126 THE DIGESTIVE FLUIDS. 



A solution of propepsin, which may bo obtained by numerating in 

 glycerin the mucous membrane of a fasting animal, is thus in itself 

 inert, but is rendered active at once when hydrochloric acid is added 

 to the extent of from 0.1 to 0.2 per cent. It is indeed supposed 

 that pepsin, which in itself is inactive like its /yinogen, combines 

 with hydrochloric acid, which alone is similarly inert, as regards its 

 digestive ability, to form a compound acid, the so-called pej)nin- 

 hydroohhrio acid. On coming in contact with albuminous mate- 

 rial this is supposedly decomposed, with the formation of nascent 

 hydrochloric acid, which then acts as the active digestive principle, 

 while the liberated pepsin combines with a new portion of hydro- 

 chloric acid, and thus serves as an acid-carrier. On this question, 

 however, a uniformity of opinion does not exist ; still, the hypoth- 

 esis is an attractive one, and has a good deal in its favor. If we 

 thus regard the action of a ferment as essentially influencing the 

 rapidity of reaction, the action of the weak hydrochloric acid of 

 the gastric juice could be compared to the effect of stronger solutions 

 upon albumins under the application of heat. 



Pepsin. In pure form pepsin has thus far not been obtained. In 

 the form we are able to isolate the substance, it occurs as an amor- 

 phous white or yellowish-white powder \vhich is not hygroscopic. 

 It is soluble in water, dilute acids, and glycerin. When acidified 

 with hydrochloric acid to the extent of from 0.1 to 0.3 per cent., it 

 is capable of dissolving albumins, with the formation of albumoses 

 and so-called amphopeptone (see below). This can readily be 

 demonstrated as follows : an artificial gastric juice is prepared by 

 dissolving a pinch of one of the commercial preparations of 

 pepsin in dilute hydrochloric acid (0.1-0.2 per cent.), to which a 

 flake of boiled beef-fibrin is then added. The mixture is kept at a 

 temperature of about 40 C., when it will be noted that after a short 

 time the fibrin begins to swell and is subsequently dissolved. In the 

 solution which thus results albumoses and peptones can be demon- 

 strated (see page 183). Other acids, such as sulphuric acid, nitric 

 acid, phosphoric acid, lactic acid, and even acetic acid, are also cap- 

 able of rendering pepsin physiologically active, but much larger 

 amounts of these are necessary to bring about the same result. In 

 the case of phosphoric acid, for example, an acidity of 1012 pro 

 mille is necessary. Carbonic acid and hydrocyanic acid, on the 

 other hand, are without effect. Unlike chymosin, pepsin does not 

 bring about coagulation of casein. 



In neutral and alkaline solutions pepsin is inactive, and, as has 

 been seen, it is rapidly destroyed by sodium carbonate, even in very 

 small amount. *Its resistance to higher temperatures is to a great 

 extent dependent upon the reaction of its solutions. In neutral 

 solution it is destroyed at 55 C. ; in the presence of 0.2 per cent, 

 of hydrochloric acid this result is reached only at 65 C., and in the 

 presence of peptones and certain salts a temperature of 70 C. is 

 necessary to bring about the same end. In the dry state, on the 



