300 Freed, Reithel, and Remmert 



tion, a number of crystalline enzymes were tested for activity in vary- 

 ing concentrations of 2,4-D. In the case of glyceraldehyde-3-phosphate 

 dehydrogenase, a marked stimulation in activity was obtained with a 

 concentration of about 100 p. p.m. of 2,4-D, and a very marked inhi- 

 bition of enzymatic activity was noted at 1,000 p. p.m. of 2,4-D. In the 

 case of glucose-6-phosphate dehydrogenase, a 40 p.p.m. 2,4-D solu- 

 tion resulted in a 22 per cent increase in activity, whereas a 1,000 

 p.p.m. concentration resulted in an appreciable loss of enzymatic ac- 

 tivity. A similar situation was found to prevail with isocitric dehy- 

 drogenase. Little stimulation was found with the peroxidase, but in- 

 hibition by 2,4-D was appreciable above 500 p.p.m. \Vhile these con- 

 centrations of chemical appear quite high in considering the small 

 amounts of 2,4-D required to bring about an auxin-like effect, it should 

 be remembered that the media in which the enzymatic assays are made 

 are not those of physiological conditions. Thus, in general the pH 

 tends to be higher than would be found in the cell, reducing the 

 efficiency of the 2,4-D because of increased ionization. 



SUMMARY 



The mechanism of action of a synthetic growth substance and the 

 structural relations to activity appear to be two very closelv inter- 

 related problems. Seemingly, the solution to one affords at least some 

 indication to the solution of the other. The data presented in this pa- 

 per would appear to indicate that the primary event in the mechanism 

 of action of tiie chlorophenoxyacetic acids is that of adsorption on a 

 protein surface, and that as a consequence of this adsorption, the 

 structure of the protein is modified with a consequent change in its 

 enzymatic activity. This theory of the molecular level mechanism of 

 action of the synthetic growth substances affords an explanation of 

 how the same molecule may both stimulate growth at a lo^v concen- 

 tration and bring about inhibition at a higher concentration. The 

 lines of evidence on which this postulate is based are as follows: 



(1) The finding that the growth regulators can reduce the vis- 

 cosity of cytoplasm and can stimulate streaming of the cyto- 

 plasm. 



(2) The surface activity and behavior of growth substances as 

 shown by Veldstra (30), Brian and Rideal (8), and Linser (19). 



(3) The finding of Marinos (20) that exposure at high concentra- 

 tions of growth substances results in the shrinkage of the cy- 

 toplasm and a leakage of the cell constituents. 



(4) Examination of the ultraviolet and infrared absorption spectra 

 suggests that the geometry of the molecule is important and 

 that interpretation of these data suggests a plane surface per- 



