Biology Reference
In-Depth Information
most laboratories. As the C-3 hydroxyl group of cholesterol is oxidized to a ketone by the
enzyme cholesterol oxidase, its UVabsorbance increases. Since the enzyme is too large to cross
a membrane, it can only oxidize cholesterol exposed on the outer surface of a cell. Therefore if
cells are mixed with cholesterol oxidase in a quartz cuvette, the UV absorbance will increase
rapidly until all of the outer leaflet cholesterol is oxidized. Subsequent, slower oxidation can
be measured as cholesterol flips from the inner to the outer leaflet. From these measurements,
cholesterol's membrane asymmetry and flip-flop rate can be determined.
Erythrocyte Lipid Asymmetry
Since the 1925 Gorter and Grendel experiment establishing the lipid bilayer, erythrocytes
have been 'the laboratory for membrane studies'. It was in the erythrocyte that lipid asym-
metry was first established.
Figure 9.11
depicts asymmetry of the basic erythrocyte lipids.
The choline-containing lipids (PC and SM) are predominantly in the membrane outer leaflet,
while the primary amine lipids (PE and PS) are found predominantly in the membrane inner
leaflet. PS is unusual in being 100% asymmetrically distributed to the inner leaflet. This is
attributed to the presence of an ATP-dependent flipase
[12
14]
whose function it is to take
any outer leaflet PS and immediately flip it back to the inner leaflet, preventing accumulation
of outer leaflet PS. PA, PI, and PI 4,5 bis phosphate are predominantly located in the inner
leaflet, while cholesterol is mainly associated with the outer leaflet, although its rapid flip-
flop rate would indicate it moves back and forth across the membrane spending more
time in the outer leaflet. The sugar-containing gangliosides are always (100% asymmetric)
in the outer leaflet. CL is evenly distributed across the membrane.
e
Exoplasmic
25
SM
PC
PS
PE
PI
CI
0
Cytosolic
25
FIGURE 9.11
Distribution of common membrane lipids between the inner and outer leaflets of erythrocytes.
Note the choline-containing lipids (SM and PC) are predominantly found in the outer (exoplasmic) leaflet, while the
primary amine lipids (PE and PS) are associated with the inner (cytoplasmic) leaflet. Also, PI is found primarily on
the inner leaflet while CL is equally distributed between the leaflets. Courtesy of TISTORY
