MKTAHOLISM OF PKOPIONIC Af ID 289 



propionate under normal conditions, after which the three-carbon residues 

 follow the same metabolic pathway as does preformed propionate. Further 

 evidence that propionate is a source of carbohydrate is to be inferred from 

 the fact that the administration of tripropionin to rats is followed by the 

 deposition of a much greater amount of liver glycogen than occurs when a 

 similar amount of triacetin or of tributyrin is fed.^^^ This is because gly- 

 cerol alone serves as a source of carbohydrate in the latter cases, while 

 both glycerol and propionic acid are potential sources of glycogen in the 

 case of tripropionin. 



The metabolism and oxidation of propionic acid would thus appear to be 

 similar to that of glucose and of glucose intermediates. Thus, Mahler 

 and Huennekens^^^ reported that, when labeled propionate was oxidized 

 in the presence of large pools of unlabeled L-lactate, D-lactate, and pyru- 

 vate, the label appeared in these intermediates. This is interpreted to 

 mean that they are in equilibrium with the actual intermediates. In a 

 study of the role of propionate as a precursor of milk constituents of the 

 intact dairy cow, Kleiber et al.^^'^ noted that the decarboxylation rate of 

 propionate appeared to be higher than that of acetate, whereas the rela- 

 tive oxidation rates of the resulting two-carbon residues w^ere similar for 

 acetate and propionate. The formation of lactose from propionate is, for 

 the most part, direct, rather than via CO2 ; this is suggested by the fact that 

 only 30% of the C^"* transferred from C^^-1-propionic acid passes through 

 the carbonate pool in the case of the cow. Less than 10% of the carbon 

 transfer from C'*-2-propionic acid, which is converted to lactose, is involved 

 in the carbonate pool. A similar pattern was noted for the C'^ transferred 

 to casein. 



«i3 H. R. Mahler and F. M. Huennekens, Biochim. Biophys. Acta, 11, 575-583 (1953). 

 ^i" M. Kleiber, A. L. Black, M. A. Brown, and B. M. Tolbert, /. Biol. Chem., 203, 

 339-346(1953). 



