The binding characteristics of 125I-labeled Leu15-gastrin and the molecular identification of the gastrin receptor was investigated in dispersed guinea pig gastric glands. The binding of [125I]gastrin to gastric glands was temperature dependent, saturable, and specific. At 37 C, the binding was rapid, became maximal within 10 min, and declined after 30 min; at 24 C, binding reached a steady state between 30 and 60 min. The dissociation of specifically bound gastrin was also rapid, with 35% of the radioligand dissociating in 5 min at 37 C. Gastrin displaced [125I]gastrin in a dose-dependent manner, with 50% displacement at 4.4 nM. Scatchard analysis of the saturation curve was best described by a one-site binding model with a Kd of 2.3 nM and maximum binding of 6.0 x 10(-10) M/mg DNA. A significant reduction of [125I]gastrin binding to glands occurred in the presence of GTP (0.1 mM), (Bu)2-cGMP (1.0 mM), and protease inhibitors. Chemical cross-linking studies using the cross-linking reagent disuccinimidyl suberate and sodium dodecyl sulfate-gel electrophoresis identified a major band with a mol wt of 78K and several other lower mol wt bands in the range of 60K, 48K, and 38K. Identical electrophoretic patterns were obtained when glands were bound and solubilized in the presence and absence of dithiothreitol, showing the lack of disulfide bonds in the gastrin receptor subunit structure. Since dithiothreitol significantly enhanced radioligand binding when present during binding, its observed actions are most likely in the intracellular processing of the radioligand and not at the receptor level.