244 1- MALONATE 



be less affinity between cationic groups on the enzyme and doubly charged 

 anions, compared to singly charged anions, and one cannot help but wonder 

 if the alterations in pH in the experiments of Kosen and Klotz were not 

 affecting the ionizable groups on the enzyme. These workers suggested that 

 the binding of these substances involves an iron ion on the enzyme and 

 correlated affinities with the ability to chelate iron. This is certainly a type 

 of binding that should be kept in mind, but it is difficult to reconcile with 

 the fact that iron-chelating agents such as 1,10-phenanthroline and 2,2'-bi- 

 pyridine do not interfere with the binding of succinate to the dehydrogenase 

 even though their attachment to the enzyme can be demonstrated spectro- 

 scopically. 



An interesting succinate dehydrogenase inhibitor, not fundamentally re- 

 lated to the compounds previously discussed, is 3-nitropropionate (hiptage- 

 nate), found to be the toxic principle of Indigofera endecaphylla (Morris 

 et al., 1954). It inhibits succinate dehydrogenase competitively with K^ == 

 0.19 n\M (Hylin and Matsumoto, 1964,) although no inhibition of the 

 enzyme is found after administration of the substance to animals, so that 

 it is not possible to correlate the toxicity with an effect on this enzyme. It 

 is rather surprising that 3-nitropropionate binds so well to succinate de- 

 hydrogenase, lacking two anionic groups, and it would be interesting to 

 know more of the nature of the interaction of the nitro group with the 

 enzyme. 



With respect to inhibitors related to malonate in one way or another, 

 it must be concluded that none possesses particular advantages over mal- 

 onate for the specific inhibition of succinate dehydrogenase, although cer- 

 tain derivatives have interesting properties and metabolic actions. Progress 

 could be made by the finding of forms of malonate, or related inhibitors, 

 that are uncharged, reasonably stable outside the cell, and easily split 

 to the active inhibitor intracellularly. It would also be valuable to have an 

 inhibitor which would initially bind to the succinate dehydrogenase at the 

 substrate site, because of its complementary configuration, and then react 

 chemically with an adjacent group, so that the inhibition of this enzyme 

 would be not only specific but slowly reversible. 



