Cholecystokinin (pancreozymin). 3. Synthesis and properties of an analog of the C-terminal heptapeptide with serine sulfate replacing tyrosine sulfate

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Abstract
The influence of tyrosine-O-sulfate, the 27th residue in the sequence of cholecystokinin (pancreozymin) (CCK-PZ), on Ca outflux in isolated pancreatic cells of guinea pigs was studied by replacing this residue in the biologically active C-terminal heptapeptide, CCK-PZ-(27-33) (I), with L-serine 0-sulfate. The synthetic analogue Ser-(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2 (IV), produced the half-maximal outflux observed with I, if applied at about 250 times higher concentration. The unsulfated form of IV was about 10 times less potent than unsulfate I. In the effect on the Ca outflux serine cannot replace tyrosine without a major loss in potency; a sulfate ester group in position 27 is important but in itself not sufficient for full potency. If the terminal amino group of the heptapeptide is left protected by a tert-butyloxycarbonyl-group, the potencies of the derivatives of both I and IV are slightly, but significantly, higher than those of the free peptides.

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