2009_Book_FoodChemistry

3.5 Lipoproteins, Membranes 183
(3.43)
Fig. 3.11. HPLC-analysis of soy “raw lethicin”
(according to Sotirhos et al. 1986) 1 Triacylglycerols,
2 free fatty acids, 3 phosphatidyl glycerol,
4 cerebrosides, 5 phytosphingosine, 6 diphosphatidyl
glycerol, 7 digalactosyldiacyl glycerol, 8 phosphatidyl
ethanolamine, 9 phosphatidyl inositol, 10 lysophosphatidyl
ethanolamine, 11 phosphatidic acid, 12 phosphatidyl
serine, 13 phosphatidyl choline, 14 lysophosphatidyl
choline
cate (florisil), silicic acid, hydrophobic dextran
gel or a cellulose-based ion-exchanger, such as
DEAE-cellulose.
Also the HPLC-analysis of phospho- and glycolipids
is of growing importance. Figure 3.11
for example demonstrates the separation of soya
“raw lecithin”.
3.4.2.3 Analysis of Lipid Components
Fatty acid composition is determined after
methanolysis of the lipid. For positional analysis
of acyl residues (positions 1 or 2 in glycerol),
phosphatidyl derivatives are selectively hydrolyzed
with phospholipases (cf. 3.7.1.2.1) and
the fatty acids liberated are analyzed by gas
chromatography.
The sphingosine base can also be determined
by gas chromatography after trimethylsilyl
derivatization. The length of the carbon skeleton,
of interest for phytosphingosine, can be
determined by analyzing the aldehydes released
after the chain has been cleaved by
periodate:
The monosaccharides in glycolipids can also
be determined by gas chromatography. The
lipids are hydrolyzed with trifluoroacetic
acid and then derivatized to an acetylated
glyconic acid nitrile. By using this sugar
derivative, the chromatogram is simplified
because of the absence of sugar anomers
(cf. 4.2.4.6).
3.5 Lipoproteins, Membranes
3.5.1 Lipoproteins
3.5.1.1 Definition
Lipoproteins are aggregates, consisting of proteins,
polar lipids and triacylglycerols, which
are water soluble and can be separated into
protein and lipid moieties by an extraction
procedure using suitable solvents. This indicates
that only noncovalent bonds are involved in the
formation of lipoproteins. The aggregates are
primarily stabilized by hydrophobic interactions
between the apolar side chains of hydrophobic
regions of the protein and the acyl residues of
the lipid. In addition, there is a contribution
to stability by ionic forces between charged
amino acid residues and charges carried by
the phosphatides. Hydrogen bonds, important
for stabilization of the secondary structure of
protein, play a small role in binding lipids since
phosphatidyl derivatives have only a few sites
available for such linkages. Hydrogen bonds
could exist to a greater extent between proteins
and glycolipids; however, such lipids have not yet
been found as lipoprotein components, but rather
as building blocks of biological membranes. An
exception may be their occurrence in wheat flour,
where they are responsible for gluten stability of
dough. Here, the lipoprotein complex consists of
prolamine and glutelin attached to glycolipids
by hydrogen bonds and hydrophobic forces.