108 IMMUNO-CATALYSIS 



found not to cause deep structural variation. One may therefore, 

 perhaps, ask the question of whether or not these changes can be 

 taken as proof that these enzyme precursors are really the forms which 

 are synthesized first. It would seem that the inactive forms could 

 easily be assumed to have been derived from the active forms under 

 in vitro or in vivo environmental influences.''' The ease with which pep- 

 sin can reversibly combine with pepsin inhibitor recovered from pep- 

 sinogen complex (Pepsin + pepsin inhibitor ^ "pepsinogen"), and 

 the ease with which both chymotrypsinogen and trypsinogen are 

 changed to their respective active forms with reagents such as calcium 

 ion, or, spontaneously, at neutrality, permits, as an alternative, the as- 

 sumption that the enzyme molecule is synthesized first and subse- 

 quently converted into certain reversibly inactive forms. As will be 

 discussed later in this work, serum proteolytic enzyme which, among 

 other proteins, also digests fibrin clot exists in an inactive form in 

 normal serum. The treatment of serum with chloroform, ether, ethyl 

 alcohol, etc., or dialysis, liberate the active enzyme; the enzyme may 

 also be liberated spontaneously, on standing in the cold. Thus, it is 

 most likely that these treatments dissociate a substance from an inactive 

 complex which blocked the activity of the serum protease. 



Northrop (1946) mentions the serological behavior of the precursor 

 and the respective active enzyme as proof that the latter is derived from 

 the former, or that a protein may be formed by an autocatalytic re- 



*One must ask the question of whether chymotrypsinogen is a precursor of the 

 chymotrypsins or an artifact derived from native chymotrypsin as the result of the 

 conditions used for their isolation from the source material. Similar question may he 

 raised with respect to the relationship between trypsinogen and trypsin. In relation 

 to these questions we must be aware of the fact that there is no evidence that 

 chymotrypsinogen (and trypsinogen) fer se exist in the pancreatic tissue. All that 

 we know is that they are obtained from extracts made by means of strongly acid, 

 0.25N H2SO4, solutions at a temperature of 5°C. After several fractionations from 

 solutions on the acid side, a crystalhne material from a solution at pH 5.0 is obtained. 

 A solution of these crystals on adjusting to pH 7.6 with a trace of trypsin, yields the 

 crystalline chymotrypsin (Northrop, et al. 1948). 



The intriguing question is the nature of the possible effect of the cold strong acid 

 on the raw source of these enzymes. Is it not possible that under the above prepara- 

 tive conditions these enzymes undergo dehydration of the free amino and carboxyl 

 groups yielding chymotrypsinogen (and trypsinogen) which reverse to the active 

 forms spontaneously in weakly acid or alkaline solutions with or without the aid of 

 activating agents? 



While correcting the page proof, S. D. Elliott (J. Exp. Med., 92:201-218, 1950) 

 reported that the maximal production of the precursor of streptococcal proteinase oc- 

 curred only in slightly acid, and none in neutral or alkaline environment. 



