Biology Reference
In-Depth Information
transitions are distinct and do not overlap (they are monotectic). If both the high melting and
low melting phospholipids were of the same class (i.e. they were both PC or both PE), low
levels of cholesterol first reduced the lower melting transition. This is depicted in
Figure 10.3
,
panel (a) for 18:1,18:1 PC (low T
m
) and 16:0,16:0 PC (high T
m
). Higher levels of cholesterol
eventually diminished the higher melting transition as well.
Figure 10.3
, panel (b) compares
a low melting PC (18:1,18:1 PC) to a high PE (14:0,14:0 PE). Cholesterol first obliterated the
low melting PC transition. This does not indicate whether cholesterol prefers PC over PE
or whether cholesterol simply prefers the lower melting transition lipid. In
Figure 10.3
, panel
(c), the transition temperatures of the PC and PE were reversed. In this experiment PE
(18:1,18:1 PE) was the lower melting component while PC (16:0,16:0 PC) was the higher
melting component. Here cholesterol first obliterated the higher melting, PC component.
From these experiments it was concluded that cholesterol associates better with PC than
PE. Later Demel et al.
[8]
extended these studies to compare several of the most common
membrane lipids and reported that cholesterol has the following affinity:
>
;
>
>
PE
This DSC study was an early indication that cholesterol associates more strongly with
SM than with other membrane lipids. A decade later Simons proposed the existence of lipid
rafts, cell signaling membrane domains that are highly enriched in SM and cholesterol
(Chapter 8).
SM
PS
PG
PC
An X-Ray Diffraction Study
Compatibility with cholesterol should depend not only on the type of phospholipid
headgroup but also on the nature of the acyl chains. The structure of cholesterol with its
4 inflexible rings predicts that the sterol may not be compatible with highly flexible, poly-
unsaturated chains. One method that has been used to test this premise involves measuring
cholesterol solubility in membranes by X-ray diffraction (XRD)
[9,10]
. In Chapter 9 it was
noted that a different application of this technique (an electron density profile, Figure 9.5)
has been used to measure membrane thickness. Cholesterol's solubility in membranes can
be determined by measuring radial intensity profiles plotted against reciprocal space
(I-q plots) that show distinct peaks indicating how much cholesterol has been excluded
from the membrane in the form of cholesterol monohydrate crystals. In the experiments
outlined in
Figures 10.4 and 10.5
and summarized in
Table 10.1
, Martin Caffrey and
co-workers made lipid bilayer vesicles from different PC and PE molecular species with
increasing mol fractions of cholesterol. At low levels, cholesterol was accommodated into
the bilayer structure and no second order scattering peaks due to excluded cholesterol
monohydrate crystals could be detected.
However, at some critical concentration, cholesterol exceeds the carrying capacity of the
bilayer and is excluded. The excluded monohydrate crystals are observed in certain regions
of the I-q plots (
Figure 10.4
). The integrated intensities of the scattering peaks 002 (0.3701
˚
1
), 020 (1.033
˚
1
), and 200 (1.044
˚
1
) were combined and plotted against the mol%
cholesterol (
Figure 10.5
). Linear extrapolation of these plots to zero gives an accurate estimate
(
1.0 mol%) of cholesterol solubility limit in bilayers made from various phospholipids. The
results are compiled in
Table 10.1
. This table confirms that PC bilayers can accommodate
