I3O PHYSIOLOGICAL CHEMISTRY. 



by extraction with acidulated water or glycerol. In the stom- 

 ach it appears to exist as a propepsin or zymogen, in which 

 condition it is known as pepsinogen. The action of acid con- 

 verts this into the true ferment. Pepsin is very sensitive to 

 the action of alkalies \vhich even in weak solution destroy it 

 or materially lessen its power of dissolving protein. In pres- 

 ence of weak acids, preferably hydrochloric acid of o.i to 0.2 

 per cent strength, it forms from the native or coagulated pro- 

 teins the derived products known as albumoses and peptones. 

 This change is unquestionably associated with the addition of 

 a number of molecules of water to the original protein group. 



Commercial " pepsin " appears in commerce in the form of 

 powder or scales. The latter are obtained by drying the ex- 

 tract from the glands of the stomach on glass plates. The 

 product is far from pure, as it contains a large excess of other 

 extractives. Yet, as now made, one part by weight of the 

 scale or powder is capable of digesting or rendering soluble 

 two to four thousand parts of coagulated albumin in the form 

 of hard-boiled eggs. In an experimental way products of 

 enormously greater activity have been prepared; it is said 

 that one part of a dry pepsin may be made to dissolve three 

 hundred thousand parts of coagulated egg albumin. The rel- 

 ative strengths of pepsin products are always compared by 

 noting the amount of egg albumin or washed fibrin which they 

 will digest in an acid medium of definite concentration. 



Pepsin, like most of the enzymes, is precipitated by alcohol. 

 In aqueous solution with a little acid it is most active at about 

 40 C, and loses its power at about 56. In the dry condi- 

 tion it withstands perfectly a much higher temperature. 

 While hydrochloric acid is usually employed as an aid to pep- 

 sin digestion, other acids may be used with equally good re- 

 sults. Oxalic acid, lactic acid and formic acid work well, but 

 the action with acetic and propionic acids is weak. In presence 

 of alkalies there is no activity and certain salts also interfere 

 with the digestive power. 



Through fractional precipitations and by other means many 



