352 FOOD AND DIGESTION 



All classes of proteins are digested by gastric juice, leading to the produc- 

 tion of proteoses and peptones. The change is indicated best by the charac- 

 ters of the new protein formed. Peptones have certain characteristics which 

 distinguish them from other proteins. They are divisible; i.e., they possess 

 the property of passing through animal membranes. In their diffusibility 

 peptones differ remarkably from egg albumin, and on this diffusibility depends 

 one of their chief uses. Egg albumin as such, even in a state of solution, 

 would be of little service as food, inasmuch as its diffusibility renders difficult 

 its absorption or in the case of insoluble proteins effectually prevents absorp- 

 tion into the blood vessels of the digestive canal. When completely changed 

 by the action of the gastric juice into peptones, albuminous matters diffuse 

 readily, and can be then absorbed. Peptones, however, are not found in 

 the blood, even of the vessels immediately concerned in absorption from 

 the stomach and intestines. As will be shown, the proteins are broken 

 down into their simpler cleavage products in the intestine. 



Products at Different Stages of Gastric Digestion. The protein 

 is first changed into syntonin, or acid protein, by the combined action of the 

 pepsin and acid. Though the acid alone is capable of accomplishing this 

 step, the fact that it does not do so physiologically is proven by the great 

 length of time required in laboratory experiments for the change. The 

 acid is absolutely essential to the action of pepsin. 



The next change is the conversion of the syntonin into proteoses which, 

 according to Neumeister, occurs in two successive stages. The first of these 

 stages is the conversion of syntonin into the primary proteoses; i. e., proto- 

 proteose and hetero-proteose. The second is the conversion of both proto- 

 proteose and hetero-proteose into the secondary proteoses; i.e., deutero- 

 proteose. The last change is the conversion of the deutero-proteose into the 

 end product peptone. This last change does not occur to any great extent 

 and the proteoses always predominate in the digesting mass. The action 

 of pepsin is one of hydrolysis and the products are hydrated forms of protein. 

 Schematically the changes in the proteins may be represented as follows: 



Protein 

 Syntonin (acid protein). 



Proto-proteose. Hetero-proteose. 



Deutero-proteose. Deutero-proteose. 



Peptone. Peptone. 



Circumstances Influencing Gastric Digestion. A temperature of 

 about 40 C. is most favorable to gastric digestion. The pepsin is destroyed 



