CHOLESTEROL IN THE BLOOD 371 



Cholesterol is ordinarily combined in the plasma with the most highly 

 unsaturated fatty acids. Thus, Kelsey and Longenecker**^ reported that 

 62% of the ester cholesterol was combined with linoleic acid in the case of 

 beef plasma. A considerable proportion of the cholesterol likewise appears 

 in loose combination with protein. In the original C.A. preparation made 

 from horse serum by Macheboeuf and Rebeyrotte,^^ as much as 17.9% was 

 made up of cholesterol esters, and no free cholesterol was present. In later 

 samples prepared by a similar procedure after World War II, only 4.5% of 

 cholesterol esters were found in the C.A. preparation. A large proportion 

 of the cholesterol in human plasma is likewise in combination with pro- 

 tein. Thus, the /3i-globulin fraction of human serum was shown by Oncley 

 et alP to contain 8% by weight of free cholesterol and 39% of cholesterol 

 esters. Although this lipoprotein accounts for only 5% of the total plasma 

 protein, it was shown to carry 75% of the total serum cholesterol.^^ Gof- 

 man and associates ^"-^^ likewise separated lipoproteins from rabbit and 

 human sera by ultracentrifugation ; these were found to contain cholesterol. 

 In the rabbit, the main cholesterol-containing lipoprotein has a flotation 

 rate (aS/) of 5 to 8 /S units, while in man it is characterized by an Sf value of 

 3 to 8. In the case of the rabbit, a cholesterol-containing lipoprotein with 

 an Sf index of 10 to 30 was sometimes found in the serum after cholesterol 

 feeding. Many workers consider that the protein combinations with 

 cholesterol represent the forms in which cholesterol can exert its athero- 

 genic action.^" Koehler and Hill^^ observed that appreciable amounts of 

 7-dehydrocholesterol also occur in human serum. The total free 7-dehydro- 

 cholesterol was found to vary between 1.8 and 7.0 mg. %, while the range 

 of the cholesterol present in combination was from 2.4 to 35 mg. %.*2 



{2) Physiologic Factors Altering Blood Cholesterol 



The level of blood cholesterol may be altered in a number of physiologic 

 and pathologic conditions. However, Mayer and Schaeffer^^ reported in 

 1913 that the lipemic constant {blood cholesterol Mood fatty acids) remains 

 uniform for each species. In fact, the constant is maintained during the 



87 F. E. Kelsey and H. E. Longenecker, J. Biol. Chem., 139, 727-740 (1941). 



*8 M. A. Macheboeuf and P. Rebeyrotte, in Lipoproteins, General Discussion Faraday 

 Soc, No. 6, 62-74, Aberdeen Univ. Press, Aberdeen, 1949. 



«' J. L. Oncley, F. R. N. Gurd, and M. Melin, /. Am. Chem. Soc, 72, 458-464 (1950). 



'« J. W. Gofman, F. T. Lindgren, H. A. Elliott, W. Mantz, J. Hewitt, B. Strisower, 

 V. Herring, and T. P. Lyon, Science, 111, 166-171, 186 (1950). 



91 J. W. Gofman, F.'T. Lindgren, and H. A. Elliott, /. Biol. Chem., 179, 973-979 

 (1949). 



92 A. E. Koehler and E. Hill, Federation Proc, 12, 232-233 (1953). 



" A. Mayer and G. Schaeffer, J. physiol. path, gin., 15, 984-998 (1913). 



