Biology Reference
In-Depth Information
FIGURE 10.2
Eugene Chevreul (1786
e
1889). Engraved by C. Cook from a drawing by Maurir.
components of animal fat. He was the first to isolate and name stearic and oleic acid, and
also, in 1815 coined the term 'cholesterine' from the Greek word chole for bile. Chevreul is
credited with creating margarine and, due to his longevity (102 years), he was also a pioneer
in the field of gerontology. Therefore he was both an investigator and a subject of
investigation!
Most of the attention given to cholesterol is due to its deleterious role in human atheroscle-
rosis (heart disease)
[2]
. Just the word cholesterol has become synonymous with bad things.
As if cholesterol has not been maligned enough, a recent report has even linked this unfor-
tunate sterol to tail-chasing in dogs
[3]
! But cholesterol is not all evil. It does play an essential
role in some, but not all, membranes. As discussed in Chapter 5, cholesterol is by far the major
polar lipid in mammalian plasma membranes, comprising up to ~60 mol% of the polar lipids,
yet it is almost totally absent in other mammalian intracellular membranes (e.g. the mito-
chondrial inner membrane). It is also hard to understand how cholesterol can play such an
essential role in life processes in mammalian cells yet be essentially missing in fungi, plants,
and bacteria. Simplistically, cholesterol is a 'membrane homogenizer'. It intercalates between
membrane phospholipids and sphingolipids and helps to control membrane lipid packing.
Lipid packing is an essential feature in membrane 'fluidity' (Chapter 9) and permeability
(Chapter 14). In a tightly packed gel state, membrane insertion of cholesterol decreases lipid
packing, thus increasing membrane 'fluidity' and permeability. In these membranes choles-
terol behaves as a lipid-soluble contaminant. In contrast, in a liquid crystalline or fluid state,
membrane cholesterol increases lipid packing, thus decreasing 'fluidity' and permeability. In
these membranes cholesterol prevents the formation of gauche kinks that are responsible for
acyl chain melting (see Chapter 9).
Mixing cholesterol and phospholipids often produces an unusual fluid membrane state
called the liquid ordered (l
o
) state
[4
6]
. The l
o
state behaves as if it were half way between
e
the gel (L
) states, having properties of both. In the l
o
state, acyl
chains are extended (they have fewer gauche kinks) and so in this sense behave like a gel state.
) and liquid crystalline (L
b
a