INHIBITORS STRUCTURALLY RELATED TO MALONATE 239 



glycine (it is carboxyglycine) and presumably inhibits 8-aminolevulinate 

 synthetase by binding to the glycine site and complexing with pyridoxal-P 

 as well. Aminomalonate condenses with aldehydes nonenzymatically in 

 the presence of pyridoxal-P. Furthermore, other pyridoxal-P-dependent 

 enzymes are inhibited, e.g. serine hydroxy methyltransferase, whereas en- 

 zymes involved in glycine metabolism but not requiring pyridoxal-P are not 

 inhibited. Aminomalonate can be formed in the tissues by transamination 

 between ketomalonate and glutamate, and can be decarboxylated by an 

 enzyme found in silkworm larvae and rat heart and liver to glycine. This 

 derivative of malonate is a good illustration of how a simple change in the 

 structure can create an inhibitor with quite different properties and inhib- 

 itory spectrum. 



Substituted Malonates 



Although the alkylmalonates are not particularly interesting as inhibi- 

 tors, there are two malonate derivatives that may warrant further investi- 

 gation. Fluoromalonate was studied by Chari-Bitron (1961) on the principle 

 that if malonate is metabolized through acetyl-CoA, fluoromalonate might 

 follow the same pathway and enter the cycle as fluoroacetyl-CoA, produc- 

 ing the same effects as fluoroacetate, namely, a block of the cycle at the 

 aconitase step. The toxicity of fluoromalonate is a good deal less than 

 fluoroacetate but the ester is as toxic in mice (see accompanying tabulation). 



LD50 (mg/kg) 



Animal 



Fluoromalonate Fluoromalonic diethyl ester Fluoracetate 



15 15 



70 5 



— 0.25 



Death is associated with a marked accumulation of citrate in the tissues 

 and it differs strongly from malonate in this respect. Accumulation of cit- 

 rate also occurs in kidney mitochondria with fluoromalonate at concentra- 

 tions around 1 mM. It was further established that decarboxylation of 

 fluoromalonate occurs in kidney preparations. Finally, fluoromalonate is 

 only about one tenth as effective as malonate in the inhibition of succinate 

 dehydrogenase. The results thus conform quite well to the predicted mech- 

 anism. Difluoromalonate and its amide inhibit quite readily the oxida- 

 tions of succinate and fumarate by Pseudomoyias (around 70% inhibition 

 at 0.7 mM), but there is no inhibition of succinate dehydrogenase in soni- 

 cates; the mechanism is unknown (Bernheim, 1963). 



