JOHN H. NORTHROP 



215 



hemoglobin, however, is isoelectric at a pH of about 6.8 (Michaelis^) 

 so that it must be quite largely present as a salt and therefore ionized 

 at a pH of 4.5. It would be predicted then, according to the hy- 

 pothesis that the amount of protein ions present determines the rate 

 of digestion of the protein, that hemoglobin should be digested 

 by pepsin at pH 4.5 more rapidly than is egg albumin or gelatin at 

 the same pH. 



12 

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 8 

 6 

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6 



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pH6 



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cc. 



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 0.5 

 04 

 0.3 

 0.2 

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 -0.5 

 -0.4 

 -0.3 

 -0.2 

 -0.1 



pH 



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Fig. 1. Influence of pH on conductivity and rate of digestion of egg albumin 

 and oxyhemoglobin solutions. 



In order to test this prediction, parallel experiments were made to 

 determine the rate of digestion and the conductivity of hemoglobin 

 and egg albumin solutions at various hydrogen ion concentrations. 

 The results of such an experiment are shown graphically in Fig. 1. 

 It is clear that the conductivity and digestion curves, for each protein, 

 as plotted against the pH of the solution are approximately parallel 



