INHIBITION OF RECEPTOR G PROTEIN COUPLING BY SURAMIN ANALOGS/

Suramin analogues act as direct antagonists of heterotrimeric G protei ns because they block the rate-limiting step of G protein activation ( i.e., the dissociation of GDP prebound to the G protein alpha subunit) . We have used the human brain A, adenosine receptor and the rat stria tal D-2 dopamine receptor, two prototypical G(i)/G(o)-coupled receptor s, as a model system to test whether the following analogues suppress the receptor-dependent activation of G proteins: 8-(3-nitrobenzamido)- 1,3,5-naphthalenetrisulfonic acid (NF007), benzamido)-benzamido)-1,3,5 -naphthalenetrisulfonic acid (NF018); o-3,1-phenylene))bis-(1,3,5-naph thalenetrisulfonic acid) (NF023); carbonylbis(imino-3,1-phenylene)carb onylimino-(3,1 -phenylene))bis-(1,3,5-naphthalenetrisulfonic acid) (NF 037); and suramin. Suramin and its analogues inhibit the formation of the agonist-specific ternary complex (agonist/receptor/G protein). Thi s inhibition is (i) quasicompetitive with respect to agonist binding i n that it can be overcome by increasing receptor occupancy but (ii) do es not result from an interaction of the analogues with the ligand bin ding pocket of the receptors because the binding of antagonists or of agonists in the absence of functional receptor/G protein interaction i s not affected. In addition to suppressing the spontaneous release of GDP from defined G protein alpha subunits, suramin and its analogues r educe receptor-catalyzed guanine nucleotide exchange. The site, to whi ch suramin analogues bind, overlaps with the docking site for the rece ptor on the G protein alpha subunit. The structure-activity relationsh ips for inhibition of agonist binding to the A(1) adenosine receptor ( suramin > NF037 > NF023) and of agonist binding to the inhibition D-2 dopamine receptor (suramin = NF037 > NF023 > NF018) differ. Thus, NF03 7 discriminates between the ternary complexes formed by the agonist-li ganded D-2 dopamine receptors and those formed by the A(1) adenosine r eceptor with > 10-fold selectivity. Therefore, our results also show t hat inhibitors can be identified that selectively uncouple specific re ceptor/G protein tandems.