Biology Reference
In-Depth Information
0.3
22:6-22:6PE
16:0-22:6PE
16:0-20:4PE
16:0-18:2PE
16:0-18:1PE
0.2
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
χ CHOL
FIGURE 10.5 Plot of the normalized scattering peaks for cholesterol monohydrate crystals excluded from the
membrane vs the mol fraction of cholesterol added to the membrane. The sum of the normalized cholesterol
monohydrate peaks are extrapolated to 0, representing the membrane carrying capacity for cholesterol. The values
are reported in Table 10.1 [9] .
Lipid Raft Detergent Extractions
In its initial form, the Fluid Mosaic model (Singer and Nicolson, 1972 [11] ) did not appre-
ciate lipid heterogeneities. However, it soon became obvious that lipid patchiness must
exist. By 1974 a series of biophysical studies started to appear supporting the basic concept
of lipid microdomains in membranes [12] . Of particular importance were cholesterol- and
sphingolipid-enriched domains studied in model bilayer membranes in the late 1970s by
Biltonen and Thompson [13] . The lipid microdomain concept was formalized in a classic
1982 paper by Karnovsky et al. [12] . Therefore cholesterol/sphingolipid microdomains pre-
dated the concept of lipid rafts by more than a decade.
The pre-raft experiments strongly implied that therewas a preferential affinity of appreciate
for sphingolipids. This observation on model membranes was given a biological link with the
discovery of lipid rafts in biological membranes (see Chapter 8) [14,15] . Rafts can be extracted
as an insoluble fraction at 4 C with non-ionic detergents such as Triton X-100 or Brij-98. Iso-
lated rafts contain about twice the amount of cholesterol and also are enriched in sphingolipids
by about 50% when compared to the surrounding plasma membrane bilayer. Cholesterol is
purported to be the 'glue' that holds rafts together, since rafts fall apart when cholesterol is
extracted by cyclodextrin. Also, cholesterol is responsible for rafts existing in the tightly
packed, liquid ordered (l o ) state and being thicker than the surrounding non-raft membrane.
Acharacteristic ofmost, but apparently not all raft lipids, is their having long, mostly saturated
acyl chains. Figure 10.6 shows molecular models of SMand cholesterol. Note that in this repre-
sentation the inverted cone shape of SM fits nicely next to the cone-shaped cholesterol,
enhancing affinity between these two lipids. Fitting lipids of different shapes together like
pieces of a jigsaw puzzle is known as 'complementarity'. However, it is not certain whether
raft-insolubility in cold non-ionic detergent results fromextraction of intact rafts or is the result
 
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