The binding of [3H]caerulein (a stable, biologically active labeled analog of cholecystokinin-pancreozymin) to semi-purified rat pancreatic plasma membranes was investigated. The binding was dependent on time and temperature, as well as saturable, specific and reversible. This process was pH-dependent and optimal at pH 7.0. Cysteine and serine residues in plasma membranes were of importance for binding. Mg2+ favored the binding. 2. The acceleration of the dissociation of [3H]caerulein in the presence of an excess of native caerulein suggests that binding was characterized by a negative cooperativity. The fast dissociation state evoked by a high degree of occupancy by caerulein was inhibited by lowering the temperature, by decreasing the pH, or by the presence of wheat germ agglutinin.
