Elsevier

Peptides

Volume 14, Issue 3, May–June 1993, Pages 621-627
Peptides

Article
Interaction of ovine pituitary adenylate cyclase-activating peptide (PACAP-38) with rat lung membranes

https://doi.org/10.1016/0196-9781(93)90154-9Get rights and content

Abstract

The binding of ovine pituitary adenylate cyclase-activating peptide (PACAP-38) to rat lung membranes was investigated using [125I]PACAP-38 as radioligand. Binding was rapid at 37°C, reversible, saturable, and time, concentration, and temperature dependent. Kinetic parameters derived from saturation experiments revealed a Kd = 100 ± 15 pM, Bmax = 310 ± 36 fmol/mg protein, and a Hill slope factor (nH) of 1.17 ± 0.12. Various chemically synthesized analogues of PACAP-38, as well as related peptides, were tested for their ability to displace [125I]PACAP-38. Of those that had an IC50 < 0.2 μM, the following order of potency was determined: PACAP-38 (IC50 = 25 nM) ≥ [Ile2]PACAP-38(IC50 = 31 nM) >PACAP-27(IC50 = 54 nM) >[Tyr1]PACAP-38(IC50 = 104 nM) >GHRH(1–29)NH2(IC50 = 108 nM) >PHI(IC50 = 181 nM) >[Ser2]PACAP(2–38)(IC50 = 198 nM). Glucagon, PHM, secretin, and GIP exhibited little affinity in the same binding assay. Vasoactive intestinal peptide (VIP) had an IC50 in excess of 1 μM. When [125I]VIP was used as radioligand, PACAP-27 had an IC50 = 0.2 nM >PACAP-38 (IC50 = 0.5 nM) >VIP (IC50 = 16 nM). A novel analog of PACAP-38, [4-Cl-d-Phe6,Leu17]PACAP-38, was able to displace [125I]VIP very efficiently (IC50 = 1 nM), but had little potency in displacing [125I]PACAP-38 (IC50 = 320 nM). The results presented suggest that PACAP-38 binds to single, noninteracting binding sites on rat lung membranes that are distinct from those for VIP, but could be shared with PACAP-27; PACAP-38 can also efficiently bind to the VIP receptor(s), but VIP has much less potency toward the PACAP-38 receptor. Moreover, modification of the PACAP-38 molecule, either through chain length or amino acid residue substitutions, results in apparent decreased binding.

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