Preparation and Characterization of a Biologically Active Gastrin Derivative Modified with an 125I-Labeled Imidoester

Both trace-labeled and high-specific activity 125I-labeled derivatives of hexadecapeptide gastrin (G) were prepared by reaction with the iodinated form of the imidoester, methyl p-hydroxybenzimidate. Reaction conditions for preparation of trace-labeled iodinated imidoester gastrin (IIE-G) were: excess imidoester to G (IIE:G, 20:1), pH 9.2, and a reaction time of 24 h. Following purification by gel filtration and silica gel chromatography, an IIE-G component was isolated which appeared homogeneous on thin-layer chromatography and retained the same biological and immunological properties as unmodified G. Somewhat different conditions were necessary to prepare high specific activity iodinated imidoester gastrin (IIE∗-G). These included reducing the volume (20 μl) and pH (7.5) at which the imidoester was iodinated and adjusting the concentrations of reactants to the same molar amounts as 5 mCi of carrier-free 125I. Sufficient amounts of IIE∗-G were obtained by reversing the ratio of G and IIE∗ and reacting with a G excess (GIIE∗, 10:1). The purified IIE∗-G had a specific activity exceeding 1500 μCi/nmol and was used to establish a specific and sensitive radioimmunoassay for gastrin.