THE NATURE OF BLOOD LIPIDS 351 



(1) Fatty Acids 



The fatty acids occur in combination with the phospholipids, and in 

 ester combination with cholesterol, as well as with the carotenols and fat- 

 soluble vitamins. According to Luck, 8 fatty acids may likewise be bound 

 by plasma proteins ; this fraction may be identical with that referred to as 

 free fatty acids. 



There is evidence that, in addition to the usual Ci6-Ci 8 saturated and 

 unsaturated acids in the blood, several of the short-chain (volatile) acids 

 may also be present, especially in the case of ruminants. Thus, Craine and 

 Hansen 9 proved that the peripheral blood of goats contained acetic, pro- 

 pionic, and butyric acids, as well as other unidentified volatilea cids. It was 

 shown that, concurrently with the development of the rumen, the level of 

 blood glucose decreased, while that of the short-chain fatty acids increased. 

 Although McClymont 10 showed that the concentration of butyrate in the 

 blood of ruminants is low, there are appreciable amounts of /3-hydroxy- 

 butyrate in blood. On the basis of arteriovenous differences, there would 

 seem to be strong evidence that the bovine udder is able to remove measur- 

 able amounts of this hydroxy-acid from the blood. u It is now believed that 

 acetate is the chief end-product of the breakdown of celluloses and other 

 polysaccharides in the rumen of the goat and cow. These molecules are 

 carried via the blood to the udder, where they are removed to be used in the 

 synthesis of long-chain fatty acids. This is discussed in Volume III. 

 The recent reviews of Popjak 12 and Folley 13 furnish considerable informa- 

 tion on this phenomenon. 



The presence of the volatile fatty acids is not, however, confined to the 

 blood of ruminants. Thus, Fonnesu 14 reported that acetic acid is present 

 in human blood. It was found to be higher in the arterial blood (7.5 

 milligram per cent) than in the venous blood (5.0 milligram per cent), 

 which would indicate that it was removed in the tissues either for oxidation 

 or for fat synthesis. The level of blood acetate rises rapidly after a meal, 

 and reaches a maximum in one hour. It then gradually decreases to the 



8 J. M. Luck, in Lipoproteins, General discussion Faraday Soc, No. 6, 44-52, Aber- 

 deen Univ. Press, Aberdeen, 1949. 



9 E. M. Craine and R. G. Hansen, J. Dairy Sri., 35, 631-636 (1952). 



10 G. L. McClymont, Biochem. J., 45, i-ii (1949). 



11 J. C. Shaw and C. B. Knodt, J. Biol. Chem., 138, 287-292 (1941). 



12 G. Popjdk, "Fat Synthesis from Small Molecules," in R. T. Williams, Lipid 

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



13 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, 1952, pp. 52-65. 



14 A. Fonnesu, Bull. soc. chim. biol, 33, 1021-1024 (1951). 



