790 PHYSIOLOGY 



activity ^superior to that of any other 'digestive juice, so that the 

 proteins of the food undergo a very thorough disintegration. The 

 different constituents of the protein molecule show a varying 

 resistance to the action of trypsin. The greater part of the molecule 

 is rapidly broken down into its proximate constituents, namely, 

 amino-acids, and the same change is undergone by the albumoses 

 and peptones resulting from the gastric digestion of proteins. Within 

 a few minutes therefore after the chyme has reached the small 

 intestine a certain amount of amino-acids will have been formed. 

 Some of the groups present, however, a resistance to disintegration. 

 After tryptic digestion for a few hours the mixture will be found 

 still to contain a considerable quantity of peptone, which in con- 

 sequence of its resistance to further alteration was designated by 

 Kiihne ' antipeptone.' The antipeptone of Kiihne certainly included 

 some of the diamino-acids, which at that time had not been isolated. 

 There is always a part, however, which gives the biuret reaction 

 and is only slightly broken down after the prolonged action of 

 trypsin into the amino-acids. Even when the trypsin has acted 

 for weeks and the biuret reaction has entirely disappeared, the mixture 

 will be found to contain, in addition to the separate amino-acids, 

 some members of the polypeptide class, composed of two or more 

 molecules of amino-acid united together. One of these polypeptides 

 has been isolated by Fisher and Abderhalden from the products 

 of tryptic digestion of the protein of silk, and has been found to 

 contain glycine, alanine, and proline. The stages therefore in tryptic 

 digestion, e.g. on fibrin, may be set out as follows : 



(1) After one hour's digestion soluble coagulable protein, deutero- 

 albumose, peptone, amino-acids, with a small amount of alkali albumen 

 produced by the action of the alkali of the juice. 



(2) After digestion for one day deutero-albumose, ' antipeptone,' 

 amino-acids, polypeptides. 



(3) After digestion for one month amino-acids, polypeptides. 

 Among the amino-acids tyrosine is one of the first to be split 



off, and this substance, with leucine, was among the earliest known 

 products of pancreatic digestion. The action of trypsin is thus seen 

 to resemble very closely the action of boiling concentrated hydro- 

 chloric acid. Like the latter it attacks the protein molecule at 

 the CO NH coupling, introducing water at this point and 

 therefore breaking up the polypeptide groupings into simple amino- 

 acids. Why it always leaves a certain remnant of the polypeptides 

 unattacked is not at present explained. The investigation of its 

 action on the polypeptides has shown that very minute differences 

 in the grouping of the molecule may determine whether or not the 

 molecule is attacked by trypsin. Apparently it will only attack 



