METABOLISM OF MALONATE 231 



CoA-transferase have been partially purified. The acetyl-CoA formed from 

 malonate may enter the cycle directly or participate in other reactions, such 

 as the formation of acetoacetate if the cycle is blocked by malonate, or 

 transfer its CoA to malonate, or simply be hydrolyzed to acetate. It is very 

 interesting that a lag period was noticed in the oxidation of malonate by 

 cell-free extracts (Wolfe et al., 1955). Little oxygen uptake occurs with 

 malonate until it is all decarboxylated; during the period of the most rapid 

 CO2 evolution, the respiration is low. Yet acetate is activated and oxidized 

 rapidly. One possible explanation is a block of the cycle by malonate so 

 that acetyl-CoA cannot enter until most of the malonate has been me- 

 tabolized. Another explanation involves a distribution of CoA in favor of 

 the malonyl derivatives with little acetyl-CoA present during the active 

 decarboxylation reaction. 



Cryptococcus terricolus can grow on malonate as the sole source of carbon 

 and malonate stimulates the endogenous respiration and CO2 formation after 

 a lag phase (Pedersen, 1963). It would appear that malonate is completely 

 oxidized to CO2 and water, since the R.Q. in the presence of malonate is 

 1.54. In other microorganisms where the oxidation is not complete, mal- 

 onate may be incorporated into a variety of substances, especially lipids 

 (Bu'Lock et al., 1962), although the rate of incorporation is seldom very 

 rapid. 



Pathways of Malonate Metabolism In Plants and Animals 



Despite the widespread occurrence of malonate metabolism in plant tis- 

 sues, little is known of the reactions involved, although perhaps they are 

 not significantly different from those described for microorganisms. Peanut 

 mitochondria supplemented with ATP, CoA, and other factors are able to 

 oxidize malonate (Giovanelli and Stumpf, 1957). Incubation with malo- 

 nate-2-C^* for 2 hr leads to the appearance of labeled citrate, malate, and 

 succinate, indicating the sequence 



Malonate ->■ malonyl-CoA -> acetyl-CoA -> CO2 + HjO 



the last step occurring in the cycle. The participation of malonyl-CoA in 

 the oxidation of propionate by peanut mitochondria is suggested by the 

 tracing of the label from propionate- 1-C^* (Giovanelli and Stumpf, 1958). 

 The following sequence involving malonic semialdehyde may be formulated: 



Propionate -> propionyl-CoA ->■ acrylyl-CoA -> |3-hydroxypropionyl-CoA -> 



^-hydroxypropionate -> malonic semialdehyde -> malonyl-CoA -> 



acetyl-CoA -> CO^ + H2O 



It is also possible that CoA derivatives are retained throughout, since a 

 malonic semialdehyde-CoA dehydrogenase which catalyzes the formation 



