Development of a Radioimmunoassay for Cyclic Somatostatin: Antibody Production, Comparative Radioiodination, and Dose-Response Curve

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Abstract
Synthetic somatostatin was conjugated with bovine serum ablumin or human serum .alpha.-globulin using glutaraldehyde as the coupling reagent. Antisera were raised in rabbits injected with the hapten-bovine serum albumin-, or with the hapten-human serum .alpha.-globulin-conjugate. Since somatostatin lacks a tyrosine residue that can be readily iodinated, 3 different radiolabeling procedures for synthetic cyclic Tyr1-somatostatin were compared with each other. Cyclic somatostatin was radiolabeled with the Bolton-Hunter reagent. The radiolabeled antigens were purified by G-25 Sephadex adsorption chromatography and thus separated from unreacted components. Maximal yields were achieved when Tyr1-somatostatin was iodinated with chloramine-T. The largest immunochemical reactivity, however, was obtained by applying the lactoperoxidase-catalyzed iodination with 125I. Iodinated somatostatin-derivatives adsorbed onto an inorganic as well as an organic matrix; the percentage of non-specific binding was 14%. Recovery of cyclic somatostatin added to human plasma, cerebrospinal bluid [CSF], pancreatic, hypothalamic and pituitary tissue ranged from 77-84% when reextracted with acid acetone. Native cyclic somatostatin in human plasma could not be detected. It was possible to detect 116 .+-. 12 ng/l somatostatin in the acid acetone extract of freeze-dried CSF from 4 healthy persons.