98 IMMUNO-CATALYSIS 



when it is replaced by a more actively metabolizing substrate (for ex- 

 ample, glucose). The increase in galactose fermenting activity with the 

 elimination of the fermentation of glucose is explained by assuming that 

 the common preenzyme is adapted now for one substrate and then for 

 another. This does not appear to offer sufficient grounds for a theory 

 of the production of a new enzyme protein from a common protein 

 when it is in contact with each member of a group of related sub- 

 strates. The application of the enzyme precursor idea to such problems 

 seems to exaggerate the significance of such changes. 



In an actively metabolizing environment with or without outside 

 nitrogen source, an enzyme can be reversibly changed to an inactive 

 form. The restoration of activity when in contact with substrates or 

 in their absence in no way justifies the assumptions that the inactive 

 precursor form originated first during the cytoplasmic synthesis of the 

 protein. The fact that the active enzyme can assume an inactive or 

 weakly active state when a substrate (galactose) is eliminated, or when 

 its metabolism is followed by the metabolism of another substrate, 

 shows clearly that the enzyme which was once active, could assume 

 an inactive form, and therefore justifies the belief that the active 

 form of the enzyme is synthesized first. There seems therefore no 

 experimental basis for hypothesizing the existence of enzyme pre- 

 cursors as being synthesized prior to the active enzyme (see further a 

 succeeding section). 



Third: Cells do not manifest the quality of producing a new, 

 "adaptive enzyme" for a substrate which does not possess an enzyme- 

 specific reactive group common to a substrate which is normally readily 

 metabolized by a given cell. A cell cannot be forced to produce an 

 enzyme to metabolize a substance for which there is no genetic pro- 

 vision in its species specific physiology and economy. Staphylococcus 

 aureus, for example, lacks the ability to metabolize formate in a manner 

 similar to that exercised by E. coli. Staphylococcus lacks the genetic 

 ability to produce the formic hydrogenylase. Hemophilus influenzae 

 has not been shown to adapt itself to synthesize the porphyrin molecule 



involves a Walden inversion of C-4 and is catalyzed by an enzyme which they call 

 "galactowaldenase." The coenzyme of this reaction has been isolated and character- 

 ized as uridine-difhos'phate-glucose (UDPG). This coenzyme has been found in 

 animal tissues and in yeast not adapted to galactose, a-1 ,6-Glucosedi'phosphate, dis- 

 covered and isolated by Cardini et al. (1949), has been found to function as co- 

 enzyme for phosphoglucomutase (reaction III). 



