542 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



Lusk 110 has calculated that, even when this change is proceeding at a 

 maximum rate, a hog weighing 13.5 kg. can synthesize a maximum of only 

 2 mg. of fat per second. If such a reaction is a generalized one which is 

 taking place simultaneously throughout the organism, as most certainly 

 must be the case, the maximum concentration of fat synthesized per second 

 would amount to 0.15 /ug. per gram of tissue. In all probability, the inter- 

 mediates would not be present in any greater amount than the end-prod- 

 uct; thus the identification of such intermediates poses an extremely 

 difficult analytical problem. 



Since the chief fatty acids which are synthesized are the Ci 6 and Ci 8 

 acids, 51 ' 123 it would seem that the building stones should consist of a com- 

 pound in which the number of carbon atoms is a common denominator of 

 these values, namely 2. One might also postulate that three glucose 

 molecules are converted to a Ci 8 acid, which can then be degraded to a Ci 6 

 acid. Such an in vivo conversion of stearic (Ci 8 ) to palmitic (Ci 6 ) acid has 

 been demonstrated by Schoenheimer and Rittenberg 124 ; the reverse 

 change, namely the synthesis of the Cis acid from a Ci6 acid, has likewise 

 been proved by these workers. One can also assume that fatty acids 

 having a shorter chain length than Ci6 can be produced in an analogous 

 manner. Thus, myristic acid (C14) might readily be synthesized in vivo 

 from palmitic acid. There is no experimental evidence to support the 

 hypothesis that a condensation of three glucose molecules is involved in 

 fat synthesis. 



Of the possible initial building stones, the fragments with 2 carbons would 

 seem to be the logical units. Moreover, the fact that even-numbered 

 carbon acids shorter than Ci 6 are found, especially in such sources as the 

 milk fats, renders the process of synthesis by steps of two carbons at a time 

 a most attractive one. In any event, a three-carbon intermediate is 

 almost certainly ruled out, inasmuch as this would result in the production 

 of odd-carbon atom acids in the intermediate stages. Since, practically 

 without exception, all animal fats are composed of fatty acids with an 

 even number of carbon atoms, the three-carbon intermediate would appear 

 to be highly improbable. 



Magnus-Levy 125 has postulated that acetaldehyde is the two-carbon 

 intermediate in the biosynthesis of fat from carbohydrate. He has pro- 

 posed that two such molecules condense in a reaction similar to the aldol 



123 H. E. Longenecker, G. Gavin, and E. W. McHenry, J. Biol. Chem., 134, 693-699 

 (1940). 



124 R. Schoenheimer and D. Rittenberg, J. Biol. Chem., 120, 155-165 (1937). 



126 A. Magnus-Levy, cited by G. Lusk, The Elements of the Science of Nutrition, 4th 

 ed., Saunders, Philadelphia-London, 1928, p. 350. 



