Cholecystokinin (pancreozymin). 5. Hormonally active desamino derivative of Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2

How a single inversion of configuration leads to a reversal of the binding mode: proposal of a novel arrangement of CCK2 ligands in their receptor, and contribution to the development of peptidomimetic or non-peptide CCK2 ligands

The implication of CCK(2) receptors in crucial physiological functions has driven the search for synthetic ligands of this receptor. A notable rationale starting from CCK-4 (minimal endogenous CCK(2) agonist), the 'dipeptoid' strategy, led to potent CCK(2) antagonists exemplified by CI-988. However, careful examination of the literature enlightened several incompatibilities between the proposed recognition mode of the receptor by such compounds (or peptide analogues) and experimental data. Thus, we hypothesised that CCK(2) 'dipeptoid' antagonists bind the receptor in a mode opposite to that previously suggested. The reexamination of numerous published data, supported by the characterisation of new 'hybrid' compounds, brought out strong evidence that this 'reverse' mode truly characterises CCK(2) 'dipeptoid' antagonists. These findings renew the perspectives of further chemical development of CCK(2) ligands, e.g. non-peptidic agonists.

Towards understanding the role of the first extracellular loop for the binding of peptide hormones to G-protein coupled receptors

By a combination of molecular modelling and site-directed mutagenesis studies, we have recently identified a key residue in the first extracellular loop which determines agonist selectivity and high-affinity binding in the V1a vasopressin receptor. Based on primary sequence analysis and structure-activity relationship studies of other neuropeptides and their receptors, the corresponding amino acid in the first extracellular loop is proposed to play a homologous role in conferring affinity and selectivity. This would seem to be the case notably for angiotensin, cholecystokinin, neuropeptide Y and neurokinin receptors.

Cholecystokinin suppresses food intake in cats: structure-activity characterization

Our experimental models in this study were cats fitted with gastric fistulae. Intravenous infusion of sulfated CCK 8 and nonsulfated Boc CCK 7 inhibited both sham-feeding and feeding in fasted cats. The threshold dose (1.2 pmol/kg.hr) required for inhibition of sham-feeding was identical to that required to inhibit feeding in the same animals. However, the gastric secretory studies indicated that this dose was 90 times lower than the threshold dose stimulating gastric acid secretion (109 pmol/kg.hr). In nonfasted animals, sulfated CCK 8 and nonsulfated Boc CCK 7 (219 and 875 pmol/kg.hr) are both capable of decreasing the food intake at different intervals following the infusion with no significant effect on daily food intake. Our findings clearly show that there is no difference in the sensitivity of CCK's ability to inhibit sham-feeding and feeding, suggesting that CCK's suppressive effect on food intake does not solely involve gastric distension mechanisms. In contrast to gastric acid secretion, the sulfate group is not a "restrictive" factor for peripherally-induced CCK satiety.

Preparation and characterization of a new cholecystokinin receptor probe that can be oxidatively radioiodinated

Oxidative iodination is the simplest, most efficient, and least expensive method for radiolabeling peptide hormones. It has not been applied to cholecystokinin (CCK), however, because this peptide lacks an unsubstituted tyrosine residue and oxidation abolishes its biologic activity. We report the synthesis, purification, and characterization of the first derivative of CCK that can be radioiodinated with an oxidative method while maintaining full biologic activity and receptor binding affinity. Boc-Tyr-[(Thr28, Nle31)CCK-25-33] was synthesized, iodinated using a solid-phase oxidant, and purified on reverse-phase high-pressure liquid chromatography to a specific activity of 2125 Ci/mmol. Both native and iodinated Boc-Tyr-[(Thr28, Nle31)CCK-25-33] exhibited efficacy and potency for stimulation of in vitro pancreatic enzyme secretion that were identical to CCK-8. Binding of Boc-125I-Tyr-[(Thr28, Nle31)CCK-25-33] to rat pancreatic plasma membranes was rapid, reversible, temperature-dependent, saturable, and specific. The abilities of various molecular forms of CCK to compete for this binding paralleled their potencies for stimulation of pancreatic secretion (Kd: CCK-8, 0.8 nM; CCK-33, 3 nM; CCK-8DS, 1 microM; CCK-4, 50 microM), whereas structurally unrelated pancreatic ligands (1 microM) demonstrated no significant competition. These findings suggest that Boc-125I-Tyr-[(Thr28, Nle31)CCK-25-33] interacts with the same high-affinity pancreatic receptor as the CCK receptor probes previously reported, i.e., the Bolton-Hunter-labeled CCK-33 and CCK-8. This probe also possesses the major advantages of simplicity, efficiency, and cost of the labeling procedure, and the stability and relative oxidation-resistance of the product. Boc-125I-Tyr-[(Thr28, Nle31)CCK-25-33] will be useful for the characterization of binding interactions between CCK and its target tissues.

