120 PHYSIOLOGICAL CHEMISTRY. 



established fact, but farther than this no positive statement can be 

 made. 



The most important of the enzymes of the gastric juice is the 

 proteolytic enzyme pepsin. The pepsin does not originate as such 

 in the gastric cells but is formed from its precursor the zymo- 

 gen or mother-substance pepsinogen which is produced by the 

 parietal cells of the fundus as well as by the chief cells of the fun- 

 dus and pyloric glands. Upon coming in contact with the hy- 

 drochloric acid of the secretion this pepsinogen is immediately 

 transformed into pepsin. Pepsin is not active in alkaline or neu- 

 tral solutions but requires at least a faint acidity before it can 

 exert its power to dissolve and digest proteins. The percentage 

 of hydrochloric acid facilitating the most rapid peptic action varies 

 with the character of the protein acted upon, e. g., 0.08 per cent to 

 o.i per cent for the digestion of fibrin and 0.25 per cent for the di- 

 gestion of coagulated egg-white. While hydrochloric acid is the 

 acid usually employed to promote artificial peptic proteolysis, other 

 acids, organic and inorganic, will serve the same purpose. Acidity 

 of the liquid is necessary to promote the activity of the pepsin, 

 but the acidity need not necessarily be confined to hydrochloric 

 acid. 



In common with many other enzymes pepsin acts best at about 

 38-40 C. and its digestive power decreases as the temperature 

 is lowered, the enzyme being only slightly active at o C. Its 

 power is only temporarily inhibited by the application of such 

 low temperatures, however, and the enzyme regains its full proteo- 

 lytic power upon raising the temperature to 40 C. As the tempera- 

 ture of a digestive mixture is raised above 40 C. the pepsin grad- 

 ually loses its activity until at about 8o-ioo C. its proteolytic 

 power is permanently destroyed. 



Our ideas regarding the nature of the products formed in the 

 course of peptic proteolysis have undergone considerable revision 

 in recent years. The former view that these products included 

 only acid albuminate (acid metaprotein), proteoses and peptones 

 is no longer tenable. From the investigations of numerous ob- 

 servers we have learned that artificial gastric digestion if permitted 

 to proceed for a sufficiently long period will yield, in addition to 

 proteoses and peptones, a long list of protein cleavage products 

 which are crystalline in character, including leucine, tyrosine, alanine, 

 phenylalanine , aspartic acid, glutamic acid, proline, leucinimide, 

 valine and lysine. A similar group of substances may result from 



