The storage time of monoiodinated gastrin affects the biological activity and binding to rat fundic plasma membranes similarly, whereas the immunoreactivity is less affected.

Monoiodinated gastrin was produced by a gentle Iodo-gen method, followed by gel and ion-exchange chromatography. During storage this tracer has previously been found to retain its immunological activity, whereas its specific binding to rat gastric fundic plasma membranes decreased rapidly. To examine whether the loss of specific binding could be reflected in loss of biological activity, we examined the tracer at regular intervals for immunoreactivity by binding to an antibody directed against the C-terminal bioactive site of the gastrin molecule; for biological activity in the totally isolated, vascularly perfused rat stomach concomitantly stimulated with a phosphodiesterase inhibitor; for specific binding ('receptor binding') to a rat gastric fundic plasma membrane fraction; and for fragmentation and intramolecular changes by fast protein liquid chromatography (FPLC). Biological activity and 'specific' binding showed a parallel decrease to zero during 4 weeks of storage, whereas the immunoreactivity was retained much longer. There was no apparent fragmentation of the gastrin molecule during the 1st month after preparation as evaluated by FPLC. This study accordingly shows that both biological activity and specific binding to a gastric fundic plasma membrane fraction of 125I-gastrin are lost before detectable loss in immunoreactivity and before FPLC-detectable fragmentation of the molecule. Thus, during the early storage period subtle changes in the 125I-gastrin molecule must take place. Since the specific binding is so closely correlated to biological activity, it suggests that the binding actually represents true receptor binding.