Effects of different radioligands on the antigen binding specificity of somatostatin antisera

The regions of the somatostatin (SRIF) molecule recognized by five antisera were systematically studied using three radioligands (125I-N-Tyr-SRIF, [125I-Tyr1]-SRIF and [125I-Tyr11]- SRIF) and SRIF analogs containing sequential substitutions with alanine or tyrosine. Antisera SA and moreso SB had N-terminal specificity when used with [125 I-Tyr11]-SRIF but central molecular specificity when studied with the two N-terminal radiolabelled analogs. The N-terminal and central specific populations of antibodies in antiserum SB were separable by immunoaffinity adsorption using immobilized [Tyr1]-SRIF. It is of practical significance that the same antiserum (SB) could be used with different radioligands to perform N-terminal and central specific radioimmunoassays (RIAs). The central specific RIA detected SRIF-14 and SRIF-28 on an approximately equimolar basis whereas the N-terminal specific RIA was selective for SRIF-14.