10] AUTODIGESTION OF PEPSIN 181 



that of the parent substance. If the activity is expressed per unit tyrosine, 

 the increase is 80 to 100 per cent. The hydrolysis of hemoglobin^ and 

 of the synthetic substrate acetyl-L-phenylalanyl-diiodotyrosine^ are affected 

 to the same extent. Preliminary amino acid analyses, kindly carried out by 

 Dr D. Spackman, revealed differences not only in the tyrosine content but 

 also in that of several other amino acids. 



60 



50 

 40 

 30 

 20 



10- 



' Activity 



> Protein (ToJin) 



2.0 4.0 6.0 8.0 10.0 12.0 

 Cm. from origin 



Fig. 2. Zone electrophoresis on Geon resin of pepsin treated with 8 0m urea at 37°C 

 for 24 hours. Electrophoresis was carried out in acetate buffer, pH 46 and O-l ionic 

 strength after predialysis in glycine-HCl, pH 2-5, 01 ionic strength. 



The present observations consist of a few experiments only, and it is not 

 possible to state whether the urea modified pepsin represents a mixture of 

 molecules which differ from each other by small changes in their amino 

 acid composition brought about by autodigestion or whether the original 

 pepsin molecules have been broken down to smaller units which reaggregate 

 to form a complex of a molecular weight of 30,000 to 35,000. 



From the results presented here it can be concluded that the apparent 

 loss of the enzymic activity of pepsin after treatment with urea is caused 

 by an enhanced autodigestion. At any given stage, however, during the 

 inactivation enzymically active modifications of the parent enzyme exist 

 which differ in some of their properties. Although an enhancement of 

 activity has been observed it has not yet been possible to change the speci- 

 ficity of the enzyme. 



REFERENCES 



1. G. E. PERLMANN, Advances in Protein Chem., 10, 1 (1955). 



2. J. STEINHARDT, J. Biol. Chem., 123, 453 (1938). 



3. G. E. PERLMANN, Arch. Biochem. Biophys., 65, 210 (1956). 



4. M. L. ANSON in Crystalline Enzymes (J. H. Northrop, M. Kunitz and R. M. 



Herriott, eds.), p. 305, Columbia University Press, New York, 1948. 



5. L. E. BAKER,/. Biol. Chem., 193, 809 (1951). 



