Identification of the origin of the growth hormone-binding protein in rat serum.

GH specifically interacts with a soluble binding protein in serum. The GH-binding protein (GHBP) has been shown to contain the extracellular portion of the cell surface GH receptor (GHR). In rats and mice there is a unique mRNA that encodes the GHBP. This mRNA contains an alternatively spliced exon that replaces the transmembrane and intracellular domains of the receptor with a short hydrophilic carboxy-terminus of 17 and 25 amino acids, respectively, in rats and mice. In humans and other species no mRNAs encoding the GHBP have been identified, suggesting that the GHBP is in these cases a proteolytically processed GHR. In this study a monoclonal antibody (GHBP 4.3) was raised to the rat GHBP using as immunogen a synthetic peptide containing the unique C-terminal 17 amino acids that are not found in the rat GHR. As predicted, this antibody is specific to rat GHBP and does not cross-react with rat GHR. In combination with polyclonal and monoclonal antibodies that recognize both GHBP and GHR, this antibody was used to show that all, or most, of the GHBP in rat serum is indeed derived from the alternatively spliced GHBP mRNA and not from proteolytic processing of the GHR. In addition, endogenous rat serum GHBP was found to exist in two forms, with apparent mol wt of 52 and 44 kDa, arising from a single protein core of 32 kDa by extensive glycosylation. The concentrations of GHBP in male and female rat plasma were also estimated to be 300 and 575 ng/ml, respectively (measured in nonglycosylated GHBP equivalents).