Food Chemistry

3.5 Lipoproteins, Membranes 185

Fig. 3.13.Plasma protein fractionation by a preparative
ultracentrifugation (UC) method (according toSeidel,
1971)

similar to that of chylomicrons. The composition
of some lipoproteins is presented in Table 3.20.
Certain diseases related to fat metabolism (hy-
perlipidemias) can be clinically diagnosed by the
content and composition of the plasma lipopro-
tein fractions.
Electron microscopy studies have revealed that
the fat globules in milk have small particles at-

Table 3.20.Composition of typical lipoproteins

Source Lipoprotein Particle Protein Glycero- Cholesterol Triacyl-
weight (%) phospho- —————— glycerols
(kdal) lipids free esterified
(%) (%) (%) (%)

Human Chylomicron 109 –10^10 1–2 4 2 .5–3 3–4 85–90
blood serum Pre-β-lipoprotein 5 − 100 · 106 8. 319. 27. 411. 154. 2
LDL (β-lipoprotein) 2. 3 · 106 22. 727. 98. 528. 810. 5
HDL (α-lipoprotein) 1− 4 · 105 58. 124. 72. 99. 25. 9
Egg yolk β-Lipovitellin 4 · 105 78 12 0. 90. 19
(chicken) LDL 2 − 10 · 106 18 22 3. 50. 258
Bovine milk LDL 3. 9 · 106 12 .952 0 0 35. 1

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

Membranes that compartmentalize the cells and

many subcellular particles are formed from two

main building blocks: proteins and lipids (phos-

pholipids and cholesterol). Differences in mem-

brane 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 pro-

foundly change the organization and functionality

of the membrane.

Model membranes are readily formed. The major

forces in such events are the hydrophobic interac-

tions between the acyl tails of phospholipids, pro-

viding a bilayer arrangement. In addition, the am-

phipathic 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.