Biomedical Engineering Reference
In-Depth Information
OUT
IN
classical
pump
vacuum
cleaner
flippase
(a)
OUT
IN
high P
lip
low K
d
low P
lip
high K
d
(b)
FIGURE 10.2.
(
a
) Classical pump, vacuum cleaner, and flippase models of Pgp action. Clas-
sical pumps, such as lactose permease, transport a polar substrate from the aqueous phase
on one side of the membrane to the aqueous phase on the other side. The substrate does not
come into contact with the lipid bilayer and moves through a hydrophilic path formed by the
TM regions of the protein. In the vacuum cleaner model, drugs (both substrates and modu-
lators) partition into the lipid bilayer and interact with Pgp within the membrane. They are
subsequently effluxed into the aqueous phase on the extracellular side of the membrane. In the
flippase model, drugs partition into the membrane, interact with the drug-binding pocket in
Pgp (which is located within the cytoplasmic leaflet) and are then translocated, or flipped, to
the outer membrane leaflet. Drugs will be present at a higher concentration in the outer leaflet
compared to the inner leaflet, and an experimentally measurable drug concentration gradient is
generated when drugs partition rapidly from the two membrane leaflets into the aqueous phase
on each side of the membrane. (
b
) The effect of membrane partitioning on drug-binding to Pgp.
The binding affinity of Pgp for a particular substrate or modulator (
K
d
) is related to the lipid-
water partition coefficient of the drug (
P
lip
). A drug with a high value of
P
lip
(left side of the
figure) will accumulate to a high concentration within the membrane. This will favor binding
to Pgp and result in a low apparent
K
d
value. In contrast, a drug with a low value of
P
lip
(right
side of the figure) will have a lower membrane concentration and a high apparent
K
d
value.
but not if added to intact cells.
156
Similarly, some peptide modulators cannot interact
with Pgp in intact cells if supplied in the extracellular medium but can do so in mem-
brane vesicles, where they can reach the cytoplasmic leaflet.
157
More recently, a FRET
approach estimated the distance separating bound H33342 from a fluorescent probe
linked covalently to the catalytic sites, and the results clearly place the drug-binding
Search WWH ::
Custom Search