EFFECTS OF MALONATE ON LIPID METABOLISM 141 



The basic procedure in these investigations was to incubate carboxyl- 

 labeled fatty acids with rat liver and kidney slices, and determine the dis- 

 tribution of C^* in acetoacetate and COg. Malonate at 5 mM depresses the 

 formation of C^^Og from octanoate-C^* 00^ around 60% and fumarate is 

 able to overcome this inhibition only partially (Geyer et al., 1950 a). Fu- 

 marate and other cycle intermediates increase the total COg formed but 

 have little effect on the C^^Oa- This was explained by the accumulation of 

 some of the C^* as succinate, this not being relieved by fumarate, and we 

 have previously cited this as an example of the importance of considering 

 what is measured in demonstrating a reversal by fumarate. 



Where does the C^* go that does not appear as C^^Oo in inhibited slices? 

 They found that in the presence of malonate much of the C^* appears in 

 acetoacetate (Table 1-24) (Geyer and Cunningham, 1950). The ratio AcAc- 

 QujQuQ^ is near 1.21 in the controls and is increased to around 4.51 by 

 malonate, averaging the results from the five fatty acids used. It may 

 also be noted that malonate generally increases the total C" recovered, even 

 though succinate was not determined, showing that malonate does not in- 

 hibit the fatty acid oxidation directly. Later they determined both the car- 

 boxyl and carbonyl C^* in acetoacetate and the more complete results are 

 summarized in Table 1-25, where I have taken the liberty of averaging 

 the data for all the fatty acids used, inasmuch as the effects are always 

 in the same direction although differences between the different fatty acids 

 are evident. These results show clearly the diversion of fatty acid metabo- 

 lism into acetoacetate by malonate. Weinhouse et al. (1949) reported that in 

 rat liver slices 10 mM malonate inhibits COo formation and no acetoacetate 

 appears, which was so contradictory to the results obtained by Geyer that 

 the latter studied malonate in concentrations up to 20 mM, but found only 

 that even more acetoacetate accumulates. Also they tested three different 

 strains of rat and the results were the same. The reason for this discrepancy 

 could not be explained. 



The differential labeling in the carboxyl and carbonyl groups of aceto- 

 acetate is difficult to explain. If acetoacetate arises by a condensation of 

 acetyl-CoA units, the labeling in these positions should be uniform. However, 

 the ratio is seldom unity as may be seen in the results summarized by Chai- 

 koff and Brown (1954). In the work of Geyer with rat liver slices, the ratio 

 CHgC^^ — / — CHoC^* 00" is less than 1 in the controls and increases with 

 the length of the fatty acid chain. Malonate increases this ratio, that is, it 

 increases relatively the labeling in the carbonyl group. Chaikoff and Brown 

 have given a detailed analysis of the possible factors determining this ratio, 

 and the explanation is based on the existence of two types of 2-carbon 

 fragment formed from fatty acids, one designated the CH3CO — fragment 

 and the other the — CH2CO — fragment. These fragments are assumed 

 to arise from different portions of the fatty acid chain and only the — CHg 



