314 ESSENTIALS OF PHYSIOLOGY. 



enzyme, trypsin, and is converted into the latter by another ferment, 

 enterokinase, produced in the mucous membrane of the small intestine. 

 Trypsinogen may also be converted into trypsin by other means, for 

 example by the action of lime salts ; or the activation will slowly take 

 place spontaneously if the juice is allowed to stand. Activation is, 

 however, effected most rapidly by enterokinase, requiring a few minutes 

 in the case of this ferment as against over twelve hours by means of 

 calcium. 



If a few flakes of fibrin are placed in a solution containing trypsin 

 and 0*5 per cent, sodium carbonate in a test tube, and kept at a 

 temperature of 37 C., the fibrin will begin to be eroded in a few 

 minutes, and gradually it will become dissolved. The products in 

 solution will vary according to the time during which digestion has 

 been allowed to proceed, but generally speaking the course of hydrolysis 

 is the same as in peptic digestion, except that, as the process takes 

 place in an alkaline medium, the metaprotein formed is the alkaline and 

 not the acid variety. A second point of difference is that the inter- 

 mediate stages are passed through more rapidly in pancreatic than in 

 gastric digestion, and thirdly, some amino-acid is produced even in the 

 time available for digestion in the intestine. 



In the normal course of digestion in the intestine, the final con- 

 version of peptone into amino-acids is largely effected by the ferment 

 erepsin, which is contained in the intestinal juice ; but almost complete 

 hydrolysis into amino-acids can be obtained in vitro by means of trypsin, 

 if the digestion is allowed to proceed for three or four days. The 

 splitting is not quite complete, for even if the digestion has been 

 allowed to go on for some weeks some amino-acids remain united in 

 groups of two or more, known as polypeptides. These latter substances 

 have much smaller molecules than peptones and do not give the biuret 

 reaction, that is, they do not give a pink colour with copper sulphate 

 and caustic soda. 



If a tryptic digestion of fibrin or casein has been allowed to proceed 

 for some days, the solution contains the amino-acids derived from these 

 substances (p. 13). Leucine and tyrosineare most easily demonstrated 

 in the fluid, and crystallise out readily if the fluid is concentrated. 

 Tyrosine appears as sheaves of colourless needles, and leucine, which is 

 the more soluble of the two, occurs in the form of yellowish balls, which 

 sometimes show concentric and radial striation. Solutions of tyrosine, 

 when boiled with Millon's reagent, give a red colour. 



The stages of tryptic digestion may be represented in tabular 

 form thus: 



