Radioimmunoassay for GH-release inhibiting hormone.

The synthetic growth hormone release inhibiting hormone (GH-RIH) was conjugated with human serum globulin using glutaraldehyde and administered to rabbits. An antiserum thus generated 70% of 125I-Tyr1-GH-RIH. The binding was inhibited by unlabelled GH-RIH and the inhibition was dose-related, enabling us to establish a radioimmunoassay method for GH-RIH. The minimum detectable dose was 4 pg. A linearity was demonstrated for immunoreactive GH-RIH of extracts of rat and pig hypothalami, indicating that they contained substance(s) indistinguishable from GH-RIH. Various hypothalamic and pituitary hormones did not interfere with the radioimmunoassay, but considerable cross-reaction was observed for linear GH-RIH and the ring portion of GIH-RIH, suggesting that the antigenic determinant involved the amino acid sequence from position 3-14 of GH-RIH or part of it. Plasma protein appears to contain substance(s) immunologically indistinguishable from GH-RIH or to interfere, in a nonspecific manner, with the radioimmunoassay system for GH-RIH.