CHAP. II.] DIFFERENCES BETWEEN PEPSINOGEN AND PEPSIN. 103 



alkaline salt, the time during wKich it is allowed to act, the tempera- 

 ture of the mixture, and the amount of proteids present. The mere 

 act of neutralising an acid pepsin solution may destroy a considerable 

 part of the pepsin. When equal volumes of a fluid containing pepsin 

 or 1 per cent, solution of sodium carbonate are well mixed, the 

 greater part of the pepsin is destroyed in fifteen seconds; in a 

 neutralised acid extract of the gastric mucous membrane of a cat 

 the amount thus destroyed may be nearly 97 per cent, of the whole. 

 Even very dilute sodium carbonate (0'005 per cent.) will cause an 

 appreciable destruction of pepsin in one or two hours at the body 

 temperature, provided proteids are present in small amount only. 



Proteids lessen the rate of destruction of pepsin, probably by 

 combining with the alkali or alkaline salt. 



Pepsin prepared from a frog is less rapidly destroyed than pepsin 

 prepared from a mammal. 



The difference between pepsinogen and pepsin on their be- 

 haviour to reagents is one of degree only, and not one of kind. 

 Pepsinogen, like pepsin, is destroyed by alkalies and alkaline salts, 

 but the destruction is much slower. Pepsinogen is very rapidly con- 

 verted into pepsin by dilute mineral acids ; at 20 C. all or nearly all 

 the pepsinogen present in an aqueous extract of a cat's gastric 

 mucous membrane, may be converted into pepsin in 60 seconds by 

 01 per cent, of HC1. In the absence of acid, pepsinogen is fairly 

 stable ; in neutral and in alkaline solutions its conversion is slow, 

 and in a glycerin extract it may remain unchanged for years. Pep- 

 sinogen is not affected by a stream of oxygen passed through it. 



Since the aqueous extract of the gastric mucous membrane of a 

 hungry animal does not lose peptic power, or loses very little, on brief 

 treatment with sodium carbonate, it follows that pepsinogen, but 

 little or no pepsin, is present in the gastric glands during abstinence. 



In consequence of the rapidity of conversion of pepsinogen into 

 pepsin, it is difficult to be certain whether pepsin is or is not present 

 in the gastric glands during digestion and after the injection of 

 peptone into the blood. In both cases, acid gastric juice is present 

 in the stomach and it is probable, since the glands are secreting at 

 the moment of death, that a little acid remains in the lumina of the 

 glands and, before it can be neutralised, soaks into the gland-cells 

 and changes some pepsinogen to pepsin. In fact, pepsin is sometimes 

 present in an extract prepared from the gastric glands of a digesting 

 animal, but it is not always so. 



Carbonic acid when passed through an aqueous extract of a frog's 

 oesophageal glands for about an hour destroys nearly the whole of 

 the pepsinogen. Certain salts increase the rate of destruction, whilst 

 peptone greatly delays the action, and albumin and globulin likewise 

 do so, though to a much less extent. Carbonic acid destroys pepsin 

 also, but less readily than pepsinogen. 



Both pepsinogen and pepsin are rapidly destroyed when heated 

 to 55 C 57 C. 



