THE GASTRIC JUICE. 127 



other hand, the ferment may be heated to 100 C., and even higher, 

 without being destroyed. At temperatures lower than 40 C. pepsin 

 is still active, but less energetically so, and at C. its action ceases 

 altogether. 



Alcohol precipitates pepsin from its solutions without affecting its 

 subsequent activity, unless the exposure has been prolonged. Some 

 of the salts of the heavy metals, such as the acetates of lead and 

 platinum chloride, as also tannic acid, magnesium carbonate, and 

 ammonium sulphate, likewise cause the precipitation of impure 

 forms, at least, but are without effect upon the ferment itself. Like 

 tho albumins, pepsin does not diffuse through animal membranes. 



To a certain extent the rapidity of digestion is dependent upon 

 the amount of pepsin that is available ; but, as in the case of all 

 ferments, very small quantities are sufficient to effect an amount of 

 chemical change that is apparently out of all proportion to the 

 amount present. Thus, Petit claims that a pepsin preparation, 

 which he prepared himself, was capable of dissolving 500,000 times 

 its weight of fibrin in seven hours. The much more impure 

 commericial forms are, of course, far less active, but many of them 

 possess remarkable digestive power. 



The ability on the part of pepsin to digest albumins is, however, 

 limited ; and with an increase in the amount of digestive products 

 formed, its activity gradually diminishes and finally ceases. This 

 can be obviated in a measure by removing these products as they 

 are formed, and may be artificially accomplished by allowing the 

 digestion to take place in a parchment tube which has been sus- 

 pended in dilute hydrochloric acid. The peptones which are formed 

 then pass from the tube by dialysis, and in this manner digestion 

 can be carried much further than under other conditions. Com- 

 plete digestion, however, may even then not be achieved, which 

 is probably to be explained by the reversible action of the fer- 

 ment, as already described (page 111). 



Little is known of the chemical nature of pepsin. At first 

 sight, it is apparently related to the albumins ; but it is to be noted 

 that the reactions of the purer forms become further and further re- 

 moved from those of the albumins as the degree of purity increases. 



An analysis of a fairly pure specimen has given the following 

 result: C, 47.75; H, 6.5; N, 14.24; O, 30.20; S, 1.31 per cent. 



Specific tests for the demonstration of the pepsin of the gastric 

 juice, as compared with other proteolytic ferments which similarly 

 act in acid solutions, are unknown. As a result, all such ferments 

 have been designated as pepsin, although it is very likely that they 

 are not identical. Such ferments have been observed in the secre- 

 tion of the glands of Brunner, in the muscles, the kidneys, the 

 brain, the saliva, and the urine. 



Isolation of Pepsin. If it is merely desired to obtain an effective 

 solution of pepsin without regard to the purity of the substance, 

 the following procedure may be employed : 



