A method for determination of the specific activity of receptor-bound human chorionic gonadotropin (hCG)

Precise knowledge of the specific activity (S.A., muc/mug) of the receptor bound radiolabelled hormone is required for study of the stoichiometry of peptide hormone-receptor interactios. A radioligand receptor assay using 131-I-hCG as tracer and transplantable mouse luteoma homogenates (Biol. Reprod. 8:550, 1973) as a source of receptor was used as a model to determine the specific activity of receptor bound 125-I-hCG. Progressive saturation of the gonadotropin receptor by 125-I-hCG suggests the presence of a high affinity-low capacity binding event (saturating between 14 and 37 ng/100 mg homogenate) that does not distinguish between non-radioactive hCG and 125-I-hCG, and a low affinity-high capacity binding event (saturating between 240 and 270 ng/100 mg homogenate) that shows a preference for non-radioactive hCG over 125-I-hCG. Parallelism between bound 125-I-hCG and non-radioactive hCG in terms of competition with tracer 131-I-hCG could only be demonstrated for the high affinity event.