Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 1 Examples of naturally occurring sterols.
droxylated steroids that retain some or all of the carbon atoms of squalene in the
side chain and partition almost completely into an ether layer when shaken with
equal volumes of water and ether [27].
Sterols are common in eukaryotic cells but rare in prokaryotes. Without exception,
vertebrates confine their sterol biosynthetic activity to producing cholesterol.
Most invertebrates do not have the enzymatic machinery for sterol biosynthesis and
must rely on an outside supply. Sterols of invertebrates have been found to comprise
most complex mixtures arising through food chains. In plants, cholesterol exists only
as a minor component. Sitosterol and stigmasterol are the most abundant and widely
distributed plant sterols, while ergosterol is the major occurring sterol in fungus and
yeast. The plant sterols are characterized by an additional alkyl group at C-24 on
the cholesterol nucleus with either � or � chirality. Sterols with methylene and
ethylidene substitutes are also found in plants (e.g., 24-methylene cholesterol, fucosterol).
The other major characteristics of plant sterols are the presence of additional
double bonds in the side chain, as in porifeasterol, cyclosadol, and closterol.
Despite the diversity of plant sterols and sterols of invertebrates, cholesterol is
considered the most important sterol. Cholesterol is an important structural component
of cell membranes and is also the precursor of bile acids and steroid hormones
[28]. Cholesterol and its metabolism are of importance in human disease. Abnormalities
in the biosynthesis or metabolism of cholesterol and bile acid are associated
with cardiovascular disease and gallstone formation [29,30]. Our discussion will
mainly focus on cholesterol and its metabolites, with a brief comparison of the
biosynthesis of cholesterol and plant sterols (see Sec. II.B.2). The biosynthesis of
plant sterols and sterols of invertebrates was reviewed by Goodwin [31] and Ikekawa
[32].
The chemistry of sterols encompasses a large amount of knowledge relating to
the chemical properties, chemical synthesis, and analysis of sterols. A detailed discussion
on all these topics is impossible in one chapter. We consider the analysis of
sterols to be of primary interest, and therefore our treatment of the chemistry of
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