35> 2 DIGESTION 



liquid before digestion has proceeded very far, we shall find chiefly 

 so-called acid -albumin in solution; later on, chiefly albumoses; and of 

 these some authors distinguish the primary albumoses (proto -albumose 

 and hetero-albumose), the first to appear in quantity, followed by 

 secondary or deutero-albumoses (p. 10).* Still later, peptones in large 

 and always relatively increasing amounts will be present along with 

 the albumoses. From this we conclude that acid-albumin is a stage 

 in the conversion of fibrin into albumose, and albumose a half-way 

 house between acid -albumin and peptone. It must not be supposed, 

 however, that all the protein is first changed into acid-albumin before 

 any of the acid-albumin is changed into albumose, or that all the 

 protein has already reached the albumose stage before peptone begins 

 to appear. On the contrary, a certain amount of albumoses and of 

 peptones are present very early in peptic digestion, while the greater 

 part of the original protein is still unaltered. Among the somewhat 

 vaguely characterized group of bodies comprised under the term 

 peptones, there are no doubt decomposition products of the proteins 

 in which the hydrolysis has been carried to different degrees. Similar, 

 but not identical, intermediate substances occur in the digestion of the 

 other proteins, including that of bodies like gelatin, which are not 

 ordinary proteins, but which pepsin can digest. The generic name of 

 proteose properly includes all bodies of the albumose type, the term 

 ' albumose ' itself being sometimes reserved for such intermediate 

 products of the digestion of albumin; while those of fibrin are called 

 fibrinoses; of globulin, globuloses; of casein, caseoses; and so on. The 

 peptones produced from different proteins are also not absolutely 

 identical. If the digestion is prolonged, the peptones first formed are 

 in turn further hydrolysed, so that eventually a considerable proportion 

 of the original protein is converted into bodies which no longer give the 

 biuret reaction. 



In the stomach, during the four or five hours for which gastric 

 digestion ordinarily lasts, none of the protein passes beyond the 

 stage of proteose and peptone, including those relatively simple 

 ' abiuret ' compounds which still consist of several ' building-stones, ' 

 chiefly, it would seem, the amino-acids, phenylalanin, and prolin, 

 linked together. When precautions are taken to prevent the 

 passage of any portion of the contents of the duodenum into the 

 stomach, no amino-acids can be detected in the gastric contents 

 during the digestion of protein. In this connection it is interesting 

 to note that none of the polypeptides hitherto prepared (p. 2) are 

 decomposed by pepsin. It is not known at what points in the link- 

 age of the groups that compose the complex protein molecule the 

 pepsin ruptures the chain, but the points of attack are different from 

 those of trypsin. The pancreatic juice, as we shall see later on, not 



* In the light of modern investigation the results of fractional precipitation 

 by salts of the products of proteolysis have lost a good deal of their interest, 

 and it is seen that undue importance has often been attached to them. The 

 student should be warned that such terms as ' albumose ' and ' peptone ' do 

 not indicate precise chemical differences between the products separated in 

 this way, nor even invariably such differences in molecular weight as the 

 current schemata of the digestive processes are apt to imply. Some so-called 

 ' peptones ' may indeed have a higher molecular weight and be more nearly 

 related to the original protein than some so-called ' albumoses.' 





