INHIBITION OF SUCCINATE DEHYDROGENASE 



33 



Preparation 



Ki {mM) 



Reference 



Thorn (1953) 



ITyhn and Matsumoto (1964) 



Ryan and King (1962a) 



Cheniae and Evans (1959) 



McDonald et al. (1963) 



Strauss and Jann (1956) 



Kearney (1957) 



Keihn and King (1960) 



Ryan and King (1962 a) 



Lee et al. (1964) 



Warringa and Giuditta (1958) 



Hiatt (1961) 



Honda and Muenster (1961) 



and 0.05 vaM, and mammalian tissues generally yield quite low constants. 

 The interest in the K, lies in its relationship to the binding energy of mal- 

 onate to the active center, so that variations in the K^, unless due to exper- 

 imental conditions, may be attributed to differences in the topography 

 of the enzyme surface. The values of £",„ for different preparations are not 

 particularly significant, except for characterizing a certain preparation 

 under specified conditions, because .K",,, does not usually equal K^.. Inasmuch 

 as the ratio of K^ to K j is of some significance in interpreting interactions 

 with the active center, it will be worthwhile to discuss the only instance in 

 which this has been determined. 



The ratio, K„,!K„ has been given by various investigators as ranging 

 from 10 to 60. Thorn (1953) pointed out that K,„ for succinate dehydroge- 

 nase is often quite different than K, (Slater and Bonner, 1952). K,„ thus 

 equals (A-j + A'2)/A:j, and A-j is dependent on the experimental conditions. 

 Thorn obtained values of KJK^ from 3 to 60 by varying the electron- 

 acceptor dye and the reaction rate. The faster the rate, the greater the de- 

 viation of ^,„ from K^. Thus extrapolation to zero rate from a series of 

 experiments at different methylene blue concentrations enabled Thorn to 

 determine the true ratio of succinate and malonate affinities; KJK, turned 

 out to be 3. Since K, is approximately 0.0076 milf . K, = 0.023 mM. a value 

 which checks well with directly determined values of the rate constants 

 (A'i = 3.35x 10^, and A-_i = 0.99). Similar low ratios would probably be found 

 in most succinate dehydrogenase preparations. The difference in binding 

 between succinate and malonate is thus not so great as previously believed. 



It is of great practical importance to realize that the degree of inhi- 

 bition of succinate oxidase by malonate varies with the rate of succinate 

 oxidation and the electron acceptors present. This may explain some of the 



