Specificity of covalently stabilized complexes of 125I-labeled human somatotropin and components of the lactogenic binding sites of rat liver

125I-labeled human somatotropin specifically bound to the lactogenic sites of microsomal membranes from pregnant rat liver, originated a radioactive covalent complex of Mr 63,000 upon reaction with dimethyl suberimidate, disuccinimidyl suberate (DSS) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The amoun of this species was closely parallel with the preexisting amount of the ligand-receptor complex. Photoactivation of a hormone derivative bound to the receptor also gave rise to the 63 K species. A ternary complex of receptor, hormone and Triton X-100 cross-linked with DSS yielded the 63 K species and a new one of 96 K. The data indicate that the 63 K complex involves the radioactive hormone and a constituent of the binding site. The 96 K species could comprise a second component of the receptor.