| Abstrakt: |
Chemical cross-linking was used to analyze the binding sites for the agonist bradykinin (BK) and the antagonists NPC17731 and HOE140 on the bovine B2 bradykinin receptor. [3H]BK and [3H]NPC17731 bound with high affinity to the same B2 receptor in bovine myometrial membranes as determined by the total number of specific binding sites and pharmacological specificity of the binding of these two radioligands. Cross-linking experiments were done using a series of bifunctional reagents reactive either primarily to amines (homobifunctional) or reactive to amines in one end and to sulfhydryls in the opposite end (heterobifunctional). All the heterobifunctional reagents plus the homobifunctional arylhalide 1,5-difluoro-2,4-dinitrobenzene were effective in cross-linking the [3H]BK N terminus specifically to a sulfhydryl in the receptor, and this cross-linking occurred at 5-100 μM reagent. In contrast, the homobifunctional N-hydroxysuccinimide ester reagents, at ≤1 mM, were only able to cross-link [3H]BK to membrane proteins nonspecifically. The sulfhydryl reagents N-ethylmaleimide, iodoacetamide, and phenylarsine oxide blocked cross-linking, whereas these reagents did not inhibit reversible specific [3H]BK binding. Immunoblotting with anti-BK antiserum revealed that low concentrations of BK (5-50 nM) were cross-linked to a receptor-specific species of 65 kDa. All cross-linking of [3H]NPC17731 was nonspecific with both homobifunctional and heterobifunctional reagents. The 65-kDa receptor-specific species was observed on anti-HOE140 immunoblots, but only when proteins were cross-linked with very high concentrations of HOE140 (≥500 nM). Our results provide direct biochemical evidence that the binding site for the agonist BK in the bovine B2 receptor is adjacent to a cysteine and is differentiated from the binding site(s) for the antagonists NPC17731 and HOE140. |
