Penultimate aspartyl unnecessary for stimulation of acid secretion by gastrin-related peptide.

Abstract

THE carboxy-terminal tetrapeptide of gastrin, Trp-Met-Asp-Phe-NH2, has the same biological activity as the parent hormone1. Morley2,3 showed that certain substitutions in the Trp, Met, and Phe positions gave active analogues and concluded that these positions are concerned only with binding at the site of action. In contrast, the only substitution in aspartic acid that gave activity was tetrazolyl for the β-carboxyl, thereby maintaining a proton donor of similar size at this position. Morley concluded that the aspartyl residue has a functional rather than a binding role and presumably is indispensable. The carboxy-terminal octapeptide of cholecystokinin (OP-CCK) contains the carboxy-terminal tetrapeptide of gastrin and has gastrin-like activity4. We report here that the synthetic analogue of OP-CCK, in which alanyl is substituted for aspartyl at the position in question , stimulates gastric acid secretion. In conscious cats with gastric fistulas5, dose-response curves were established for gastric acid response to OP-CCK, 7-Ala OP-CCK, and pentagastrin6. The peptides were given by continuous intravenous infusion and response is taken as peak 10 min output during a 30 min infusion at each dose level (Fig. 1). Assigning a potency of 1 to OP-CCK the relative molar potency of 7-Ala OP-CCK is about 1/110 and of pentagastrin is about 1/4. We also tested the analogue of gastrin tetrapeptide in which alanyl is substituted for aspartyl and found no detectable stimulation of acid secretion at doses as high as 15 mg/kg h, confirming similar negative findings by Morley2 in rat. This suggests that the weak action of alanyl substituted analogues cannot readily be detected without the enhancement of potency conferred by the sulphated tyrosyl of OP-CCK7. Ondetti et al.7 showed that 7-Ala OP-CCK contracts guinea-pig gallbladder with a potency about 1/150th that of OP-CCK, comparable with that reported here for acid secretion. This suggests that the same part or parts of the molecule are required for cholecystokinetic action and for gastric secretory action; the aspartyl residue in the penultimate position is dispensable for both of these actions. On the assumption that gastrin and CCK act at the same site8, we propose that the corresponding aspartyl residue of gastrin is similarly dispensable. For a direct test of this hypothesis, studies are needed of the synthetic analogue in which alanyl is substituted for the penultimate aspartyl in gastrin hepta-decapeptide, perferably gastrin II with sulphated tyrosyl because it is more potent than gastrin I in certain species9.