Synthesis of analogues of the Des-Phe-NH2 C-terminal hexapeptide of cholecystokinin showing gastrin antagonist activity

Four analogues of Z-CCK-27-32-NH2, Z-Tyr(SO3-)-Met-Gly-Trp-Met-Asp-NH2, a cholecystokinin receptor antagonist have been synthesized by solution methodology. In these analogues, Z-Tyr(SO3-)-Nle-Gly-Trp-Met-Asp-NH2 16, Z-Tyr(SO3-)-Nle-Gly-Trp-Nle-Asp-NH2 17, BOC-Tyr(SO3-)-Met-Gly-Trp-Met-Asp-NH2 24 and Boc-Tyr(SO3-)-Met-Gly-Trp-Nle-Asp-NH2 25 methionyl residues were replaced by norleucyl residues. Preliminary biological activity on gastrin-induced acid secretion, in rat, are reported. These derivatives proved to antagonize the action of gastrin, with ED 50 of between 0.5 and 3 mg/kg.

Evidence for dopaminomimetic effect of intrastriatally injected cholecystokinin octapeptide in mice

The behavioural effect of intrastriatally injected cholecystokinin sulphated octapeptide (CCK-8S), and its interactions with the antagonists Z-CCK-(27-32)NH2 and proglumide, were investigated in mice. When injected into the right striatum, CCK-8S (0.05-1 ng) induced contralateral rotations, as did the dopamine agonist apomorphine. Non-sulphated CCK-8 was inactive and sulphated desamino-CCK-7 was only weakly active in this respect. CCK-8S-induced turning was antagonized by co-injected Z-CCK-(27-32)NH2 (0.01-10 ng) or proglumide (0.1-1 micrograms), as well as by intraperitoneal injection of the neuroleptic drug haloperidol. These data suggest that CCK-8S may, in these conditions, stimulate dopamine-mediated neurotransmission, and that Z-CCK-(27-32)NH2, in addition to its peripheral effect, is also a very potent CCK antagonist at the striatal level.

Preparation of an N-acetyl-octapeptide of cholecystokinin. The role of N-acetylation in protecting the octapeptide from degradation by smooth muscle tissues

The C-terminal octapeptide of cholecystokinin (CCK-8) was acetylated on its lone N-terminal amino group using acetic anhydride in N,N-dimethylformamide. The acetylated derivative (Ac-CCK-8) and unreacted CCK-8 were separated from acetic anhydride and other reaction products by fractionation on Sephadex LH-20. Final purification was by thin-layer isoelectric focusing in a pH 2.5--4.0 gradient. The immunochemical properties of the octapeptide were unaffected by acetylation as measured by radioimmunoassay. The N-acetylated-octapeptide was equally as effective as unmodified CCK-8 in producing concentration-dependent isometric tension development in isolated cat gallbladder strips. Acetylation did, however, protect CCK-8 from N-terminal degradation by soluble peptidases that eluted from gallbladder and other smooth muscle tissues of the cat. Unmodified CCK-8 was degraded rapidly in the presence of these tissues and in buffers previously exposed to the same tissues. In contrast, the Ac-CCK-8 was resistant to N-terminal degradation under the same conditions. Degradation of CCK-8 from its N-terminus produces biologically inactive derivatives and could adversely affect in vitro studies. Since the acetylated-CCK-8 retained full biological and immunological activity, its use would eliminate the effect of extracellular proteolysis on CCK-8 action.

The importance of the amino acid in position 32 of cholecystokinin in determining its interaction with cholecystokinin receptors on pancreatic acini

In the C-terminal heptapeptide of cholecystokinin, replacement of the penultimate residue, aspartic acid, by beta-alanine caused a 300-fold decrease in the potency with which the peptide stimulated enzyme secretions, whereas replacement by glutamic acid caused a 1000-fold decrease in potency. The beta-alanine-substituted peptide was approximately ten times more potent when the n terminus was blocked with t-butyloxycarbonyl than when it was blocked with benzyloxycarbonyl, and the glutamic acid-substituted peptide was approximately twice as potent when the N terminus was blocked with t-butyloxycarbonyl than when it was blocked with benzyloxycarbonyl. Changes in the ability of the peptide to stimulate amylase secretion were accompanied by corresponding changes in the ability of the peptide to inhibit binding of 125I-labeled cholecystokinin. The magnitude of stimulation of enzyme secretion caused by a maximally effective peptide concentration was the same with each analogue as it was with the unaltered peptide. Replacing the aspartyl residue by beta-alanine or glutamic acid or replacing the N-terminal t-butyloxycarbonyl moiety by benzyloxycarbonyl caused an equivalent decrease in the ability of the peptide to stimulate enzyme secretion and its ability to cause residual stimulation of enzyme secretion. In contrast, the N-terminal desamino analogue of cholecystokinin heptapeptide was ten times less potent than the unaltered peptide in stimulating amylase secretion, but 100 times less potent than the unaltered peptide in causing residual stimulation of enzyme secretion.