MINERALS IN NUTRITION 



33 



Pepsin and Trypsin. — The activity of proteolytic enzymes was estimated by 

 measuring the increase of soluble non-protein nitrogen in milk after it 

 had been digested with the enzymes. For the study of pepsin, an arti- 

 ficial gastric juice (0.5 percent hydrochloric acid and 1 percent pepsin) was 

 added to the milk. The mixture was incubated at 37° C, with constant 

 shaking. Nitrogen determinations were made at the beginning of the 

 experiment, and after 2, 4, and 6 hours of incubation. The undigested 

 protein nitrogen was precipitated and removed from the mixture by filtra- 

 tion, and the soluble non-protein nitrogen in the filtrate was determined 

 by the official Kjeldahl method. The results are shown in Table 3 as 

 milligrams of non-protein nitrogen per 100 milliliters of milk. 



The same general procedure was followed for trypsin, except that a 2 

 percent commercial trypsin solution was used and the reaction was ad- 

 justed to pH 8.0 to 8.2. Results shown in Table 3 are on the same basis as 

 those for pepsin digestion. 



Comment. — Inorganic iodine inhibited the proteolytic activity of both pepsin 

 and trypsin, and the efifect of organic iodine was less marked. The amount 

 of non-protein nitrogen resulting from tryptic digestion was greater than 

 that from peptic activity. This may be accounted for by the fact that 

 trypsin carries the digestion of protein to completion and pepsin does not. 



Table 3. — Effect of Iodization on Pepsin and Trypsin 



Milligrams non-protein nitrogen 

 in 100 milliliters 



Sample 



Pepsin 



Control, untreated pasteurized 



milk 18.15 



Milk with 100 p.p.m. tincture 



of iodine 18.14 



Milk with 100 p.p.m. organic 

 iodine 18.14 



Trypsin 

 Control, untreated pasteurized 



milk 17.82 



Milk Avith 100 p.p.m. tincture 



of iodine 17.81 



!NIilk with 100 p.p.m. organic 



iodine 17.81 



24.23 

 21.05 

 23.51 



36.70 39.55 



25.25 28.40 



31.74 34.42 



Discussion 



The sensitivity of enzymes to their chemical environment is said to 

 be due to their colloidal nature. A slight change in the reaction of the 

 substrate causes marked efifects on their activities. In the absence of 

 bufifers, for 'instance, proteolytic digestion may cease, whereas in the 

 presence of bufifers, it may continue for some time. In addition to the 

 action of bufTers, the following factors may decrease the velocity of 

 enzyme action: 



