EFFECTS ON METABOLIC PATHWAYS 165 



Succinate and propionate are formed anaerobically in Ascaris muscle 

 from glucose and lactate, presumably by the following pathway: 



Glucose ^ 



^>fc^ +CO, +4H „ -COj ^ 



Pyruvate »=- Oxalacetate *- Succinate *~ Propionate 



Lactate -""^ 



Malonate at 20 mM does not appreciably inhibit the decarboxylation of suc- 

 cinate to propionate (about a 13% reduction in total radioactivity) but the 

 small inhibition indicates a possible competition with succinate for the 

 enzyme. However, the incorporation of lactate-2-C'-^ into succinate is inhibited 

 almost 90%. If succinate is formed by reduction of fumarate derived from 

 oxalacetate, malonate would be expected to inhibit well, not only because 

 of the effect on the succinate dehydrogenase but also by an inhibition of 

 oxalacetate formation. Malonate inhibits the formation of labeled propionate 

 from lactate-2-C^* 65%. The smaller inhibition compared to that for suc- 

 cinate formation implies another less important pathway for the formation 

 of propionate, perhaps by direct reduction, as shown in several bacteria. 



The metabolism of glyoxylate by Kver mitochondria is rather complex; 

 it is decarboxylated to formate by a devious route, it may be oxidized to 

 oxalate, or it may be aminated to glycine (Crawhall and Watts, 1962). 

 Malonate inhibits the decarboxylation competitively but does not interfere 

 with the formation of oxalate or glycine; indeed, the latter may be stimul- 

 ated slightly due to diversion in a branched chain. The decarboxylation 

 reaction, which requires glutamate, is quite sensitive to malonate, around 

 50% inhibition occurring at 0.15 mM, both substrates being at 3 mM. 

 This would certainly appear to be one system in which a marked effect can 

 be exerted by malonate at low concentrations and which is unrelated to 

 succinate oxidation. 



The synthesis of acetylocholine is an endergonic process and is related 

 to the cycle both for the supply of energy and with respect to the utilization 

 of acetyl-CoA. The effects of inhibitors on acetylcholine synthesis and hy- 

 drolysis are particularly important when considering the mechanisms by 

 which malonate can alter nerve and muscle function. Unfortunately, only 

 one study of the action of malonate has been made (Torda and Wolff, 

 1944 a). The formation of free acetylcholine in minced frog brain, in the 

 presence of physostigmine to prevent hydrolysis, is inhibited 32% by 0.08 

 mM, 46% by 0.8 mM, and 49% by 8 mM; the inhibition of total acetyl- 

 choline is about the same. Succinate, fumarate, and citrate increase acetyl- 

 choline formation. It would be interesting to investigate the effects of mal- 

 onate on the purer enzyme systems now available for acetylcholine synthe- 

 sis to determine if the inhibition is a direct effect or secondary through 

 ATP depletion. The effects of malonate in the intact cell may be quite 

 complex, because malonate might suppress the incorporation of acetyl-CoA 



