INHIBITION OF SUCCINATE DEHYDROGENASE 



25 



tested with malonate. However, it was demonstrated quite early that the 

 inhibition of rabbit muscle succinate dehydrogenase is reversible by washing; 

 the preparation was incubated for 30 min. with 100 milf malonate anaerobi- 

 cally and then washed 3 times on a filter — the reduction times were 15 

 min for the control, 61 min with the malonate, and 17 min for the washed 

 preparation (Hopkins et al., 1938). A trypanosomal succinate dehydroge- 

 nase, however, showed no reversal of the inhibition, even when succinate 

 was added to 50 times the malonate concentration, which is particularly 

 surprising since in the living cells a good reversal was observed (Agosin and 

 von Brand, 1955). The concentrations of malonate used here were not 

 unduly high and so these data are unexplainable. 



Fig. 1-6. Curves showing the calculated reductions 

 of the malonate inhibition of succinate dehydrogen- 

 ase by various concentrations of succinate, using 

 K„ = 0.366 mM, and K^ = 0.0076 mM, as given 

 by Thorn (1953). 



Constants of the Inhibition 



It is not surprising that the values of the inhibitor constant, K^, for 

 malonate inhibition are quite variable in the literature, because not only 

 do the experimental conditions affect this constant markedly but the suc- 

 cinate dehydrogenase varies in its properties from species to species. From 

 the values in the accompanying tabulation and from reasonable estimates 

 based on assumed or determined substrate constants for the data in Table 

 1-6, it is seen that the K^'s for most preparations vary between 0.005 mM 



