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J. Biol. Chem., Vol. 259, Issue 20, 12431-12436, Oct, 1984
PJ Leigh, WA Cramp and J MacDermot
Evidence has been obtained for a specific protein receptor for prostacyclin
on cells of the NCB-20 somatic hybrid. A new stable prostacyclin analog,
5-[(E)-(1S,5S,6R,7R) - 7 - hydroxy-6-[(E) - (3S,4RS) -3-hydroxy-4-methyl-1
-octen-6-inyl]bicyclo[3.3.0]-octan-3- ylidene]pentanoic acid (Iloprost,
ZK36374) activates adenylate cyclase of NCB-20 cell membranes to an extent
similar to prostacyclin and with a comparable high affinity. The binding of
[3H]Iloprost to NCB-20 membranes was rapid with an association rate
constant (k+1) of 2.01 X 10(5) M-1 s-1 at 20 degrees C. The rate constant
for the dissociation of the ligand-receptor complex (k-1) was 1.19 X 10(-3)
s-1, giving a dissociation constant (k-1/k+1) of 5.9 nM. The equilibrium
dissociation constant was 29.9 nM, and the membranes had a maximum binding
capacity of 347 fmol mg-1 protein. Radiation inactivation has been employed
to determine the molecular weights of the functional prostacyclin receptor
and components of the adenylate cyclase system in the plasma membrane of
the NCB-20 cells. Cell membranes were lyophilized prior to irradiation,
which lead to the formation of high-molecular-weight aggregates. The
aggregation was avoided, however, when membranes were prepared in an
isotonic Tris-HCl buffer containing sucrose. Molecular weight values of
111,000 for the catalytic subunit of adenylate cyclase, 89,000 for the
regulatory subunit, and 83,000 for the prostacyclin receptor were obtained.
Loss of [3H]Iloprost binding capacity after irradiation of lyophilized
membranes yielded a molecular weight value (mean +/- S.E.) for the
prostacyclin receptor of 82,800 +/- 12,900 (n = 3).
Identification of the prostacyclin receptor by radiation inactivation
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