144 1. MALONATE 



We have seen that the accumulation of acetoacetate in the presence of 

 malonate can be attributed to an increased rate of formation of the aceto- 

 acetate. Is the accumulation due entirely to this or can malonate also inhibit 

 the utilization of acetoacetate in some tissues? The rise in the acetoacetate 

 in liver slices in the presence of malonate was believed by Jowett and 

 Quastel (1935 c) to be due to the inhibition of the decomposition of aceto- 

 acetate, since at that time the pathways for the formation of acetoacetate 

 were not understood. However, Quastel and Wheatley (1935) soon provided 

 evidence that malonate can interfere with the disappearance of acetoacetate 

 in rat liver and kidney slices. In kidney slices, malonate at 8 mM inhibits 

 around 42% and at 16 mM 64%, and in liver slices an inhibition of 74% 

 was observed with 40 mM malonate. Fumarate is able to counteract this 

 inhibition partially and it was concluded that acetoacetate oxidation must 

 be coupled with other oxidations inhibited by malonate. Very similar results 

 were reported by Edson and Leloir (1936); indeed, 20 mM malonate inhibits 

 disappearance of acetoacetate in rat kidney slices 93% and it was stated, 

 "Malonate is a powerful and relatively specific inhibitor of respiration and 

 of aerobic disappearance of acetoacetic acid in kidney." Both Handler 

 (1945) and Mookerjea and Sadhu (1955) in their work with whole animals, 

 favored the concept that malonate interfered with acetoacetate metabolism 

 accounting for the rises in blood acetoacetate. Inasmuch as several different 

 pathways are open to acetoacetate and these vary with the tissue used, it is 

 difficult to interpret accurately these results. In some tissues, acetoacetate 

 can be split into acetyl-CoA fragments that enter the cycle and here malo- 

 nate might inhibit by blocking the cycle and the formation of oxalacetate, 

 which is, of course, esentially the same mechanism adduced to explain the 

 increased formation of acetoacetate. There is evidence that malonate does 

 not inhibit the reduction of acetoacetate to /?-hydroxybutyrate (Edson and 

 Leloir, 1936), nor does it seem to interfere with the formation of sterols from 

 acetoacetate (Mookerjea and Sadhu, 1955). It is probably best in the 

 present state of our knowledge to attribute the accumulation of acetoacetate 

 in the presence of malonate primarily to a diversion of 2-carbon units away 

 from oxidation through the cycle, without eliminating the possibility that 

 malonate may interfere in other pathways for the utilization of acetoacetate. 



Effects on Propionate Metabolism 



Propionate arises terminally from the /^-oxidation of odd-numbered fatty 

 acids and in certain tissues, such as the liver, can be oxidized completely 

 through the cycle. However, the oxidation of propionate differs from that 

 of other fatty acids. The following sequence of reactions has been suggested: 



CO, 



+ 



. ATP ATP B,j 



Propionate — > propionyl-CoA — > methylmalonyl-CoA — > succinyl-CoA 



