THE CHEMISTRY OF DIGESTION 



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Proteoses and peptones are classified in accordance with their physical character- 

 istics, such as their solubility and salting out. In the following table a native 

 protein albumin is contrasted in this respect with its peptic end-products : 



Action of 



Upon the constituents of connective tissue and other allied protein substances, 

 the pepsin-hydrochloric acid combination acts as follows: 



(a) Collagen, a constituent of bone and white fibrous and areolar tissue, is 

 converted into gelatin, gelatoses and gelatin-peptones. Since these tissues con- 

 tain much fat, this foodstuff is separated from its investments. 



(6) Elastin, a constituent of elastic tissue, is not acted upon under ordinary 

 conditions. 



(c) Mucin, a constituent of the ground-substance of connective tissue, is con- 

 verted into peptone-like substances. 



(d) Nucleo-proteins are changed into a protein portion and a nuclein portion. 

 The former is then converted into proteoses and peptones, whereas the latter is 

 precipitated in an insoluble form. On phospho-proteins it acts in a somewhat simi- 

 lar manner. 



The Function of Pancreatic Juice. — This secretion plays an even 

 more important part in digestion than the gastric juice, because it 

 contains several powerful enzymes. Its function may be summarized 

 as follows: 



(a) Proteolytic. — This property is imparted to it by its enzyme trypsin which 

 differs materially from pepsin, because it gives rise to a more rapid as well as more 

 thorough catalysis. To begin with, it is to be noted that trypsin acts in an alka- 

 line medium, whereas pepsin acts in an acid medium. Moreover, while the former 

 produces the same initial conversions of the protein molecule as the latter, it does 

 not stop here but reduces the peptones still further into their constituent amino- 

 acids, such as leucine, tyrosine, alanine, aspartic acid, glutamic acid, arginine, 

 tryptophane, and others. It is also to be observed that this conversion is effected 

 so rapidly that the formation of the primary proteoses can scarcely be detected, 

 while the secondary derivatives come into prominence almost immediately. In 

 place of acid-metaprotein, however, we now obtain alkaline-metaprotein. In 

 addition, a reduction of elastin takes place which is not effected at all by the gastric 



