164 1. MALONATE 



is difficult to fit the data into the modern concepts of the carbon pathway in 

 photosynthesis, which does not directly involve the cycle, and particularly 

 to understand the mechanism whereby the Hill reaction is inhibited. 



Malonate is also able to interfere in the metabolism of glycerol. Glycerol 

 is fermented to succinate, accompanied by the uptake of CO2, in Propioni- 

 hacterium pentosaceum, and 30 mM malonate inhibits both the glycerol 

 fermentation and the CO2 uptake around 10% (Wood and Werkman, 1940). 

 It is impossible to attribute this to an action on succinate dehydrogenase. 

 The oxidation of glycerol may involve an initial phosphorylation with 

 subsequent formation of pyruvate and entry into the cycle: 



Glycerol + ATP -> glycerophosphate -> pyruvate -> cycle 



The phosphorylation is inhibited in rat liver homogenates (Ruffo and D'A- 

 bramo, 1952), but the total oxidation is not markedly affected in the 

 mycobacteria (G. J. E. Hunter, 1953). Malonate at 10 mM inhibits 5% in 

 M. stercoris and stimulates 4-6% in M. smegmatis and M. butyricum. It is 

 surprising that greater inhibition is not observed if the oxidation does 

 involve the cycle. In castor bean cotyledons, malonate at high concentra- 

 tions has very marked effects on the utilization of glycerol (Beevers, 1956). 

 At 70 mM the C^^Oa formation from labeled glycerol is inhibited 97% and 

 the sucrose formation is inhibited 82%, while at 130 mM the oxygen 

 uptake is inhibited 60%. Such high concentrations may interfere with the 

 formation of ATP and hence depress phosphorylation and also block the 

 cycle CO2 release. 



The stimulation of muscle respiration by insulin is inhibited potently by 

 malonate (Stare and Baumann, 1940). Insulin almost doubles the respiration 

 of minced breast muscle from depancreatized pigeons and 1 mM malonate 

 inhibits this increase 92%. Fumarate is able to overcome both this inhibi- 

 tion and the inhibition of nonstimulated respiration completely. This 

 interesting action in vitro led to a study of the antagonism in the whole 

 animal. Solutions of sodium malonate were injected subcutaneously in 

 rabbits either before or with insulin and the drop in blood glucose was much 

 less than with insulin alone. Insulin (4 units) decreases the blood glucose 65% 

 in 4 hr, whereas with malonate present the reduction is only 13% and none 

 of the rabbits goes into convulsions. Malonate alone increases the blood 

 glucose 26%. The respiratory inhibition in the muscle mince could be 

 explained on the basis of a typical cycle block (although the degree of inhi- 

 bition is surprising for a concentration of 1 mM), but the inhibition of glu- 

 cose utilization in the animal is more complicated. The initial phosphoryla- 

 tion of glucose and its uptake could have been inhibited indirectly by a re- 

 duction of the available ATP, or it could have resulted, at least in part, 

 from a hyperglycemic action of malonate unrelated directly to the insulin 

 stimulation. 



