Interaction of lipophilic VIP derivatives with recombinant VIP1/PACAP and VIP2/PACAP receptors

Stearyl vasoactive intestinal polypeptide has been reported to be a VIP (vasoactive intestinal polypeptide) receptor agonist of high potency with an original bioavailability and action. We synthesized three fatty acyl derivatives, myristyl-, palmityl- and stearyl-[Nle17]VIP, and tested their capacity to recognize recombinant rat- and human VIP1- and VIP2/PACAP (pituitary adenylate cyclase-activating polypeptide) receptors and to stimulate adenylate cyclase activity. The three lipophilic analogues bound with high affinity (from 0.5 to 20 nM) to both receptor subtypes but did not distinguish between them. In preparations expressing a high density of human VIP1/PACAP receptors, the three lipophilic analogues had the same efficacy as VIP and [Nle17]VIP. In preparations expressing the rat receptors, stearyl-[Nle17]VIP had a lower efficacy than the other peptides tested. In preparations expressing a low level of VIP1/PACAP receptors and in those expressing VIP2/PACAP receptors, all analogues behaved like partial agonists. The lowest efficacy was observed for stearyl-[Nle17]VIP on the VIP2/PACAP receptor subclass. Based on our results, a complex pattern of in vivo biological effects of the lipophilic VIP derivatives should be expected: these compounds might behave as full agonists, partial agonists, or antagonists of the VIP response, depending on the number and the subtype of receptor expressed.

Vasoactive intestinal peptide modification at position 22 allows discrimination between receptor subtypes

Secretin and growth hormone releasing factor (GRF) have a weak affinity for VIP (vasoactive intestinal peptide)/PACAP (pituitary adenylate cyclase activating polypeptide) receptors, but discriminate between VIP1/PACAP and VIP2/PACAP receptors. This previously allowed us to develop modified secretin and GRF derivatives as high affinity and highly selective VIP1/PACAP receptor ligands. We tested the hypothesis that the presence of a Gln residue at position 24 and a Leu residue at position 22 was responsible for their VIP1/PACAP receptor selectivity. [Gln24]VIP was not different from VIP but [Leu22]VIP had a 100-fold lower affinity for VIP2/PACAP receptors as compared to VIP1/PACAP receptors. The substitution of Tyr22 by Phe22 in VIP had no significant effect on the recognition of both receptors but [Ala22]VIP had a reduced affinity for the VIP2/PACAP receptor. This indicated that an aromatic residue at position 22 of VIP was required for a high affinity for the VIP2/PACAP receptor but not for the VIP1/PACAP receptor.

C-terminally shortened pituitary adenylate cyclase-activating peptides (PACAP) discriminate PACAP I, PACAP II-VIP1 and PACAP II-VIP2 recombinant receptors

Pituitary adenylate cyclase-activating polypeptide (PACAP) analogues were tested for their ability to occupy the recombinant selective PACAP receptors (PACAP type I receptors) and the non-selective PACAP-vasoactive intestinal polypeptide (VIP) receptors (PACAP type II, VIP1 and PACAP type II, VIP2 receptors), stably transfected and expressed in Chinese hamster ovary cells. Their capacity to stimulate the adenylate cyclase activity was also measured. The synthetic analogues tested were peptides shortened at the carboxyl terminus by the removal of 1-4 amino acids (PACAP-26 to PACAP-23). All the peptides discriminated the 3 receptor subtypes and had the highest affinity for the VIP1 receptors, and the lowest affinity for the VIP2 receptors; PACAP-25 having the highest ability to discriminate the VIP1 and VIP2 receptors. All the peptides tested were full agonists on the PACAP I and VIP1 receptors; PACAP-25 and -26 were partial agonists on VIP2 receptors and may be appropriate tools to establish the receptor subtype involved in a given cellular response.

