A radioreceptor assay for purified teleost growth hormone.

Highly purified ¹²⁵I-labelled tilapia (Sarotherodon mossambicus) growth hormone (GH) binds to membrane fractions prepared from tilapia liver. Specific (displaceable) binding occurred with the 600, 15,000, and 90,000g fractions with the greatest binding observed with the 90,000g microsomal membrane fraction. Binding was dependent on time and membrane concentration. Specificity studies showed that up to 60% of the ¹²⁵I-labelled tilapia GH bound to the liver microsomal membrane fraction could be displaced by 500 ng of unlabelled hormone. Scatchard analysis of the binding of ¹²⁵I-labelled tilapia GH revealed a single class of binding site with a binding capacity of 125 ± 7.7 fmol/mg protein and a binding affinity of 1.5 × 10¹⁰ ± 0.4 × 10¹⁰ (SEM) l/mol. GH preparations from several vertebrate classes and tilapia prolactin and ovine prolactin at high concentrations displayed competition for the tilapia GH-binding site.¹²⁵I-Labelled tilapia GH demonstrated specific binding to liver microsomal membrane fractions of the teleosts Gillichthys mirabilis, Salmo gairdneri, and Oncorhynchus tschawytscha but not to those of an amphibian Necturus maculosus or a bird Coturnix japonica. Slight, but significant, specific binding was observed with the microsomal membrane fraction of tilapia kidney and gill and of rat liver. These data suggest that this tilapia GH radioreceptor assay may have an application in the detection of teleost GH-like activity.