INHIBITION OF SUCCINATE DEHYDROGENASE 



21 



enzymes common to the succinate pathway. Malonate does not usually 

 interfere with ascorbate oxidation, but in the silkworm it was found both 

 spectroscopically and manometrically that malonate inhibits the oxida- 

 tions of succinate and ascorbate equally (Sanborn and Williams, 1950). 

 Observations such as these, coupled with those showing greater inhibition 

 of succinate oxidase compared to the dehydrogenase, make it necessary 

 to exert some caution in assuming a completely specific action in all cases. 



Competitive Nature of the Inhibition 



It has been often stated that competitive inhibition was first demonstrat- 

 ed by Quastel and Wooldridge (1928) for the inhibition of E. coli succinate 

 dehydrogenase by malonate. However, inhibitions were not calculated 

 and the data presented do not lend themselves to quantitative interpreta- 

 tion. Indeed, when their data are plotted on a l/w, — 1/(S) graph (Fig. 1-2) a 

 straight line is not obtained. Since no comparable control studies were done 

 in the absence of malonate, the fact alone that increasing the succinate con- 

 centration increases the rate in the presence of malonate does not prove 

 competition. These points are brought out not to criticize pioneering work 

 but to illustrate that conclusions about the type of inhibition cannot be 

 made so readily as many imagine. 



Fig. 1-2. A double reciprocal plot for the 

 inhibition of E. coli succinate dehydrogenase 

 by malonate at 1.43 mM. Succinate concen- 

 trations are in vaM. (Data from Quastel and 

 Wooldridge, 1928). 



