704 PHYSIOLOGY 



ammo-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 consequence 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 ammo-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 poly- 

 peptides 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 metaprotein 

 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 hydrochloric 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 such molecules as are present in the 

 naturally occurring proteins. Thus under the action of trypsin the following 

 polypeptides undergo hydrolytic dissociation : alanyl glycine, alanyl alanine, 

 alanyl leucine A ; while the closely similar polypeptides glycyl alanine, 

 glycyl glycine, alanyl leucine B are left untouched. 



CONDITIONS OF TRYPTIC ACTIVITY 



Since the pancreatic juice is strongly alkaline it might be expected that 

 trypsin would be most effective in an alkaline medium. It must be remem- 

 bered, however, that the alkaline juice when secreted meets the correspond- 

 ingly acid contents discharged from the stomach, and that the resulting 

 mixture is practically neutral. This neutrality exists throughout the small 

 intestine, the reaction of the contents of the gut being similar to that of a 

 fluid containing alkali which has been saturated by the passage of carbonic 



