THE NATURE OF BLOOD LIPIDS 373 



viously in pure form. An excellent discussion of the lipoproteins in human 

 plasma is the article of Gurd, Oncley, Edsall, and Cohn. 160 



(a) ^-Lipoproteins in Plasma. About 75% of the lipid in the plasma of a 

 normal fasting man is present in the /3-lipoprotein fraction. This consti- 

 tutes only approximately 5% by weight of the total plasma proteins. 161 

 The /3-lipoproteins are euglobulins, since they are insoluble in water at the 

 isoelectric point in the absence of salt. Small amounts of salt have a pro- 

 found influence on their solubility near the isoelectric point (pH 5.4). The 

 /3-lipoproteins in human plasma interact with other proteins in the plasma, 

 such as 7-globulins, to form insoluble complexes. In a pH range at which 

 both the 7-globulins and the /3-lipoproteins are soluble (the 7-type as 

 cations, the /3-type as anions), a complex is formed on mixing, the solubility 

 of which is reduced a hundred-fold. 



The /3-lipoproteins are large molecules. On the basis of the demon- 

 stration of an intrinsic viscosity 75 of 0.041, and of a marked difference in 

 partial specific volume between the anhydrous protein 75 and the hydrated 

 protein, 162 Oncley et a/. 75 suggest that this lipoprotein possesses a spherical 

 shape. A molecular weight of 1,300,000 has been postulated for the an- 

 hydrous protein. Assuming the hydrated molecule to be a sphere, each 

 molecule would have a diameter of 185 A. The /3-lipoprotein has a close 

 relationship to the X-protein isolated by Pedersen. 162 



According to Gurd et al., m the average composition of three separate 

 lots of /3-lipoprotein gave the following figures: protein, 25%; phos- 

 pholipid, 30%; and cholesterol (esterified and free), 45%. Nearly all of 

 the carotenoid of the plasma has been located in this /3-lipoprotein frac- 

 tion. 106 The absolute amount was found to be only 0.02 to 0.03%, which 

 would be less than one mole per molecule of protein. According to Roberts 

 and Szego, 163 one of the hormones, the common estrogen, estriol, is com- 

 bined with the /3-lipoprotein fraction and not with the a-lipoprotein. 



(b) a-Lipoproteins in Plasma. The a-lipoproteins constitute only about 

 3% of the plasma proteins, but they include 35% of the total plasma lipids. 

 Their molecular weight is approximately one-sixth of that of the /3-lipo- 

 protein; it has been estimated at 200,000. 75 Oncley, Scatchard, and 

 Brown 75 suggested that the a-lipoprotein molecule has an ellipsoid form, 

 with a length of 300 A. and a 50 A. cross-section. 



160 F. R. N. Gurd, J. L. Oncley, J. T. Edsall, and E. J. Cohn, Lipoproteins, General 

 Discussion Faraday Soc, No. 6, 70-74, Aberdeen Univ. Press, Aberdeen, 1949. 



161 E. J. Cohn, Experieniia, 3, 125-136 (1947). 



162 K. O. Pedersen, Ultracenirifugal Studies on Serum and Serum Fractions, Almqvist 

 & Wiksells, Upsala, 1945, p. 167. 



163 S. Roberts and C. M. Szego, Endocrijwlogy, 39, 183-187 (1946). 



