Metabolism and mode of action 



measured as shown in Figure 9. It is evident that in the control a rapid 

 decline in starch was obtained as indicated with the iodine test. With 

 increasing concentrations of TIBA both the amount of starch disappearing 

 and the rate of disappearance were reduced. These data suggest then that 

 the ability of TIBA to react with sulphydryl groups can actually bring about 

 an alteration of activity of an enzyme reciuiring free sulphydryl groups. 



500 



\ 



■^ v-oo 



•5 



300 



CI 

 to 



200 



100 



A my lose activity 



TIBA concn. 



0-5 



Time 



5 

 min 



Figure 9. Effect of TIBA on amylase activity. 

 Order of addition of reagents : 



1-0 ml acetate buffer pH 6-0 



0-1 ml 10-='M CaSOi 



De-ionized water to final volume of 10-0 ml 



TIBA 



Malt a-amylase at final concentration of 0-0\5 rngjml. 

 Incubation time: 2 hrs. 

 Temperature : ca. 25'. 

 Enzyme reaction initiated by additions of substrate (0-05 per cent final concentrations of soluble. 



starch). 

 Reaction stopped by removal of samples to KIg reagent. 

 Starch-iodine complex estimated by E^^b- 



One of the most important compounds in plant metabolism known to 

 require a free sulphydryl group is CoA. If some growth substances inactivate 

 sulphydryl groups, they may be able to inactivate CoA similarly. Experi- 

 ments were therefore set up to investigate the possible inactivation of CoA 

 with the growth substances TIBA and 2:4-D. 



The method used involved the incubation of CoA and growth substance 

 for periods up to an hour. The subsequent activity of the CoA in terms of 

 its ability to form an acetyl ester was measured through the /ran^acetylase 

 system of Stadtman (1952). The reactions involved were as follows: 



, , , , /-, A trawsacetylase 

 Acetyl phosphate + t-^oA — -. 



t 



-> Acetyl CoA + phosphate 

 arsenate 



CoA + Acetyl arsenate 



Acetate + arsenate 



278 



