364 V. BLOOD LIPIDS 



by Macheboeuf, 5,69,71,72 the purified lipoprotein contained 17.9% of chole- 

 sterol esters ; however, no free (unesterified) cholesterol was present. On 

 the other hand, in the preparations of C.A. from horse serum made since 

 World War II, only 4.5% of cholesterol esters were present. 



Blix and co-workers 73 showed that, although all electrophoretic frac- 

 tions of human serum contained lipids in at least a one-to-one molecular 

 ratio, the a- and /3-globulin fractions had a much higher lipid content; 

 cholesterol made up 8.6% and phospholipids comprised 10% of this pro- 

 tein. The high lipid content of the ^-globulin fraction is in line with 

 observations of Longsworth et al. u that the large fractions of this protein 

 occurring in nephrosis and in jaundice are markedly reduced by ether 

 extraction. McFarlane 95 reported a similar phenomenon in the case of 

 normal human serum, when the ether extraction was accompanied by 

 freezing at low temperatures. The ft-globulin fraction prepared by Oncley 

 and associates 76 from human serum in a relatively homogeneous form was 

 shown to contain 8% by weight of free cholesterol, and 39% of cholesterol 

 esters. This lipoprotein accounts for 5% of total plasma protein, but it 

 carries 75% of the total serum cholesterol. 



Gofman and his collaborators 96,97 reported the separation of a cholesterol- 

 containing lipoprotein from the sera of man and of rabbit; this can be 

 separated by ultracentrifugation, since it possesses a lower density than do 

 other components of the serum. This fraction was shown to vary only 

 slightly with a normal dietary. 



The lipoproteins were identified by their flotation rates (expressed as 

 Svedberg units). Gofman et al. 96 introduced this term, flotation rate, in 

 place of the more cumbersome expression, negative sedimentation rate. 

 This index is expressed in negative Svedberg units. A Svedberg unit (S) 

 equals 10~ 13 cm./sec./dyne/g. Thus, the molecules described as Sy 5 to 8 

 are those which have flotation rates consistent with 5 to 8 S units. The 

 flotation rates are not necessarily an index of molecular size. They de- 

 pend not only upon density but also upon the molecular shapes. In rabbit 

 serum, the major lipoprotein has a flotation rate (Sj) of 5 to 8 S units; in 

 man, the corresponding lipoprotein is characterized by an S/ value of 3 to 8. 

 When cholesterol is fed to the rabbit, an increase in the S/ 5 to 8 component 



94 L. G. Longsworth, T. Shedlovsky, and D. A. Machines, /. Exptl. Med., 70, 399- 

 413 (1939). 



95 A. S. McFarlane, Nature, 149, 439 (1942). 



96 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). 



97 J. W. Gofman, F. T. Lindgren, and H. A. Elliott, J. Biol. Chem., 179, 973-979 

 (1949). 