Fragments of pituitary adenylate cyclase activating polypeptide discriminate between type I and II recombinant receptors

Pituitary adenylate cyclase activating polypeptide (PACAP) analogues were tested for their ability to occupy the recombinant selective PACAP receptors (PACAP type I receptor) or the non-selective PACAP-vasoactive intestinal polypeptide (VIP) receptors (PACAP type II, VIP1 and VIP2 receptors) stably transfected and expressed in Chinese hamster ovary (CHO) cells. The synthetic analogues consisted of N- and/or C-terminally shortened peptides. Thus, peptides starting at amino acid 1, 2, 3 or 6 and terminating at amino acid 27, 29, 30, 32 or 38 were compared on the three receptors studied. The shortening of PACAP-(1-38) to PACAP-(1-27) was of little influence. However, in N-terminally deleted peptides the PACAP-38 derivatives were of higher affinity than the PACAP-27 fragments on PACAP type I and PACAP type II, VIP2 receptors but not on PACAP type II, VIP1 receptors. The presence of the sequence 28-32 was in all cases sufficient to reproduce the data obtained with the PACAP-38 analogues. PACAP-(3-32) is able to discriminate the PACAP type II, VIP2 subtype from the other two subtypes, and PACAP-(6-30), PACAP-(6-32) and PACAP-(6-38) can discriminate the PACAP type II, VIP1 receptors from the other two subtypes. These molecules may help in the quantitative detection of receptor subclasses in complex systems when two or more receptor subtypes are found.

Purification and sequence determination of a new muscarinic toxin (MT4) from the venom of the green mamba (Dendroaspis angusticeps)

A toxin which partially inhibited [3H]N-methylscopolamine binding to rat brain muscarinic receptors was purified from the venom of green mamba, Dendroaspis angusticeps. The N-terminal sequence (up to 45 amino acids) was determined by automated Edman degradation of the whole molecule. The complete sequence was elucidated after enzymatic cleavage with endoproteinase Arg-C or endoproteinase Lys-C and peptide fragments purification. The identity of the C-terminal amino acid was confirmed by hydrazinolysis. The new toxin (MT4) had eight half-cystines and 66 amino acids. It differed from muscarinic toxin MT1 by a single substitution in position 57 (arginine in MT1, histidine in MT4), proximal to the sixth half-cystine.

Purification and amino acid sequence of vasoactive intestinal peptide, peptide histidine isoleucinamide (1-27) and secretin from the small intestine of guinea pig

The neuropeptides vasoactive intestinal peptide (VIP) and peptide histidine isoleucinamide (1-27) (PHI) and the hormone secretin were purified from the small intestine of guinea pig, being detected by radioimmunoassay and radioreceptor assay throughout six to seven chromatographic steps. After elution on a reverse-phase C18 column, the three peptides were separated on a Fractogel column. After cation-exchange chromatography of each peptide on Mono S, the final steps were performed using a reverse-phase RP8-e column. Guinea pig PHI differed from porcine PHI in having Tyr and Arg residues instead of Phe and Lys in, respectively, position 10 and 20. We confirmed the original sequence of guinea pig VIP previously documented (with Leu5, Thr9, Met19 and Val26). We also established the similarity of the primary structure of guinea pig secretin with that of porcine and bovine.

Isolation and primary structure of rat secretin

A major form of rat secretin was purified to homogeneity from small intestine, being detected with a porcine secretin radioimmunoassay throughout 7 chromatographic steps. The sequence of the heptacosapeptide amide H-S-D-G-T-F-T-S-E-L-S-R-L-Q-D-S-A-R-L-Q-R-L-L-Q-G-L-V-NH2 shows that rat secretin has a glutamine residue in position 14 instead of arginine as in pig secretin.

Lack of adaptation of pancreatic colipase in rats and mice

A new automated potentiometric method for the determination of colipase was developed, taking advantage of the reactivation of purified lipase, in the presence of bile salt and at pH 6.5. High-fat and high-starch diets induced an opposite regulation of lipase and amylase in the rat pancreas. At the same time, the level of colipase was not influenced by nutrition. During fasting and in alloxan diabetes, the specific activity of lipase almost doubled, that of amylase decreased sharply, and colipase was not affected in the rat pancreas. In obese-hyperglycemic mice, suffering from obesity, hyperinsulinism, and moderate diabetes, there was also no regulation of pancreatic colipase. Thus, at variance with a number of hydrolases, there was no dietary or hormonal adaptation of colipase. However, this was probably without any bearing on intraluminal lipolysis. Indeed, comparison of lipase and colipase activities in pancreas and in small intestine suggests that colipase concentration is not a limiting factor of intraluminal lipolysis. The molecular mechanism of this assumption is discussed on the basis of in vitro studies.