2009_Book_FoodChemistry

3.5 Lipoproteins, Membranes 185
tached to their membranes; these are detached
by detergents and have been identified as LDL
(cf. Table 3.20).
3.5.2 Involvement of Lipids in the Formation
of Biological Membranes
Fig. 3.13. Plasma protein fractionation by a preparative
ultracentrifugation (UC) method (according to Seidel,
1971)
similar to that of chylomicrons. The composition
of some lipoproteins is presented in Table 3.20.
Certain diseases related to fat metabolism (hyperlipidemias)
can be clinically diagnosed by the
content and composition of the plasma lipoprotein
fractions.
Electron microscopy studies have revealed that
the fat globules in milk have small particles at-
Membranes that compartmentalize the cells and
many subcellular particles are formed from two
main building blocks: proteins and lipids (phospholipids
and cholesterol). Differences in membrane
structure and function are reflected by the
compositional differences of membrane proteins
and lipids (see examples in Table 3.18).
Studies of membrane structure are difficult since
the methods for isolation and purification profoundly
change the organization and functionality
of the membrane.
Model membranes are readily formed. The major
forces in such events are the hydrophobic interactions
between the acyl tails of phospholipids, providing
a bilayer arrangement. In addition, the amphipathic
character of the lipid molecules makes
membrane formation a spontaneous process. The
acyl residues are sequestered and oriented in the
nonpolar interior of the bilayer, whereas the polar
hydrophilic head groups are oriented toward the
outer aqueous phase.
Another arrangement in water that satisfies both
the hydrophobic acyl tails and the hydrophilic
polar groups is a globular micelle. Here,
thehydrocarbon tails are sequestered inside,
while the polar groups are on the surface of the
sphere. There is no bilayer in this arrangement.
Table 3.20. Composition of typical lipoproteins
Source Lipoprotein Particle Protein Glycero- Cholesterol Triacylweight
(%) phospho- —————— glycerols
(kdal) lipids free esterified
(%) (%) (%) (%)
Human Chylomicron 10 9 –10 10 1–2 4 2.5–3 3–4 85–90
blood serum Pre-β-lipoprotein 5−100 · 10 6 8.3 19.2 7.4 11.1 54.2
LDL (β-lipoprotein) 2.3 · 10 6 22.7 27.9 8.5 28.8 10.5
HDL (α-lipoprotein) 1−4 · 10 5 58.1 24.7 2.9 9.2 5.9
Egg yolk β-Lipovitellin 4 · 10 5 78 12 0.9 0.1 9
(chicken) LDL 2−10 · 10 6 18 22 3.5 0.2 58
Bovine milk LDL 3.9 · 10 6 12.9 52 0 0 35.1