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The peptide hormone gastrin 17 , 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 gastrin 17  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 gastrin 17 , gastrin 17 SO 4  and gastrin 17 PO 4  on addition of Fe 3+  ions were monitored. Binding of gastrin 17 , gastrin 17 SO 4  and gastrin 17 PO 4  to the human CCK1 and CCK2 receptors was assessed by competition with [ 125 I]-Bolton and Hunter-labelled cholecystokinin 8  in transiently transfected COS cells. Tyrosine sulphation or phosphorylation increased the affinity of gastrin 17  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 gastrin 17  for the second ferric ion bound was reduced from 94 pM to 7.32 µM and 671 nM, respectively. While sulphation of gastrin 17  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.

Tyrosine modification increases the affinity of gastrin for ferric ions