THE NATURE OF BLOOD LIPIDS 375 



the pressure normally caused by serum proteins, by reducing the volume of 

 solvent available to the proteins, with the result that the effective concen- 

 tration of the latter is increased. 



Tayeau and Rolland 171 observed that, in aging serum, the amount of 

 biliary salts adsorbed diminishes markedly. It is believed that this re- 

 duction of adsorption results because the surface of the protein molecules is 

 being modified by transformation of lipid-protein linkages as a result of the 

 action of serum lecithinase on phospholipids. 



The hypothesis that varying types of union occur between the lipids and 

 protein in the lipoproteins is largely based upon extraction studies. Thus, 

 Delsal 172 noted that phosphatides could not be extracted from the blood 

 serum by the McFarlane method, 95 but only by means of an ether-alcohol 

 mixture. When cholesterol was removed, the albumin : globulin ratio re- 

 mained unchanged, while the elimination of phosphatides resulted in a 

 marked diminution of this ratio. This would indicate that the linkages of 

 protein with phosphatides and with cholesterol were different. In fact, 

 Ardry and Fontaine 173 have postulated the presence of three types of lipo- 

 protein linkages in horse serum, namely, loose association, cenapse, and a 

 very strong bond in ^-lipoprotein. In the last case, the bond is so strong 

 that the lipid cannot be separated without destruction of the protein. 

 Moreover, it was noted that, on aging, a rupture of the loose associations, 

 and molecular transfers, occur which partially block the extraction of other 

 lipids. The variation in linkages is also shown by the use of cationic 

 soaps. 174 In the presence of ether, these soaps assist in the solubilization 

 of almost all cholesterol, but not of phospholipids, while in the absence of 

 ether these soaps precipitate most of the lipids, including phospholipids. 

 However, there is a fraction of lipids which is not precipitated by any 

 quantity of cationic soaps. According to this method of differentiation, the 

 serum lipids are divided into two distinct groups. Feldman and Wein- 

 berg 175 reported that a consistent layering of hyperlipemic and cholesteremic 

 sera occurs on standing, and that this process is accentuated by centrifuga- 

 tion. The cholesterol content of the two layers was shown to vary mark- 

 edly. 



c. Factors Altering the Level of Lipoproteins in Blood. A considerable 

 species variation obtains in the nature of blood lipoproteins. Thus, Lewis, 



171 F. Tayeau and R. Rolland, Cornpt. rend. soc. biol., US, 394-395 (1949). 



172 J. L. Delsal, Bull. soc. chim. biol, 31, 122-125 (1949). 



173 R. Ardry and M. Fontaine, Bull. soc. chim. biol, 33, 1497-1503 (1951). 



174 M. Macheboeuf and J. Polonovski, Bull. soc. chim. biol, 31, 125-128 (1949). 

 176 M. Feldman, Jr., and T. Weinberg, Science, 113, 697 (1951). 



