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

Summary: A radioimmunoassay for the measurement of cyclic somatostatin is described. Synthetic somatostatin was conjugated with bovine serum albumin or human serum-globulin using glutaraldehyde as the coupling reagent. Anti-sera were raised in rabbits injected with the hapten-bovine serum albumin-, or with the hapten-human serum a-globulin-conjugate. Since somatostatin lacks a tyrosine residue that can be readily iodinated, three different radiolabeling procedures for synthetic cyclic Tyr^som'atostatin were compared with each other. In addition, cyclic somatostatin was radiolabeled with the Bolton-Hunter reagent. The radiolabeled antigens were purified by G-25 Sephadex adsorption chromato-graphy 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 lactoper-oxidase-catalysed iodination with 125I. Iodinated somatostatin-derivatives adsorbed onto an inorganic as well as an organic matrix; the percentage of non-specific binding was 14%. The radioimmunoassay was performed with an antiserum dilution of 1:5000. Displacement of labeled cyclic Tyr1-somatostatin by cold cyclic somatostatin was optimal with the use of 25—33.3 kBq/1 and was in the range of 6 pmol/1—6 nmol/1. Ocytocin, (lys)-vasopressin, (arg)-vasopressin, valinomycin, polymyxin, insulin, glucagon, human growth hormone (somatotropin), luteinizing hormone-releasing hormone (luliberin), and thyrotropin releasin