Purification and partial characterization of a nonprimate growth hormone receptor

Pregnant rabbit liver membranes have been shown to possess two types of receptors by displacement analysis, a growth hormone (GH) receptor which binds bovine growth hormone with an affinity constant (KA) of 3 x 10(9) M-1 and ovine prolactin with a KA of 3 x 10(8) M-1, and a prolactin (Prl)-specific receptor which binds ovine prolactin with a KA of 5 x 10(9) M-1. The prolactin-specific receptor when solubilized with Triton exhibits a 4-fold increase in the its KA while the KA of the growth hormone receptor decreases slightly to 2 x 10(9) M-1 after solubilization. The 10-fold difference in affinity which results has been exploited to facilitate the separation of these two receptors by differential affinity chromatography on human growth hormone (hGH) affinity gels. The growth hormone receptor is eluted from the gel with 4 M urea while 5 M MgCl2 is required to elute the prolactin receptor. Conditions of affinity chromatography have been optimized, and further purification of the GH receptor by preparative isoelectric focusing and Sepharose 6B gel filtration resulted in a more than 8000-fold purification of the receptor. This material had a Stokes radius of 62 A, consistent with a molecular weight of 300,000 and gave one main band (75,000 to 80,000) and two minor bands on sodium dodecyl sulfate (SDS) polyacrylamide gels, which could be interpreted as indicating a tetrameric receptor. The GH receptor was shown to be a sialoglycoprotein (or closely associated with sialoglycoprotein) by analytical isoelectric focusing with an isoelectric point of 4.6. Specificity studies with the highly purified receptor confirmed the initial hypothesis that this receptor is capable of binding bovine growth hormone (bGH) with high affinity and ovine prolactin (oPrl) with low affinity, in contrast to the prolactin-specific receptor.