Characterization of a novel binding site for 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]bradykinin on epithelial membranes of guinea pig ileum.

We have recently shown that (a) 125I-Tyr8]bradykinin (BK) recognized bradykinin binding sites in guinea pig epithelium membranes with a Kd value of 1.6 nM and a Bmax of 156 fmol/mg protein, and (b) B2 agonists and some B2 antagonists, such as D-Arg-Hyp3,D-Phe7,Leu8]BK, inhibited this specific binding with a Ki value of 32 nM. In the present study, we have radioiodinated the B2 antagonist Tyr-D-Arg-Hyp3,D-Phe7,Leu8]BK and have performed a full characterization of the binding properties of this tracer in the same membrane preparation. Equilibrium experiments performed in the absence or presence of an excess of BK (10(-5) M) showed that 125I-Tyr-D-Arg-Hyp3,D-Phe7,Leu8]BK specifically labelled two different sites. One of these is the same as the site labelled by 125I-Tyr8]BK, and this indicates that 125I-Tyr-D-Arg-Hyp3,D-Phe7,Leu8]BK interacts specifically with kinin B2 receptors. Equilibrium experiment performed in the presence of an excess of BK (10(-5) M) indicated that specific binding of 125I-Tyr-D-Arg-Hyp3,D-Phe7,Leu8]BK to the second site is also saturable and Scatchard analysis showed that the site is of high affinity with a Kd of 16.8 nM and a Bmax of 2.08 pmol/mg protein. Surprisingly, unlabelled B2 agonists such as bradykinin, Tyr8]BK, Leu8]BK, Hyp3,Tyr8(OMe)]BK, D-Arg-Hyp3]BK and kallidin were found to be inactive on this second site. A series of B2 receptor antagonists, Tyr-D-Arg-Hyp3,D-Phe7,Leu8]BK, D-Arg-Hyp3,D-Phe7,Leu8]BK, D-Arg-Hyp3,Leu5,8,D-Phe7]BK, D-Arg-Hyp3,Gly6,D-Phe7,Leu8]BK and D-Arg-Hyp3,Thi5,8,D-Phe7]BK inhibited 125I-Tyr-D-Arg-Hyp3,D-Phe7,Leu8]BK binding with Ki values of 25.0, 20.9, 15.8, 64.6 and 6606.9 nM respectively. On the other hand, Thi5,8,D-Phe7]BK did not interfere with 125I-Tyr-D-Arg-Hyp3,D-Phe7,Leu8]BK but was found to be a potent inhibitor of 125I-Tyr8]BK binding (Ki = 53.7 nM). As expected, B1 receptor agonists, antagonists and peptides non-related to BK such as substance P, neurokinin A, neurokinin B, angiotensin II, bombesin, vasopressin and the calcitonin gene related peptide were unable to compete with 125I-Tyr-D-Arg-Hyp3,D-Phe7,Leu8]BK. The results show that 125I-Tyr-D-Arg-Hyp3,D-Phe7,Leu8]BK is interacting with two distinct binding sites in the guinea pig epithelium: one is the well known bradykinin B2 receptor and the other is a new, non-characterized binding site that interacts exclusively with some bradykinin receptor antagonists.