THE INFLUENCE OF GROWTH SUBSTANCES 

 UPON SULPHYDRYL COMPOUNDSf 



A. C. Leopold and C. A. Price 



Department of Horticulture, University of Purdue 



A WIDE variety of theories concerning the mechanism of auxin action have 

 been proposed which inchide suggestions that sulphydryl materials may be 

 specifically involved in auxin action within plants. It is the intention of this 

 discussion to examine experimentally two of these theories in an effort to 

 determine whether a positive role for sulphydryl reactions might be suggested. 



W'^V[ 



10 



10 



Concn. of auxin 



W 



10' 



10-^V\ 



Figure 1. Coenzyme A disappearance as measured by the nitroprusside test {Leopold and Guernsey, 

 1953). 



THE AUXIN CoA THEORY 



In 1953 Leopold and Guernsey proposed a theory that the mechanism of 

 auxin action might involve the formation of a thio-ester between auxins 

 and CoA. Since the most active auxins are derivatives of acetic acid, it was 

 suggested that they might form esters homologous to acetyl-CoA. Evidence 

 was presented that such a reaction might occur, although the evidence was 

 indirect in nature. Measurements were made showing a disappearance of 

 sulphydryl groups in the presence of auxins, tomato mitochondrial prepara- 

 tions, and ATP. More direct evidence would result from the measurement 

 of the actual formation of thio-esters. In the present study we have set 

 about to make such measurements. 



Turning first to the sulphydryl disappearance in the presence of auxins, 

 some difficulties were encountered in repeating the experiments reported in 

 1953. In the earlier experiments the most active auxins brought about 

 decreases of sulphydryl concentrations in the order of 10"'* M as shown in 

 Figure 1. In some more recent experiments the extent of enzymatic sulphydryl 



f This paper was read at the Conference by A. C. Leopold. 



271 



