MECHANISM OF ANTIBODY FORMATION 107 



molecule which autocatalytically can be polymerized or aggregated to 

 yield the largest molecular species one can find in a species. 



a. Consideration of Enzyme Precursors. Pepsinogen is changed 

 into pepsin under the following conditions: (a) by hydrogen ion on the 

 acid side of pH 6.0 (maximal activity at pH 2.0); (b) by addition of 

 pepsin; or, (c) increasing the salt concentration to increase the rate 

 of conversion. This change from pepsinogen to pepsin in each case 

 is associated with the dissociation of a polypeptide, with a molecular 

 weight of 6,000, which is capable of recombining with pepsin at pH 

 5 to 6 to form a dissociable pepsin-inhibitor complex. In this combina- 

 tion the activity of pepsin is blocked but not destroyed. On long stand- 

 ing with pepsin between pH 2.0 and 5.0, the inhibitor is destroyed. 

 The pepsin inhibitor has exposed basic groups. 



The change from chymotrypsinogen to chymotrypsin by slightly 

 acid solution, or by trypsin, would appear to be the result of a limited 

 hydrolytic reaction without any apparent effect on the molecular 

 weight of the inactive protein. According to Northrop (1937), the 

 chymotrypsin molecule contains five amino groups* more than chy- 

 motrypsinogen, indicating the opening of a peptide ring. There is also 

 an associated shift in isoelectric point from pH 5.0 to 5.4. No change 

 was observed in elementary analysis, and the tyrosine plus tryptophane 

 content. 



In the change from trypsinogen into trypsin, no measurable increase 

 in amino groups was detected. As suggested, it might be that the 

 hydrolysis of a peptide link, too small to be detected, might have taken 

 place. The change from trypsinogen to trypsin takes place at pH 7.0 

 to 9.0 without the aid of any outside activator, at pH 3.0 to 4.0 by 

 means of a mold kinase (Kunitz, 1938), in the presence of enteroki- 

 nase at pH 6 to 9.0 where spontaneous activation of trypsinogen occurs 

 readily, or, simply, in the presence of calcium ion at pH 8.0 and 5°C 

 (Kunitz, 1945). Under this latter condition, trypsinogen is quantita- 

 tively converted into trypsin vdthout the formation of "inert protein" 

 formed under other conditions. 



In the above cited typical examples of enzyme precursors which are 

 studied more completely than any other crystalline enzymes isolated, 

 the change from inactive to active form involves treatments which are 



* Amino nitrogen as per cent of total nitrogen: chymotrypsinogen, 4.7; chymo- 

 trypsin, 6.0 (Northrop, Kunitz and Herriott, 1948). 



