2O PHYSIOLOGICAL CHEMISTRY. 



containing 10 c.c. of a o.i per cent solution of pure, fat-free, casein, 1 

 which has been heated to a temperature of. 40 C. Add to the 

 contents of the series of tubes increasing amounts of the trypsin 

 solution under examination, 2 and place them at 40 C. for fifteen 

 minutes. At the end of this time remove the tubes and acidify 

 the contents of each with. a few drops of dilute (i per cent) acetic 

 acid. The tubes in which the casein is completely digested will 

 remain clear when acidified while those tubes which contain undi- 

 gested casein will become more or less turbid under these condi- 

 tions. Select the first tube in the series which exhibits no turbidity 

 upon acidification, thus indicating complete digestion of the casein, 

 and calculate the tryptic activity of the enzyme solution under ex- 

 amination. 



Calculation. The unit of tryptic activity is an expression of the 

 power of i c.c. of the fluid under examination exerted for a period 

 of fifteen minutes on 10 c.c. of a o.i per cent casein solution. For 

 example, if 0.5 c.c. of a trypsin solution completely digests 10 c.c. 

 of a o.i per cent solution of casein in fifteen minutes the activity of 

 that solution would be expressed as follows : 



Tryptic activity = I -f- 0.5 = 2. 



Such a trypsin solution would be said to possess an activity of 

 2. If 0.3 c.c. of the trypsin solution had been required the solution 

 would be said to possess an activity of 3.3 i. e. } i -7-0.3 = 3.3. 



1 Made by dissolving one gram of Griibler's casein in a liter of o.i per cent 

 sodium carbonate. A little chloroform may be added to prevent bacterial action. 



2 The amount of solution used may vary from o.i-i c.c. The measurements 

 may conveniently be made by means of a i c.c. graduated pipette. 



