2009_Book_FoodChemistry

184 3 Lipids
Fig. 3.12. Separation of lipid classes by thin layer chromatography using silicagel as an adsorbent. R f values 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 ultracentrifugation.
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 lipoprotein
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 lipoproteins”
(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 as β-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 triacylglycerol
stabilized in the aqueous medium by
a membrane-like structure composed of protein,
phosphatides and cholesterol. The role of chylomicrons
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