The radioimmunoassay for somatostatin using 125I-[Tyr8]-somatostatin as a labelled antigen

The peptides such as somatostatin analogs tend to degrade by oxidation in radioiodization procedure and are labile in storage. In this study, the authors developed the radioimmunoassay for GIF using 125I-[Tyr8]-GIF as a labeled antigen which is a far more stable tracer. [Tyr8]-GIF was radioiodized by lactoperoxidase method, and the 125I-[Tyr8]-GIF was purified with CMC column chromatogram. The fraction containing 125I-[Tyr8]-GIF in ammonium acetate buffer was stored in polystyrene tubes at -20 deg C. Just before the use, the stored solution was thawed, and the ammonium acetate buffer was discarded, then the 125I-[Tyr8]-GIF adsorbed on tube wall was recovered in GBPBS. The repurification with CMC column chromatogram of the recovered tracer in GBPBS was carried out, and it was found that 88.2% of the applied radioactivity was eluated. The autoradiogram of the enzymatic hydrolyzate of 125I-[Tyr8]-GIF showed that the labeled amino acid was 3-iodotyrosine. With specific antiserum R501, the binding of the tracer to the antibody reached near the maximum level 19 hours after incubation at 4 deg C. Based on these results, the assay was performed. The antigenic determinant for the antiserum R501 involved the central region of amino acid sequence including the ring structure of GIF. (Kako, I.)