Physical-Chemical Aspects of Synthetic Auxins 



299 



0.25 



H 0.05- 

 O 



< 



25 



35 45 



TEMPERATURE "0 



55 



Fig. 2. The effect of 2,4-D on the enzymatic activity of peroxidase as a function 

 of temperature. 



It is immediately apparent from this graph that not only has 

 2,4-D accelerated the loss of activity of the peroxidase, but also the 

 optimal activity of the peroxidase has been shifted from 45° C. down 

 to 35° C. in the presence of 2,4-D. These data lend support to the 

 assumption that the mechanism of action of 2,4-D in modifying en- 

 zymatic activity of a protein is a result of the change of structure of 

 the enzyme molecule. 



Consideration of these data raises the speculation as to how 2,4-D 

 and other synthetic auxins may at one concentration cause growth 

 stimulation and at higher concentrations inhibition. Reports in the 

 literature, indicating that low concentrations of 2,4-D or other syn- 

 thetic growth regulators may stimulate the activity of a certain en- 

 zyme and at higher concentrations cause inhibition of this enzyme, 

 reinforce interest in this speculation. It is postulated that the en- 

 zymatic stimulation associated with low concentrations of 2,4-D could 

 come about through a slight reversible modification of the enzyme 

 structure which would make the enzyme a more efficient catalyst, but 

 that as additional molecules of 2,4-D are added to this surface the 

 modification of the structure becomes increasingly severe with a con- 

 sequent loss of catalytic property. 



In order to ascertain whether or not 2,4-D is capable of both 

 stimulation and inhibition of an enzyme by variation in concentra- 



