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 Northrop [1932, 2] has shown that trypsin can similarly be obtained in a crystalline form and has the properties of a protein, whilst Sumner [1926] claims that the crystalline urease prepared by him is also a protein. Work has been proceeding in this Department for a number of years with a view to determining the mechanism of the process of the denaturation of proteins. In this connection the conclusions mentioned above are of great importance, for if the processes of inactivation and denaturation of the crystalline enzymes do go hand in hand, then an investigation of the heat-inactivation of the enzymes might yield valuable information regarding the mechanism of the process of denaturation, especially as the rate of inactivation can be determined experimentally with great accuracy by a number of different methods. Consequently an investigation of the heat-inactivation of crystalline pepsin as prepared by Northrop was undertaken, and the results obtained are given in the present communication. EXPERIMENTAL.