24 



1. MALONATE 



chrome pathway. The usual Ijv - 1/(S) plots (Fig. 1-5) show apparently 

 completely competitive inhibition, and values of the substrate and inhibi- 

 tor constants, to be discussed in the next section, were calculated. Using 

 average values of K„^ and K^ obtained by Thorn, the inhibition curves in 



(Malonate) = 0089mM 



No Malonate 



Fig. 1-5. Double reciprocal plots for the inhibition 

 of pig heart succinate dehydrogenase by malonate, 

 showing pure competitive inhibition. Succinate con- 

 centrations are in mM and v in spectrophotometric 

 units. (Data from Thorn, 1953). 



Fig. 1-6 were plotted. These curves show the expected reduction in inhi- 

 bition as the succinate concentration is increased at constant values of 

 malonate concentration. It may be noted that overcoming the inhibition 

 is much more difficult at high inhibitor concentrations and this must be 

 taken into account in experiments designed to show a competitive type of 

 action. K„i was used rather than K,. because under the usual experimental 

 conditions it is this constant that determines the behavior. 



A word must now be said about malonate reversibility. One of the cri- 

 teria of competitive inhibition is that the inhibitor should leave the active 

 site readily when its concentration is reduced, or that it should be displaced 

 rapidly when more substrate is added. These points have seldom been 



