THE NATURE OF BLOOD LIPIDS 363 



There was no correlation between the amount of cholesterol and of 7- 

 dehydrocholesterol, nor could any interrelationship be noted between the 

 concentration of this sterol and any pathological condition. 



a. Cholesterol and Cholesterol Esters. Cholesterol is present in the 

 blood both in the form of the free alcohol (unesterified) and as the fatty 

 acid ester. Cholesterol esters account for about two-thirds of the total 

 cholesterol in the plasma of man. On the other hand, the cholesterol in the 

 red blood cells is almost exclusively unesterified. However, it has been 

 stated that appreciable amounts of cholesterol esters occur in the erythro- 

 cytes of children. 47 Pfeiffer 8S also reported that cholesterol esters may occur 

 in the blood cells in the summer when, in fact, the total blood cholesterol 

 is increased. In winter, cholesterol esters and total cholesterol are at a 

 minimum level in the erythrocytes. Both Knudson 89 and Bodansky 90 

 reported an increase in the cholesterol ester content of the corpuscles dur- 

 ing fat absorption. 



Cholesterol has been found to be largely in the free form in the leucocytes. 

 In fact, Boyd and Stevenson 91 found that approximately 80% of the total 

 cholesterol was unesterified in the white blood cells of rabbits. In the case 

 of young women, Boyd 92 reported that the free cholesterol accounts for 

 about 60% of the total present in the leucocytes. 



About one-third of the stromata of the red blood cells of sheep consists of 

 cholesterol, while the remaining two-thirds are composed of equal amounts 

 of sphingomyelin and of other phospholipids. 67 On the other hand, most 

 of the stroma of human red corpuscles is in the form of sphingomyelin. 67 

 Erickson et a/. 93 reported that approximately 22% of the dried stroma of 

 sheep, cow, and horse corpuscles was lipid ; in the case of man, the figure 

 was 10 to 15% while, in birds, it was only 3%. This lipid was made up of 

 about 60% phospholipid (75% for avian erythrocytes), 30% free choles- 

 terol, and 10% fat and cholesterol esters. In human corpuscles, the stroma 

 weighed about 2 or 3% of the total of the whole corpuscle. 



(a) Fatty Acids Combined with Cholesterol. As has been discussed earlier, 

 the most highly unsaturated acids in the plasma are in ester combination 

 with cholesterol. For a discussion of this, see pages 353-354. 



(b) Cholesterol Combined with Protein. In the original C.A. preparation 



88 G. Pfeiffer, Biochem. Z., 220, 210-216 (1930). 



89 A. Knudson, J. Biol. Chem., 82, 337-346 (1917). 



90 M. Bodansky, Proc. Soc. Exptl. Biol. Med., 28, 628-630 (1931). 



91 E. M. Boyd and J. W. Stevenson, J". Biol. Chem., 117, 491-500 (1937). 



92 E. M. Boyd, J. Biol. Chem., 101, 623-633 (1933). 



93 B. N. Erickson, H. H. Williams, S. S. Bernstein, I. Avrin, R. L. Jones, and I. G. 

 Macy, J. Biol. Chem., 122, 515-528 (1937-1938). 



