Differential Solubilization of Placental Lactogen (PL)-and Growth Hormone-Binding Sites: Further Evidence for a Unique PL Receptor in Fetal and Maternal Liver*

Abstract

Previous studies from this laboratory provided evidence for the existence of a specific placental lactogen (PL) receptor in tissues of fetal lambs and pregnant sheep. The PL receptor is structurally and functionally distinct from somatotropic (GH) and lactogenic (PRL) receptors, and there are conspicuous differences in the expression of the three receptors during ontogeny. The results of the present study indicate striking differences in the solubilization of PL- and GH-binding sites in maternal and fetal sheep liver. Radiolabeled ovine PL (oPL) bound specifically and with high affinity (Kd, 0.97 nM) to soluble detergent extracts of ovine fetal liver, but there was no specific binding of radiolabeled ovine GH (oGH) or oPRL to soluble extracts or insoluble fractions of fetal liver. When liver microsomes of pregnant sheep were extracted with Triton X-100, 80% of the [125I]oPL-binding sites were recovered in the soluble fraction, but 76% of the [125I]oGH binding sites were recovered in the insoluble pellet. Soluble extracts of maternal liver had high affinity for oPL (Kd, 1.45 nM), but low affinity for oGH (Kd 33 nM) and oPRL (Kd, 1-2 .mu.M). On the other hand, Triton-insoluble fractions of maternal liver had high affinity for oGH (Kd, 0.95 nM) as well as oPL (Kd, 0.91 nM), but low affinity for oPRL (Kd, 1-2 .mu.M). The subunit structure of the [125I]oPL-binding site in soluble fractions of fetal and maternal liver (mol wt, 38-47K) was distinct from that of the [125I]oGH-binding site in Triton-insoluble fractions of maternal liver (mol wt, 54/118K). These findings indicate that treatment of microsomal fractions of fetal and maternal sheep liver with Triton X-100 solubilizes the oPL receptor but not the oGH receptor. The differential solubilization of PL- and GH-binding sites may facilitate purification of the two distinct receptors and clarification of their respective roles in the regulation of fetal and postnatal growth.