Importance of sulfation of gastrin or cholecystokinin (CCK) on affinity for gastrin and CCK receptors

We investigated the importance of sulfation of gastrin or cholecystokinin (CCK) on influencing their affinity for gastrin or CCK receptors by comparing the abilities of sulfated gastrin-17 (gastrin-17-II), desulfated gastrin-17 (gastrin-17-I), CCK-8 and desulfated CCK-8 des(SO3)CCK-8] to interact with CCK or gastrin receptors on guinea pig pancreatic acini. For inhibiting binding of 125I-gastrin to gastrin receptors, gastrin-17-II (Kd 0.08 nM) greater than CCK-8 (Kd 0.4 nM) greater than gastrin-17-I (Kd 1.5 nM) greater than des(SO3)CCK-8 (Kd 28 nM). For inhibiting binding of 125I-Bolton Hunter-labeled CCK-8 to CCK receptors the relative potencies were: CCK-8 much greater than des(SO3)CCK-8 = gastrin-17-II greater than gastrin-17-I. Each peptide interacted with both high and low affinity CCK binding sites. The relative abilities of each peptide to interact with high affinity CCK receptors showed a close correlation with their abilities to cause half-maximal stimulation of enzyme secretion. These results demonstrate that, in contrast to older studies, sulfation of both CCK and gastrin increase their affinities for both gastrin and CCK receptors. Moreover, the gastrin receptor is relatively insensitive to the position of the sulfate moiety, whereas the CCK receptor is extremely sensitive to both the presence and exact position of the sulfate moiety.