The widespread neuropeptide vasoactive intestinal peptide (VIP) has two receptors VPAC 1 and VPAC 2 . Solid-phase syntheses of VIP analogs in which each amino acid has been changed to alanine (Ala scan) or glycine was achieved and each analog was tested for: (i) threedimensional structure by ab initio molecular modeling; (ii) ability to inhibit 125 ]VIP analog which constitutes the first highly selective (>1,000-fold) human VPAC 1 receptor agonist derived from VIP ever described. The vasoactive intestinal peptide (VIP)1 is a prominent neuropeptide with wide distribution in both peripheral and central nervous systems and a large spectrum of biological actions in mammals (1, 2). VIP-containing nerves and VIP effects have been described in digestive tract, cardiovascular system, airways, reproductive system, immune system, endocrine glands, and brain (1). Besides its short-term actions on exocrine secretions, hormone release, muscle relaxation, and metabolism (1, 2), VIP has been also characterized as a growth regulator for fetuses and tumor cells and during embryonic brain development (3). There are recent evidences for an important role of VIP in the perception of pain (4) and suppression of inflammation (5). Finally, VIP has been involved in diseases such as the watery diarrhea syndrome and clinical applications of VIP have been already suggested in impotence, asthma, lung injury, a variety of tumors and neurodegenerative diseases (1-3).VIP belongs to a large family of structurally related peptides (2, 6, 7) that comprises VIP, pituitary adenylate cyclase-activating peptide PACAP-27, and its C-terminal extended form PACAP-38, secretin, glucagon, and glucagon-like peptides-1 and -2, gastric inhibitory polypeptide, peptide histidine methionine amide, growth hormone-releasing factor (GRF), and peptides isolated from the venom of the Gila Monster. VIP and PACAP are the most closely related peptides in terms of structure and function (2, 6). They share two common receptors, VPAC 1 and VPAC 2 , which display high affinity for both VIP and PACAP (2,8). These receptors together with receptors for VIP-related peptides (see above) clearly constitute an original subfamily within the superfamily of G protein-coupled receptors (2, 9, 10). This subfamily referred to as class II (2) also comprises receptors for parathyroid hormone, calcitonin, corticotropin-releasing factor, and the so called EGF-TM7 receptors (11). Class II family of receptors for peptides display several common properties including large N-terminal extracellular domains containing highly conserved cystein residues, N-terminal leader sequences, and complex gene organization with many introns (2).Although the structure-function relationship of VIP receptors, including VPAC 1 and VPAC 2 , has been recently documented (2,9,(12)(13)(14)(15)(16)(17)(18)(19)(20), the structure-function relationship of VIP itself is still poorly understood. Some old studies carried out before the characterization and cloning of VIP receptor subtypes (21-23) indicated that: (i) th...

In the present study, hepta- and octapeptide analogues of the C-terminal part of cholecystokinin, modified on the C-terminal phenylalanine residue, were synthesized. CCK analogues were prepared in which the peptide bond between aspartic acid and phenylalanine had or had not been modified and were lacking the C-terminal primary amide function. These CCK derivatives were able to cause full stimulation of amylase release from rat pancreatic acini but without a decrease in amylase release at supramaximal concentrations. There was a close relationship between the abilities of these derivatives to stimulate amylase release and their abilities to inhibit binding of 125I-BH-CCK-9 to CCK receptors on rat and guinea pig pancreatic acini. These CCK analogues were also able to recognize the guinea pig brain CCK receptors, some of them being particularly potent. The findings indicate that the aromatic ring of phenylalanine is important for the binding to brain and pancreatic CCK receptors, whereas the C-terminal primary amide function is not essential for the binding to pancreatic CCK receptors but is crucial for biological activity of rat pancreatic acini.

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ACE (angiotensin-converting enzyme; peptidyl dipeptidase A; EC 3.4.15.1), cleaves C-terminal dipeptides from active peptides containing a free C-terminus. We investigated the hydrolysis of cholecystokinin-8 [CCK-8; Asp-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2] and of various gastrin analogues by purified rabbit lung ACE. Although these peptides are amidated at their C-terminal end, they were metabolized by ACE to several peptide fragments. These fragments were analysed by h.p.l.c., isolated and identified by comparison with synthetic fragments, and by amino acid analysis. The initial and major site of hydrolysis was the penultimate peptide bond, which generated a major product, the C-terminal amidated dipeptide Asp-Phe-NH2. As a secondary cleavage, ACE subsequently released di- or tri-peptides from the C-terminal end of the remaining N-terminal fragments. The cleavage of CCK-8 and gastrin analogues was inhibited by ACE inhibitors (Captopril and EDTA), but not by other enzyme inhibitors (phosphoramidon, thiorphan, bestatin etc.). Hydrolysis of [Leu15]gastrin-(14-17)-peptide [Boc (t-butoxycarbonyl)-Trp-Leu-Asp-Phe-NH2] in the presence of ACE was found to be dependent on the chloride-ion concentration. Km values for the hydrolysis of CCK-8, [Leu15]gastrin-(11-17)-peptide and Boc-[Leu15]gastrin-(14-17)-peptide at an NaCl concentration of 300 mM were respectively 115, 420 and 3280 microM, and the catalytic constants were about 33, 115 and 885 min-1. The kcat/Km for the reactions at 37 degrees C was approx. 0.28 microM-1.min-1, which is approx. 35 times less than that reported for the cleavage of angiotensin I. These results suggest that ACE might be involved in the metabolism in vivo of CCK and gastrin short fragments.