Somatostatin radioimmunoassay with 125I-Nalpha-tyrosyl-somatostatin.

Nalpha-Tyrosyl-somatostatin was synthesized and proved to be homogeneous. Radioiodination of this tyrosine-containing somatostatin analogue by either the lactoperoxidase method or the chloramine T method led to the formation of crude iodinated compound, which was purified by ion exchange chromatography on CM-Sephadex C-25 using a linear ammonium acetate buffer gradient. This purification process was found to be satisfactorily reproducible and suitable for the preparation of 125I-Nalpha-tyrosyl-somatostatin. Using the purified 125I-somatostatin analogue, radioimmunoassay for somatostatin was performed and the assay system was proved to be sensitive and specific for somatostatin. Immunoassays of hot-water extracts of porcine and tupaia brain, pancreas, stomach and various regions of the intestine in the system revealed that those tissues contained immunoreactive somatostatin at various concentrations. Of the results, it was remarkable that somatostatin immunoreactivity was found in the ileum, middle colon and rectum in both animals, although the concentration were lower when compared with those in the stomach, duodenum and jejunum.