Biomedical Engineering Reference
In-Depth Information
Several unconjugated amphiphilic anions are also transported by ABCC1 inde-
pendently of GSH, such as folate and its antimetabolite methotrexate
76
,
97
as well
as the pentaanionic fluorescent dye fluo-3 and the monoanionic
p
-aminohippurate
(PAH).
98
Several inhibitors of ABCC1-mediated transport have been described, al-
though their specificity for ABCC1 is not very well defined and they may also inhibit
other ABCC transporters. Structural analogs of cysteinyl leukotrienes, mostly devel-
oped as leukotriene D
4
receptor antagonists such as the quinoline derivative MK571,
are potent inhibitors of ABCC1-mediated ATP-dependent LTC
4
transport, with a
K
i
value of 0.6
M for MK571.
78
However, MK571 also inhibits the ATP-dependent
transport mediated by other ABCC members (i.e., ABCC2 and ABCC4).
41
,
53
Cy-
closporin A inhibits the ABCC1-mediated ATP-dependent LTC
4
transport, with a
K
i
value of 5
μ
M
78
and the ABCC2-mediated ATP-dependent transport of monoglu-
curonosyl bilirubin with a
K
i
value of 21
μ
M.
104
However, cyclosporin A is a known
MDR1 P-glycoprotein inhibitor.
133
Its nonimmunosuppressive derivative PSC833,
also an inhibitor of MDR1 P-glycoprotein, is a relatively weak inhibitor of ABCC1-
mediated transport.
78
Several tricyclic isoxazoles have been described to be potent
and specific inhibitors of the ABCC1-mediated ATP-dependent transport in a GSH-
dependent manner, in particular LY475776 and LY402913.
134
−
137
At present, it is
known that these compounds do not inhibit ABCC2 and ABCC3, and can therefore
for the time being, be considered as ABCC1 specific.
μ
11.4.2. ABCC2
Before the molecular identification of ABCC2, the function and substrate specificity
of rat Abcc2 were studied by comparison of normal and hyperbilirubinemic mutant
rats: the Eisai hyperbilirubinemic (EHBR) rats and the GY/TR
−
mutant rats.
8
,
36
,
138
These mutant rats are deficient in the secretion of anionic conjugates into bile
37
−
40
because they lack a functional Abcc2 in the hepatocyte canalicular membrane.
8
,
41
,
42
ATP-dependent transport measurements using inside-out-oriented canalicular mem-
brane vesicles from these rats
33
−
36
,
139
contributed retrospectively to our knowledge
on the substrate specificity of Abcc2.
8
,
140
,
141
In addition, the ABCC2 substrate speci-
ficity has been explored more extensively in inside-out membrane vesicles from cells
stably expressing recombinant human ABCC2
70
,
105
,
141
and with ABCC2 purified to
homogeneity.
142
Its substrate specificity comprises several endogenous substrates that
are also prototypic for ABCC1, especially LTC
4
as a high-affinity substrate,
70
as well
as other conjugates with glutathione, glucuronate, or sulfate, such as DNP-SG,
S
-
glutathionyl ethacrynate, mono- and bisglucuronosyl bilirubin, E
2
17
G, lithocholyl-
taurine sulfate, chenodeoxycholyltaurine sulfate, and E
1
3S.
70
,
104
−
106
In addition, sev-
eral unconjugated anionic substances have been identified as ABCC2 substrates using
inside-out membrane vesicles, such as bromosulfophtalein (BSP), PAH, ochratoxin
A, the sulfated cholecystokinin octapeptide CCK-8, and methotrexate.
97
,
98
,
107
,
143
Al-
though the substrate specificity and transport efficiency of ABCC2 and ABCC1 are
similar, differences in the kinetic properties have been established, for example the
K
m
values of ABCC2 for LTC
4
and E
2
17
β
β
G are 10- and fivefold higher, respec-
tively, than those for ABCC1.
70
In contrast, ABCC2 shows a higher affinity for
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