172 J. F. PECHÈRE AND HANS NEURATH [10 



trypsin an additional lysyl-peptide bond had been hydrolyzed by trypsin- 

 Because of the specificity requirements of trypsin, these negative results also 

 seem to exclude the above mentioned possibiUty of hydrolysis during activa- 

 tion of a peptide bond involving the e-amino group of a lysyl residue. 



On the basis of all evidence considered, therefore, the hydrolysis of the 

 lysyl-isoleucine bond in the iV-terminal region of the single polypeptide 

 chain of trypsinogen seems to be the sole chemical event occurring during 

 autocatalytic activation; the configurational changes, demonstrated by 

 measurements of the optical rotation, must then be considered to be the 

 consequence of this hydrolytic cleavage. It would be difficult to conceive of 

 a mechanism which could account for these observations if trypsinogen were 

 to consist simply of an open polypeptide chain. However, the following con- 

 siderations suggest that the molecule is not of such a simple structure : (a) the 

 absence of a reactive C-terminal group ; (b) the presence of 6 intramolecular 

 disulfide bonds; and (c) the relatively low specific levorotation of native 

 trypsinogen ( — 30°) which suggests a high degree of helical configuration. 



A suggested scheme' representing the structural changes involved in acti- 

 vation is represented in Fig. 3, which incorporates the following considera- 



ACTIVATION OF TRYPSINOGEN 



(VlAlAjMWllJilVlG) 



ElectrostaficJi^r^-'Yl^ 

 Interaction or '■ ' -^ 

 Hydrogen 

 bonding 



Qsmm 



-Active center 



Free octivation 



P^ptX'" 0®©®®© 



V = Vol 

 X = specificity 

 site 



Fig. 3. Proposed scheme for the conversion of trypsinogen to trypsin. For details see 

 the text. 



tions: (a) The C-terminal region of the molecule is not involved in the 

 activation process and, hence, is omitted from this diagram, (b) A large 

 fraction of the trypsinogen molecule is in a helical configuration, but the 

 A^-terminal peptide sequence does not follow the helical pattern, presumably 

 because the electrostatic repulsion between the four adjacent aspartic acid 

 residues tends to keep this segment in a more or less extended configuration. 



