Abstract
The peptide hormone gastrin17, which occurs naturally in both tyrosine sulphated and unsulphated forms, binds two ferric ions with pM affinities. The aim of this study was to investigate the hypothesis that sulphation or phosphorylation of gastrin17 altered ferric ion binding, and/or affinity for the CCK1 or CCK2 receptor. To investigate the effect of tyrosine modification on ferric ion binding, the changes in absorbance of gastrin17, gastrin17SO4 and gastrin17PO4 on addition of Fe3+ ions were monitored. Binding of gastrin17, gastrin17SO4 and gastrin17PO4 to the human CCK1 and CCK2 receptors was assessed by competition with [125I]-Bolton and Hunter-labelled cholecystokinin8 in transiently transfected COS cells. Tyrosine sulphation or phosphorylation increased the affinity of gastrin17 for the first ferric ion bound from 267 to 83 pM and 14 pM, respectively, but had no effect on the stoichiometry of ferric ion binding. In contrast the affinity of gastrin17 for the second ferric ion bound was reduced from 94 pM to 7.32 µM and 671 nM, respectively. While sulphation of gastrin17 increased its affinity for the CCK2 receptor approximately 50 fold, phosphorylation had no effect on receptor binding. These results demonstrate that tyrosine modification may have profound effects on the interaction of gastrins with ferric ions and with the CCK2 receptor.
Highlights
The classical gastrointestinal hormone gastrin17 (ZGPWLEEEEEAYGWMDFamide, amidated gastrin17 (Gamide)) was first recognized by its ability to stimulate gastric acid secretion (Dockray et al 2001)
Human gastrin is initially synthesized as a 101 amino acid preprohormone, which is processed via the 80 amino acid prohormone progastrin, to various non-amidated precursors including glycineextended gastrin17 (ZGPWLEEEEEAYGWMDFG, glycine-extended gastrin17 (Ggly)) (Dockray et al 2001)
Changes in absorbance on binding of ferric ions to tyrosine‐modified gastrin17 The effect of addition of Fe3+ ions on the absorption spectrum of gastrin17, gastrin17SO4 and gastrin17PO4 was first investigated by absorption spectroscopy
Summary
The classical gastrointestinal hormone gastrin (ZGPWLEEEEEAYGWMDFamide, Gamide) was first recognized by its ability to stimulate gastric acid secretion (Dockray et al 2001). While Gamide stimulates proliferation in the gastric mucosa (Koh et al 1999), progastrin and Ggly are important growth factors for the colorectal mucosa (Aly et al 2004). Sulphation of CCK8 does not greatly affect its affinity for the CCK2 receptor, which is found in the gastric mucosa and in the brain (Shulkes and Baldwin 1997). Neither the CCK1 nor the CCK2 receptor recognizes non-amidated forms of CCK or gastrin, but the receptors can be readily distinguished with several selective antagonists (Shulkes and Baldwin 1997)
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