PREPARATION AND CHROMATOGRAPHIC PURIFICATION OF 125(I)-[TYR0]-SOMATOSTATIN-14 FOR THE USE IN RADIOIMMUNOASSAY AND RECEPTOR-BINDING EXPERIMENTS

[Try(0)]-somatostatin-14 (SST-14) was converted to the corresponding r adiolabeled I-125-[Tyr(0)]-SST-14 derivative by use of the chloramine- T technique. After solid-phase extraction (SPE) of the crude radiopept ide with microfine silica gel and desorption with acetone-water-acetic acid 39:40:1, the label was subjected to size exclusion chromatograph y (SEC) on Sephadex G-25 fine. Fractions attributable to the target co mpound were collected and investigated with respect to their specific as well as non-specific binding to a specific anti-somatostatin antibo dy. All fractions exhibiting an optimum ratio of specific versus nonsp ecific binding were pooled and lyophilized for their further use in bo th radioimmunoassay (RIA) measurements of somatostatinlike immunoreact ivity (SLI) and receptor-binding experiments. A specific activity of a pprox. 1.6 x 10(6) Ci/M was calculated for the radiolabel prepared in this manner. Approximately 85-90 % of radioactivity attributable to la beled [Tyr(0)]-SST species was incorporated into the desired mono-iodi nated component. When I-125-[Tyr(0)]-SST-14 was purified by reversed-p hase high performance liquid chromatography (RP-HPLC) using isocratic elution with 0.1 M triethylammonium formate (TEAF) buffer of pH 2.2 in 22 % acetonitrile after prior SPE, specific binding decreases to abou t 80 % compared with the value obtained for the radiopeptide subjected to SEC. Nevertheless, RP-HPLC proves as an efficient tool for rapid p urity control of I-125-[Tyr(0)]-somatostatin-14 samples at different s torage time intervals.