Interaction of gastrin with transferrin: effects of ferric ions

The sedimentation behavior of 125I-labeled gastrin has been studied as a function of Fe3+ ion concentration and pH. Both sedimentation velocity and sedimentation equilibrium experiments indicated that high-molecular-weight Fe3+-gastrin complexes were formed at pH 5.0 and pH 7.4. Self-association of gastrin alone was observed at pH values below 5.0. 125I-labeled gastrin bound to human serum apotransferrin at pH 7.4. Scatchard analysis of the gastrin-apotransferrin complex gave a Kd of approximately 6.4 microM at 37 degrees C, with two binding sites per molecule of apotransferrin. No significant binding of gastrin to diferric transferrin was observed under the same conditions. The binding of gastrin to apotransferrin was inhibited by NaCl. The results are consistent with the hypothesis that gastrin and transferrin act synergistically in the uptake of dietary iron by the gastrointestinal tract.