Interaction of tryptophan-modified analogues of cholecystokinin-octapeptide with cholecystokinin receptors on pancreatic acini.

None of six different tryptophan-modified analogues of the C-terminal octapeptide of cholecystokinin differed from the unaltered peptide in terms of their efficacies for stimulating amylase secretion from dispersed acini prepared from guinea-pig pancreas. Replacement of hydrogen with fluorine in position 5 or 6 on the indole ring of the tryptophan residue did not alter the potency with which the peptide stimulated amylase secretion; however, replacement of hydrogen by fluorine in positions 4, 5, 6 and 7 of the indole ring, or modifying or replacing the indole nitrogen caused a 30- to 300-fold decrease in potency. Changes in the ability of the peptide to inhibit binding of 125I-labeled cholecystokinin. Our findings indicate that reducing the ability of the tryptophan residue to donate electrons produced a greater decrease in the affinity of the peptide for cholecystokinin receptors than did abolishing the ability of tryptophan to form hydrogen bonds, and modification that altered both abilities caused a greater decrease in affinity than did modification of only one ability. Finally, in the tryptophan residues of cholecystokinin octapeptide, tetrafluorination of the indole ring or replacing the indole nitrogen by oxygen reduced the ability of the peptide to cause residual stimulation of enzyme secretion, probably by accelerating the rate at which bound peptide dissociated from its receptors when the acini were washed and resuspended in fresh incubation solution.

Role of the tryptophan residue in the interaction of pancreozymin with its receptor.

1. Analogues of the C-terminal octapeptide and tetrapeptide of pancreozymin with a modified tryptophan residue have been tested on the rat pancreas adenylate cyclase activity, on the enzyme and fluid secretion of the rat pancreas in vivo and on the amylase release from rabbit pancreatic fragments. 2. Fluorination of the tryptophan residue in position 5 or 6 does not influence the effect of the peptides on any of the measured parameters. 3. Methylation of the nitrogen atom in the indolyl ring, which eliminates hydrogen bond formation, markedly reduces the affinity of the peptides for the adenylate cyclase activity and for the amylase release in rabbit pancreatic fragments. The effects on fluid and enzyme secretion in the rat pancreas in vivo are reduced nearly as much. 4. Tetrafluorination of the tryptophan residue, which reduces its charge donor capacity, causes a still larger reduction in activity and affinity of the octapeptide. 5. The tetrafluorinated tetrapeptide stimulates the adenylate cyclase activity and the enzyme and fluid secretion in vivo more than the unmodified tetrapeptide, which may be due to its increased hydrophobicity. 6. Replacement of the nitrogen atom in the indolyl ring of tryptophan by a sulfur or an oxygen atom, which also reduces the charge donor capacity, leads again to a large reduction in the affinity and activity of both the octapeptide and the tetrapeptide. 7. These findings suggest that the charge donor capacity of the tryptophan residue is of primary importance for the biologic activity of pancreozymin, while hydrogen bond formation and hydrophobicity are of secondary importance.

Tryptophan replacement in the C-terminal octapeptide of CCK-PZ. Syntheses of six peptide analogues bearing tryptophan congeners.

Six sulphated octapeptide amide analogues, representing the C-terminal sequence of pancreozymin have been prepared in addition to the natural sequence. The tryptophyl residue which is essential for biological activity was replaced in this series by structurally, closely related analogues each of which resemble the properties of the original moiety to varying extents. The syntheses were so conducted that the C-terminal tetrapeptide amides (modified tetragastrins) could be secured. The results of the measurement of one of the biological parameters are indicated. The activity was determined to elucidate which of the physicochemical properties of the tryptophyl residue is most essential for exertion of biological activity in the described test.

Synthesis, resolution and charge-donor properties of six tryptophan analogues.

In order to investigate the special function of tryptophan in peptide hormones, six tryptophan analogues have been synthesized, in which structural resemblance has been grossly retained and potentially essential properties have only partially been varied. The L-enantiomers have been isolated after enzymatic digestion of racemic dipeptide derivatives, and charge-transfer properties of the compounds have been studied.