| Abstract: | In the radioreceptor assay for growth hormone (RRA-GH) using 125I]iodo-hGH, hGH and human liver membrane particulate fractions as tracer, hormone standard and receptors, respectively, ovine placental lactogen (oPL) is capable of inhibiting the binding of 125I]iodo-hGH in a parallel manner with hGH and in equipotency. Similarly, in the RRA-GH by employing 125I]iodo-oPL, oPL and human liver membrane particulate fractions as tracer, hormone standard and receptors, respectively, hGH is also equipotent as oPL in inhibiting the binding of 125I]iodo-oPL in a parallel fashion. The addition of monoclonal antibodies against oPL in the assay was effective in inhibiting the binding of 125I] iodo-oPL to human liver, but could not, however, inhibit the binding of 125I]iodo-hGH to human liver. Furthermore, the addition of the monoclonal antibodies in the RRA-GH did not affect the parallelism of the oPL standard but lowered the total binding of oPL. Our studies indicate that the structure of the binding sequence in oPL which binds to the GH receptor of human liver is not identical to the equivalent sequence of hGH and that the monoclonal antibodies compete with GH receptors in human liver for the binding of oPL. |
