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

The influence of tyrosine O-sulfate, the 27th residue in the sequence of cholecystokinin (pancreozymin) (CCK-PZ) on calcium 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 O-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 ten times less potent than unsulfated I. Thus, in the effect on the calcium outflux, serince 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. Interestingly, 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 were slightly, but significantly, higher than those of the free peptides.