Evidence for two distinct classes of high affinity growth hormone binding proteins in pregnant rat serum

These studies have established the presence of two major classes of high affinity growth hormone binding proteins in pregnant rat serum, designated GHBPa and GHBPb, with apparent native Mr of 257 K and 98 K respectively. GHBPa, which has not been identified previously, exhibits a binding affinity (2-5 nM(-1)) that is up to 20-fold higher than GHBPb (0.2-0.8 nM(-1)) and is the least abundant form, being approximately 15-20% of total serum GH-binding capacity. Western immunoblot analysis revealed that each GHBP is composed of several immunoreactive proteins which were reactive with carboxy-terminal (RB1615) and/or N-terminal (MAb263) domain antibodies, suggesting the presence of GHBPs with and without the hydrophilic tail. Of importance is that GHBPa exhibited significantly higher Mr (78-182 K, +DTT) than that predicted by GHBP cloning, suggesting that they may be covalently bound to other non-GH-binding proteins or may be distinct entities. GHBPb, on the other hand, was composed of smaller Mr (43/48 K, +DTT) "hydrophilic" tail-containing proteins, some of which were disulphide linked to a larger complex of approximately 110 K. These novel findings challenge the current view of the mechanism for generation of the rat serum GHBP and raise the intriguing possibility that the two classes of GHBP may play distinct and important roles in GH physiology.