352 V. BLOOD LIPIDS 



original preprandial level, which is reached after four to five hours. Fon- 

 nesu 14 suggests that the acetic acid in the blood is derived partly from in- 

 testinal absorption and partially from the intermediary metabolism of 

 tissues. Although the presence of butyric acid has not been demonstrated 

 in human blood, large amounts of /3-hydroxybutyric acid and acetoacetic 

 (diacetic) acid occur normally during fasting, as well as during the ingestion 

 of a carbohydrate-free regimen. Ketone bodies (/3-hydroxybutyrate, 

 acetoacetate, acetone) may likewise occur in large quantities in the blood 

 in such diseases as diabetes mellitus. For a discussion of ketosis and keto- 

 nemia, the reader is referred to pages 449-451. 



No information is available as to whether the volatile fatty acids occur 

 in the blood as triglycerides, as other esters, as free acids, or as salts. Most 

 of the evidence would seem to indicate that they are present in water- 

 soluble form. This would presumably suggest that they occur as free 

 acids, salts, or in combination with water-soluble proteins. 



a. Fatty Acids in Phospholipids. The fatty acids present in the phos- 

 pholipid fraction of liver are ordinarily composed of 2 to 3 parts of liquid 

 (unsaturated) fatty acids to 1 part of solid (saturated) acids. 15 The com- 

 position of blood lipids presumably follows the same pattern. The satu- 

 rated fatty acids in phospholipids consist chiefly of palmitic and stearic 

 acids, while oleic and linoleic acids largely comprise the unsaturated fatty 

 acid fraction. 



The fatty acids of phospholipids are not highly unsaturated. In the 

 case of both lactating and non-lactating cows, Schaible 16 found that the 

 average iodine value was lowest in the phospholipid fatty acids, inter- 

 mediate in those from neutral fat, and highest in those from the cholesterol 

 esters. The iodine values in lactating cows averaged 68, 103, and 145 

 for the phospholipid, neutral fat, and cholesterol fatty acids, respectively; 

 in non-lactating cows, the figures were 85, 99, and 130, respectively. On 

 the other hand, Bloor and associates 17 reported that the iodine numbers of 

 the fatty acids from human plasma are lowest for neutral fat (102), inter- 

 mediate for the phospholipid fraction (125), and highest for the fatty 

 acids combined with cholesterol (158). Bloor 18,19 and Channon and Col- 

 linson 20 have shown that the phospholipid fatty acids of species other than 

 man have a considerably lower iodine value than the figures for human 



15 R. H. Snider and W. R. Bloor, J. Biol. Chem., 99, 555-573 (1932-1933). 



16 P. J. Schaible, J. Biol. Chem., 95, 79-88 (1932). 



17 W. R. Bloor, A. G. Blake, and S. S. Bullen, J. Allergy, 9, 227-230 (1938). 



18 W. R. Bloor, J. Biol. Chem., 59, 543-556 (1924). 



19 W. R. Bloor, /. Biol. Chem., 63, 1-15 (1925). 



20 H. J. Channon and G. A. Collinson, Biochem. J., 28, 1212-1221 (1929) 



