212 HYDROGEN ION CONCENTRATION AND PEPSIN DIGESTION 



theory. Pekelharing and Ringer,^ however, showed that in solutions 

 of pure pepsin (prepared by Pekelharing's method from gastric 

 juice) the pepsin was always negatively charged. This objection 

 may of course be met by the statement that the pepsin under the 

 actual conditions of hydrolysis {i.e. when in the protein solution) 

 is not pure but is combined with some other substance and it is the 

 ionization of this compound which determines the activity of the 

 enzyme. An explanation similar to this has been offered by Michaelis.® 

 The author'' has shown, however, that pepsin combined with peptone 

 or other decomposition products of the proteins is inactive and that 

 it is only the free pepsin which takes part in the reaction. It was 

 also found^ that no positively charged pepsin could be found on the 

 alkaline side of pH 3.3. Pepsin retains its activity up to pH 5, 

 however, so that it seems unhkely that only positively charged pepsin 

 is active, as assumed by Michaehs. 



A second objection to Michaelis' view is the fact that the optimum 

 hydrogen ion concentration for the activity of pepsin is found to 

 vary with the substrate. This point has been emphasized by Long 

 and HulP (for trypsin) and by Ringer. i° From Michaelis' point of 

 view it is difficult to see how this can be. Neither of these objections, 

 however, can in the author's opinion be considered as conclusive evi- 

 dence against Michaehs' hypothesis. It could be stated for instance 

 that pepsin contained several enzymes, one for each substrate and 

 each with a different optimum. It seems simpler, however, to assume 

 that the hydrogen ion concentration afTects the condition of the sub- 

 strate rather than the enzyme. This hypothesis has the advantage 

 that it also accounts for the peculiar relation between the concen- 



^ Pekelharing, C. A., and Ringer, W. E., Z. physiol. Chem., 1911, Ixxv, 282. 



'^ Michaelis, L., Deutsch. med. Woch., 1920, xlvi, 1. 



^Northrop, J. H., /. Gen. Physiol., 1919-20, ii, 471. 



« Northrop, J. H., /. Gen. Physiol, 1919-20, ii, 468. 



^Long, J. H., and Hull, M., J. Am. Chem. Soc, 1917, xxxix, 1051. The same 

 statement is made by Hedin and Hammerstein. The author has been unable to 

 find the original work on which this statement is based. Cf. Hammerstein, O., 

 and Hedin, S. G., A text-book of physiological chemistry, translated by Mandel, 

 J. A., New York, 8th edition, 1915, 471. See also Abderhalden, E., and Fodor. 

 A., Fermentjorsch., 1914-16, i, 591. 



10 Ringer, W. E., Kolloid. Z., 1916, xix, 253. 



