The heat-inactivation of crystalline pepsin; the critical increment of the process

IN a recent series of publications Northrop [1930, 1, 2; 1931] has advanced very strong evidence that pepsin is a protein. Starting from commercial pepsin, by suitably regulating the pH and precipitating with the aid of magnesium or ammonium sulphate a crystalline product was obtained which had the general properties of a protein. In particular, solutions of this crystalline pepsin were coagulated by heat, and Northrop states that the inactivation of the enzyme either by heat or by alkali is quantitatively proportional to the denaturation of the protein. A successful effort was made [1931] to reactivate denatured inactivated pepsin along the lines used by Anson and Mirsky [1929; 1931, 1, 2] for the reversal of the denaturation of certain proteins, and the product obtained was identical with the original freshly prepared material, both in general properties and in its specific proteolytic activity. It seems safe to conclude therefore that the crystalline pepsin is in fact a protein. More recently