Physical-Chemical Aspects of Synthetic Auxins 



Table 6. Effect of temperature on the 2,4-D inhibition of 

 oxygen uptake by mitochondria. 



297 



not the chemical was being adsorbed by the protein of the mitochon- 

 dria by measuring the oxygen uptake at two different temperatures. If 

 the 2,4-D is adsorbed, one would expect less inhibition of oxygen up- 

 take at a higher temperature. This proposition was tested by using 

 a mitochondrial preparation from cabbage, one aliquot of which was 

 run at 25^^ C, the other at 30° C. In each case a control was com- 

 pared with 2,4-D treated samples. The results of this study are re- 

 ported in Table 6. 



The data in Table 6, which represents an average of triplicate 

 runs, clearly demonstrate that the rate of inhibition decreases with 

 temperature. It is interesting to note that the decrease in activity, 

 which is 13.7 per cent, is in good agreement with the increase in 

 water solubility of 2,4-D over this same temperature range (18.5 per 

 cent). The increase in the solubility again would make for decreased 

 adsorption by virtue of the change of chemical potential of the solute 

 species. These findings afford further support of the theory that the 

 primary event involved in the action of growth regulators is one of 

 adsorption. 



ACTION OF 2,4-D ON CRYSTALLINE ENZYMES 



Since it would appear from the foregoing data that physical ad- 

 sorption of the chemical by an enzyme surface is involved in the 

 biological action of these chemicals, the question arises as to the 

 consequence to the protein of this adsorption. While many of the 

 more common cases of enzyme inhibition come about by the chemi- 

 cal reacting with a particular functional group of the protein or 

 competing with the substrate for a specific site, cases of inhibition 

 by adsorption are less well known. While the adsorption probably 

 occurs at sites adaptive to the structure of the compound and is 

 therefore specific, these sites are not those occupied by the substrate. 

 This is borne out by the noncompetitive nature of the inhibition of 



