48 II. BIOSYNTHESIS 



viewpoint. Moreover, Balmain and FoUey^^^ found that the glycerol and 

 insulin effects are not additive; this would also support the thesis that the 

 action of insulin is related to a stimulation of glycerol synthesis. Although 

 this explanation does not seem to apply to sheep udder, in which no stimu- 

 lation obtains when glycerol is added to tissue metabolizing acetate plus 

 glucose, it is possible that glucose can be broken down rapidly enough in 

 the udder of this species so that glycerol is present in maximal concentra- 

 tion without the intervention of insulin. 



6. The Rate and Extent of Biosynthesis of Fatty Acids and of 

 Glycerol from Acetate and Glucose 



The rate of synthesis of fatty acids from acetate is fairly rapid. Pihl, 

 Bloch, and Anker,-^^ making use of labeled acetic acid, demonstrated that 

 the synthesis of saturated acids had a haK-life of one day in the liver, and 

 of sixteen to seventeen days in the carcass. The corresponding periods 

 for unsaturated fatty acid (oleic) were one and two and nineteen to twenty 

 days, respectively. Although acetic acid is capable of furnishing every 

 carbon of long-cham fatty acid molecules, -''•^'^ the maximal concentration 

 of isotope in the liver fatty acids was found to be about 25% of that of 

 acetyl groups; this would indicate that at least one-fourth of the carbon 

 of fatty acids is furnished by acetic acid.^^^ 



In experiments on rabbits, Popjak, Hunter, and French-^^ demonstrated 

 that, within six hours, 30 to 70% of the short-chain milk fat acids origi- 

 nated from acetate, and about 25% from carbohydrate. However, only 

 one-fifth to one-tenth of these long-chain fatty acids were synthesized in 

 the mammary gland from either of these sources. ^^^ On the other hand, 65 

 to 95% of the glycerol moiety of fat was derived from carbohydrate in the 

 mammary gland over a six-hour period. It is concluded that the glycerol 

 fraction of fat is more rapidly metabolized by the animal than is the fatty 

 acid part. In contradistinction to this Popjak et al.^^^ noted that about 

 20% of the body acetate used for acetylation of p-aminobenzoic acid was 

 derived from glucose in fully-fed lactating rabbits. 



7. The Biosynthesis of Phospholipids 



Phospholipids can be synthesized with relative ease in the animal. Thus, I 

 a number of investigators demonstrated that animals develop normally 

 when their diet contains no preformed phospholipids and all the phos- 



"5 A. Pihl, K. Bloch, and H. S. Anker, /. Biol. Chem., 183, 441-450 (1950). 



2'6 G. Popjdk, G. D. Hunter, and T. H. French, Biochem. J., 54, 238-247 (1953). 



