In the presence of iodine at pH 5.0–6.0 a solution of pepsin absorbs iodine and the specific proteolytic activity of the solution decreases. The activity is less than 1 per cent of the original activity when the number of iodine atoms per mol of pepsin is 35–40.

If the pH is 4.5 or less, iodine reacts very slowly and there is a correspondingly slower loss in activity. Glycyl tyrosine reacts with iodine in a manner similar to pepsin.

Experiments were performed to determine the extent to which oxidation of pepsin by iodine occurs during iodination, and if such oxidation were responsible for the loss in enzymatic activity. Although the results were not absolutely decisive, there seems to be no appreciable oxidation taking place during iodination and no relationship between the slight oxidation and loss in peptic activity.

From a dialyzed preparation of completely iodinated pepsin which was inactive and contained 13.4 per cent bound iodine, 82 per cent of the iodine was obtained in a solution which analyzed as a solution of diiodo-tyrosine. Because of the presence of a material which contained no iodine and prevented quantitative crystallization, only 53 per cent of the iodine containing substance could be crystallized. This 53 per cent was, however, identified as diiodo-tyrosine.

The part of the titration curve which in pepsin and most proteins represents the phenolic group of tyrosine was, in the curve for iodinated pepsin, shifted toward the acid region as expected.

From these results, it appears that the loss in proteolytic activity of pepsin, when treated with iodine under the specified conditions, is due to the reaction of the iodine with the tyrosine in pepsin.

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