610 EFFECT OF ACIDS ON DIGESTION OF PROTEINS 



and not on the enzyme. These experiments also show that the physi- 

 cal properties of the solution, such as viscosity, have little or no 

 effect on the rate of digestion since Loeb has shown that there is a 

 striking difference in the viscosity of gelatin in sulfuric as contrasted 

 with hydrochloric acid solution. This is still more strikingly shown 

 in the experiments with edestin, which is practically insoluble in 

 sulfuric acid and yet digests under these conditions at the same rate 

 as when dissolved in hydrochloric acid. The simplest explanation of 

 these results would seem to be that the rate of digestion of the pro- 

 tein is determined by th'C amount of acid protein salt formed. As 

 Loeb^'^ has shown, the physical properties of a gelatin solution are 

 also functions of this same quantity. 

 f 



II. EXPERIMENTAL. 



Method of Keeping Samples. — Samples were withdrawn from the 

 bottles containing the digestion mixtures at intervals of 4 and 24 

 hours and placed in iced bottles containing three drops of saturated 

 ferric chloride solution. They were kept at 2-4°C. until analyzed. 

 No change could be noted in the amino nitrogen content in 24 hours 

 under these conditions. 



Temperature. — Some of the experiments were conducted at 37° ± 0.1 , 

 and some at 35° ±0.1. 



Analysis. — The technique was the same as described by Van Slyke 

 except that 10 cc. of solution were analyzed in a large size apparatus 

 and "the gas was measured in a small (3 cc.) burette. This was 

 necessary owing to the small amount of amino nitrogen present. 

 The protein solution was allowed to stand 15 minutes in contact 

 with the nitrous acid and then shaken rapidly for 5 minutes. The 

 reaction was complete under these conditions. 



Pepsin Solution A. — 30 gm. of Fairchilds pepsin were dissolved in 

 500 cc. of water and allowed to digest at 37° for 24 hours. The solu- 

 tion was dialyzed and filtered, and made up to 3 liters. It contained 

 0.1 cc. of amino nitrogen per 10 cc. This quantity did not further in- 

 crease digestion in 24 hours and therefore does not enter into the 

 figures obtained for the increase of amino nitrogen in the solutions. 



