INHIBITION OF SUCCINATE DEHYDROGENASE 



23 



different plotting procedure {K^ = 0.13 mM, and KJK^ = 200). In most 

 studies insufficient data are available for plotting. It must be emphasized 

 that a change in inhibition observed at two or three succinate concentra- 

 tions is not adequate to prove a purely competitive inhibition. It seems 

 to be rarely considered that an inhibition may not be either completely 

 competitive or completely noncompetitive. The conditions for partially 

 competitive inhibition were given (Eqs. 1-3-14 and 1-3-15) and the types 

 of plot to be expected discussed (Chapter 1-5). An interesting example 

 of this is provided by the work of Honda and Muenster (1961) on the inhi- 

 bition of succinate oxidation in lupine mitochondria. Here the osmolarity 

 of the preparation and assay media was varied with sucrose, and it was 

 found that the interaction constant, a, defined in Eqs. 1-3-5 and 1-3-6, 



Fig. 1-4. A single-curve plot for the inhibition of 



Crithidia succinate dehydrogenase by malonate at 



1 mM. Ki = 0.22 mM, and KJE^ = .53. (Data from 



Hunter, 1960). 



varies quite markedly from values indicating nearly completely competitive 

 inhibition to those showing noncompetitive inhibition. This work will be 

 discussed in greater detail in a later section (see page 46), but it suffices 

 to show that partial competitive inhibition by malonate is possible and that 

 the type of inhibition may vary with the experimental conditions. 



The most elegant treatment of malonate inhibition is by Thorn (1953) 

 at the St. Thomas's Hospital Medical School in London, using succinate 

 oxidase preparations from pig heart muscle. The activity was measured by 

 the reduction of ferricyanide in the presence of cyanide to block the cyto- 



