Metabolism and mode of action 



cysteine may be able to restore the sulphydryls of CoA which have been 

 oxidized to the disulphide form, but can not restore those sulphydryls which 

 have been actually substituted by TIBA. The reversal effect of cysteine is 

 quantitatively suggestive of the auto-oxidation effect evident in Figure 2. 



V-SjcW^Y^ 



30 60 



Incubation time min 



Figure 11. Time course of CoA inactivation with TIBA. Conditions as in Figure 10. 



100 



80 





60 



o 

 CO 



fO 



53 



.1 



^ 20 



\'^-ExW'^V\ 



Reversal 



by 



cysteine 



As 



<3. 

 O 



to 



Figure 12. Effect of cysteine follow- 

 ing incubation of CoA with growth 

 substances. Conditions as in Figure 10, 

 except that 10~- M cysteine added 1 

 minute before assay of CoA. 



TIBA 2--^-D 



feactj atSx10'^V\) 



Van Overl)eek el at. (1955) devised a series of compounds which were 

 assumed to be sulphydryl inactivators: the maleimides. The effects of these 

 compovmds upon abscission were of a much higher order of magnitude than 

 the effects of TIBA. They did not measure the actual disappearance of 

 sulphydryls in the presence of these maleimides but they did obtain a complete 

 reversal of the effects with cysteine. Their findings suggest that the malei- 

 mides may inactivate some system in a chain of events which controls 

 abscission but perhaps not inactivating directly the reaction at the precise 

 site of abscission induction, for then one would not expect a complete reversal 

 with cysteine. 



280 



