Mary Lynn Fink

Interaction of Peptides Related to Secretin with Hormone Receptors on Pancreatic Acinar Cells

Three analogues of the carboxyl-terminal tricosapeptide of secretin (S5-27), one with glutamine replacing glutamic acid in position 9 (9-Gln-S5-27), a second with asparagine substituted for aspartic acid in position 15 (15-Asn-S5-27), and a third with both replacements (9-Gln-15-Asn-S5-27) were tested for their ability to interact with hormone receptors on dispersed pancreatic acinar cells. Each of these analogues inhibited binding of 125I-labeled vasoactive intestinal peptide (VIP). None of the analogues increased cellular cyclic AMP but each inhibited the increase in cellular cyclic AMP produced by secretin or VIP. At the high affinity VIP receptor (the low affinity secretin receptor) each analogue had an apparent affinity which was greater than that for S5-27, whereas at he low affinity VIP receptor (the high affinity secretin receptor), each of the analogues had an apparent affinity which was the same as that for S5-27. Thus, in S5-27, substituting glutamine in position 9 or asparagine in position 15 makes the fragment more VIP-like but not less secretin-like. These results also provide additional evidence that the receptor having a low affinity for secretin and a high affinity for VIP is functionally distinct from the receptor having a high affinity for secretin and a low affinity for VIP.

A RE‐EXAMINATION OF THE CONFORMATION OF SECRETIN IN WATER

An examination of a series of peptides corresponding to partial sequences of secretin and the application of empirical conformational parameters to the sequence has reopened the question about the position of the short helical stretch in the folded chain. In earlier studies, this was tentatively placed near the Nterminus, while newly accumulated evidence points to the C‐terminal area.

Studies on the conformation of secretin: The position of the helical stretch

Abstract An analog of the C-terminal tricosapeptide of secretin, with aspartic acid replacing glutamic acid in position 9 and lysine substituted for arginine in position 21, was prepared. The synthesis was carried out in solution by stepwise chain lengthening with the application of the in situ technique. The ord-cd spectra of this new analog closely resemble the spectra of the tricosapeptide with the unaltered secretin sequence and of the analog in which only arginine-21 was replaced by lysine and of secretin itself. The incorporation of aspartic acid instead of glutamic acid-9 resulted in an N-terminal sequence that has a considerably reduced probability of assuming a helical conformation. The observation that the helix content remained unchanged adds support to a model of secretin in which the helical stretch is near the C-terminus. The role of an acidic residue in position 9 is also discussed.

The Role of Some Polar Amino Acid Residues in Secretin

Three analogues of S5-27, the tricosapeptide with the carboxyl-terminal sequence of secretin, were studied. In the analogues, the acidic residues at positions 9 and 15 of S5-27 were replaced by the neutral residues glutamine and asparagine. These changes resulted in a decrease in immunoreactivity. Binding to an antibody against secretin could be correlated with the changes in the conformation of the synthetic analogues.