544 VI. OCCURKENCE OF LIPIDS IN THE ANIMAL 



in the experiments of Ring. 135 This investigator reported that, when 

 glucose and thiamine were administered together, the specific dynamic ac- 

 tion was twice that resulting from the administration of a similar dose of 

 glucose without the vitamin. Inasmuch as no effect was noted when a 

 like amount of thiamine was given alone or with fat, the hypothesis is 

 suggested that the additional heat set free when thiamine and glucose are 

 given together, as contrasted with the effect when the vitamin is omitted, 

 is to be ascribed to the waste energy produced when carbohydrate is con- 

 verted to fat. 



The possibility that aldehydes may act as intermediates in the synthetic 

 production of fats from carbohydrates is strengthened by the demonstration 

 that the aldehydes of the higher fatty acids occur normally in the tissues. 

 Thus, palmityl and stearyl aldehydes have been proved to be natural 

 components of tissues. 136,137 Moreover, Mockel 138 reported that such 

 lipoaldehydes are present in increased amounts whenever fat synthesis or 

 degradation occurs. Smedley 139 ' 140 postulated an aldol condensation of 

 pyruvic acid in the synthesis of fatty acids. However, in view of our more 

 recent knowledge of the function of carboxylase and co-carboxylase, this 

 hypothesis would conform to the earlier one, in considering acetaldehyde as 

 an intermediate product. 



Acetic acid is another two-carbon compound which has been suggested 

 as a possible intermediate in fat synthesis. Rittenberg and Bloch 141 ' 142 

 demonstrated that C 13 appears in the body fat when it is administered to 

 mice in either the methyl or the carboxyl group of acetic acid. It is now a 

 well-recognized fact that acetic acid can be formed in tissues from carbo- 

 hydrates. Bloch and Rittenberg 143 have shown, by the aid of deuterium, 

 that acetic acid can be derived from pyruvic acid. It is possible that a 

 condensation of acetic acid molecules takes place directly, or after reduc- 

 tion to acetaldehyde by the mechanism originally suggested by Magnus- 

 Levy. 125 The role of acetic acid in the synthesis of body fat, and especially 

 of milk fat, has been reviewed by Popjak 144 and by Folley. 145 



135 G. C. Ring, Am. J. Physiol, 138, 488-490 (1942-1943). 



136 R. Feulgen, K. Imhauser, and M. Behrens, Z. physiol. Chem., 180, 161-179 (1929). 



137 R. Feulgen and M. Behrens, Z. physiol. Chem., 256, 15-20 (1938). 



138 G. Mockel, Z. physiol. Chem., 277, 135-146 (1943). 

 139 1. Smedley, J. Physiol, 45, xxv-xxvii (1912-1913). 



140 1. Smedley and E. Lubrzyhska, Biochem. J., 7, 364-374 (1913). 



141 D. Rittenberg and K. Bloch, J. Biol. Chem., 154, 311-312 (1944). 



142 D. Rittenberg and K. Bloch, J. Biol. Chem., 160, 417-424 (1945). 



143 K. Bloch and D. Rittenberg, J. Biol. Chem., 155, 243-254 (1944). 



144 G. Popjak, "Fat Synthesis from Small Molecules," in R. T. Williams, Lipid Me- 

 tabolism, Biochem. Soc. Symposia, No. 9, Univ. Press, Cambridge, 1952, pp. 37-51. 



148 S. J. Folley, "Aspects of Fat Metabolism in the Ruminant with Special Reference 

 to the Biosynthesis of Milk Fat," in R. T. Williams, Lipid Metabolism, pp. 52-65. 



