesterin. 

 The rather complicated methods of BRUCKE and SUNDBERG are based 

 upon this property. PEKELHARING makes use of a prolonged dialysis 

 and precipitation with 0.2 p. m. HC1. 



Very permanent pepsin solutions, from which the enzyme with con- 

 siderable protein can be precipitated by alcohol, may be prepared by 

 extraction with glycerin. Solutions having a strong action may also 

 be prepared by making an infusion of the gastric mucosa of an animal 

 in acidified water (2-5 p. m. HCJ). This is unnecessary, as we can obtain 

 pure gastric juice according to PAWLOW'S method, and also because very 

 active commercial preparations of pepsin can be bought in the market. 



The Action of Pepsin on Proteins. Pepsin is inactive in neutral or 

 alkaline reactions, but in acid liquids it dissolves coagulated protein 

 bodies. The protein always swells and becomes transparent before 

 it dissolves. Unboiled fibrin swells up in a solution containing 1 p. m. 

 HC1, forming a gelatinous mass, and does not dissolve at ordinary tem- 

 perature within a couple of days. Upon the addition of a little pepsin, 

 however, this swollen mass dissolves quickly at ordinary temperatures. 

 Hard-boiled egg albumin, cut in thin pieces with sharp edges, is not per- 

 ceptibly changed by dilute acid (2-4 p. m. HC1) at the temperature of 

 the body in the course of several hours. But the simultaneous presence 



1 Zeitschr. f . physiol. Chem., 56. 2 Ibid., 55. 



