Specific binding sites for LH/chorionic gonadotrophin, low-density lipoprotein, prolactin and FSH in homogenates of human corpus luteum. I: Validation of methods.

The specific binding of 125I-labelled human chorionic gonadotrophin (hCG), human low-density lipoprotein (hLDL), human FSH (hFSH) and human prolactin (hPRL) to homogenates of human corpus luteum tissue was measured. Specific binding of 125I-labelled hCG was dependent on the temperature and duration of incubation, was inhibited by divalent metal ions or chelating agents, and increased linearly with homogenate concentration. Recovery of bound hormone was more effective using Millipore filtration or polyethylene glycol precipitation compared with centrifugation alone. Binding of 125I-labelled hCG was inhibited specifically by low levels of hCG and human LH (hLH) but not by ovine LH or bovine LH. Incubation of human luteal tissue with ice-cold citrate buffer (pH 3) released more than 90% of specifically bound 125I-labelled hCG within 5 min. This treatment inactivated LH receptors, but did not affect the immunoactivity of hLH released, enabling the measurement of released hormone by radioimmunoassay. Scatchard plots of binding of 125I-labelled LDL to human corpus luteum demonstrated a single class of binding sites. Binding was saturable, increased linearly with increasing concentration of homogenate, and was displaceable by low concentrations of unlabelled LDL. Binding of 125I-labelled hPRL to human luteal homogenates was increased by Mg2+ and was specific for lactogenic hormones (human prolactin, human growth hormone and ovine prolactin). Binding of 125I-labelled hFSH was not dependent on divalent metal ion concentration (in marked contrast to hFSH binding to immature pig granulosa cell receptors) and was displaced by hFSH preparations but not by hPRL, ovine LH or hCG at 1 microgram/ml. These results establish optimal conditions and hormone specificities for the measurement of human luteal gonadotrophin and LDL receptors, and methods for the estimation of hLH/hCG endogenously bound to human corpus luteum tissue.