184 3 Lipids
Fig. 3.12.Separation of lipid classes by thin layer chromatography using silicagel as an adsorbent. Rfvalues in
two solvent systems
Thus, lipoproteins are held together only by
non-covalent bonds.
3.5.1.2 Classification...........................................
Lipoproteins exist as globular particles in an
aqueus medium. They are solubilized from
biological sources by buffers with high ionic
strength, by a change of pH or by detergents in
the isolating medium. The latter, a more drastic
approach, is usually used in the recovery of
lipoproteins from membranes.
Lipoproteins are characterized by ultracen-
trifugation. Since lipids have a lower density
(0.88–0.9g/ml) than proteins (1.3–1.35 g/ml),
the separation is possible because of differences
in the ratios of lipid to protein within a lipopro-
tein complex. The lipoproteins of blood plasma
have been thoroughly studied. They are separated
by a stepwise centrifugation in solutions of
NaCl into three fractions with different densities
(Fig. 3.13). The “very low density lipopro-
teins” (VLDL; density< 1 .006 g/ml), the “low
density lipoproteins” (LDL; 1.063 g/ml) and the
“high density lipoproteins” (HDL; 1.21 g/ml)
float, and the sediment contains the plasma
proteins. The VLDL fraction can be separated
further by electrophoresis into chylomicrons (the
lightest lipoprotein, density < 1 .000 g/ml) and
pre-β-lipoprotein.
Lipoproteins in the LDL fraction from an
electrophoretic run have a mobility close to
that of blood plasma β-globulin. Therefore,
the LDL fraction is denoted asβ-lipoprotein.
An analogous designation of α-lipoprotein is
assigned to the HDL fraction.
Chylomicrons, the diameters of which range
from 1000–10,000 Å, are small droplets of tria-
cylglycerol stabilized in the aqueous medium by
a membrane-like structure composed of protein,
phosphatides and cholesterol. The role of chy-
lomicrons in blood is to transport triacylglycerols
to various organs, but preferentially from the
intestines to adipose tissue and the liver. The
milk fat globules (cf. 10.1.2.3) have a structure