New analogues of secretin

For the evaluation of structure/activity relationships, some porcine secretin analogues, modified in the N-terminus, have been synthesized by segment condensation in solution. The secretin activity of the analogues was defined as the volume of pancreatic juice secreted in rats and dogs. The exchange of the N-terminal pentapeptide for the N-terminal pentapeptide of human somatotropin releasing factor (h-SRF) resulted in a peptide ([1-Tyr,2,4-Di-Ala,5-Ile]secretin) with practically no SRF-activity (less than 1% SRF-activity up to 100 micrograms/kg in the rat), but surprisingly high secretin activity (almost 100% in the rat, but only 1150 CU/mg (27%) in the dog). [3-L-Cysteic acid]secretin showed 1750 CU/mg (39%) in the dog, but a less activity (23%) in the rat. [6-D-Phe]secretin and [5-D-allo-Thr]secretin are again strongly species specific. They exhibited an activity of less than 1% in the dog, but about 10-15% in the rat. The smallest secretin activity was observed with [1-Cys,6-Cys]secretin in the oxidized form. The activity in the rat with this analogue was only about 0.2%.