EFRAIM RACKER 



of interaction may actually be far removed) ; and (3) partial 

 inhibition due to interaction with sites removed from the active 

 center, "activating groups" in the sense of Langenbeck (27). 

 The fact that only a limited number of specific blocking agents 

 is available which can be used without leading to protein 

 denaturation has seriously hindered progress in this area of in- 

 vestigation. 



MODIFICATIONS OF PROTEINS BY PARTIAL DIGESTION 



The effect of partial proteolytic digestion on the activity of 

 enzymes has not been extensively explored. It has been apparent 

 for many years that, in some instances, biological activity of 

 proteins may be retained after fragmentation, as was first 

 demonstrated in the case of proteolytic digestion of diphtheria 

 antitoxin (cf. 35). More recently (17), phosphorylase was 

 cleaved into two inactive fragments and the lost activity was 

 restored by addition of adenosine-5-phosphate. After exposure 

 to proteolytic enzymes, TDH lost its activity, which was restored 

 by adding SH compounds (23). ATPase activity of myosin was 

 retained after proteolytic digestion (11). Ribonuclease, in- 

 cubated with carboxypeptidase, contained over 70% of its 

 original activity, although 20% of its nitrogen was split off. 

 Further exposure to carboxypeptidase had no effect (29, cf. also 

 la and 15a). The activation of proenzymes such as chymo- 

 trypsinogen is another example of proteolytic modification which 

 has yielded information about the active center. The splitting of 

 a single peptide bond between arginine and isoleucine appears to 

 suflfice for the activation of chymotrypsinogen (6b) . Of special 

 interest is the demonstration of enzymatic activity in dialyzable 

 fragments of autodigested pepsin (34). These small "enzymes" 

 were found to contain only 3% of the specific proteolytic activity 

 of native pepsin, but to retain 64% of its catalytic activity toward 

 the synthetic substrate acetyl-L-phenylalanyldiiodo-L-tyrosine. 

 Elucidation of the structure and amino acid sequence of the active 

 fragments may yield significant information about the active 



220 



