Glibenclamide binding to sulphonylurea receptor subtypes: dependence on adenine nucleotides

1: ATP-sensitive K(+) channels are composed of pore-forming subunits Kir6.2 and of sulphonylurea receptors (SURs); the latter are the target of the hypoglycaemic sulphonylureas like glibenclamide. Here, we report on the negative allosteric modulation by MgATP and MgADP of glibenclamide binding to SUR1 and to SUR2 mutants with high glibenclamide affinity, SUR2A(Y1206S) and SUR2B(Y1206S). 2: ATP, in the presence of an ATP-regenerating system to oppose hydrolysis during incubation, inhibited glibenclamide binding to SUR1 and SUR2B(Y1206S) by approximately 60%, to SUR2A(Y1206S) by 21%). Inhibition curves for the SUR2(Y1206S) isoforms were monophasic with IC(50) values of 5-10 microM; the curve for SUR1 was biphasic (IC(50) values 4.7 and 1300 microM). 3: Glibenclamide inhibition curves for ADP, performed in the presence of an ATP-consuming system to oppose ATP formation from ADP, were generally shifted rightwards and showed positive cooperativity, in particular with the SUR2(Y1206S) isoforms. 4: In the absence of the coupled enzyme systems, inhibition curves of MgATP or MgADP were generally shifted leftwards. This indicated synergy of MgATP and MgATP in acting together. 5: Coexpression of SUR1 and SUR2B(Y1206S) with Kir6.2 reduced both potency and efficacy of ATP in inhibiting glibenclamide binding; this was particularly marked for Kir6.2/SUR1. 6: The data show (a) that the inhibitory effects of ATP and ADP on glibenclamide binding differ from one another, (b) that they depend on the SUR subtype, and (c) that they are weakened by coexpression with Kir6.2.