GROWTH HORMONE-BINDING AND PROLACTIN-BINDING PROTEINS IN MAMMALIAN SERUM

The present study was undertaken to characterize the species specificity and diversity of lactogenic and somatogenic binding to serum among mammals and to classify GH-binding protein (GH-BP) in this group of species. Animal sera were characterized on the basis of human (h) GH and bovine (b) PRL binding levels, binding specificity toward GHs and PRLs, binding affinity constant (K(a)), and the ability of a monoclonal antirabbit GH receptor antibody (MAb-7) to inhibit the binding to serum. Analyses of the results yielded a classification of the mammalian sera into five types of GH binding, which we elected to call GH-BP. The guinea pig had undetectable levels of GH-BP and was labeled type 0. Type I GH-BP was found in the mouse and the rat. hGH binding to GH-BP of type I serum was of low affinity (1.2-3.9 x 10(8) m-1) with high IC50 (approximately 100 ng/tube) and was purely somatogenic in nature. Type II GH-BP was found in the goat, sheep, and cow. Their IC50 of hGH binding was -4-fold lower than that of type I GH-BP. Their binding of hGH was relatively specific and only marginally displaced by the PRLs. Type III GH-BP was found in the rabbit, horse, cat, dog, and pig. These animals' sera had high affinity binding toward hGH (4.7-9.2 x 10(9) M-1), with low IC50 (approximately 2 ng/tube) and dominant lactogenic binding. The great similarity of type III GH-BPs was further stressed by the ability of MAb-7 to inhibit [I-125]hGH binding to all type III sera, but not to the other mammalian sera tested. Type IV GH-BP was found in the human and rhesus monkey sera. These were characterized by binding affinities that were intermediate between those for types II and III GH-BP and by the inability of nonprimate GHs to compete with hGH binding. To directly confirm the potent effect of the lactogenic hormones in type III GH-BP, the specific binding of bPRL to sera of type III animals was studied. [I-125]bPRL-specific binding was determined under optimized assay conditions and was the highest in rabbit serum, with the following rank order being observed: rabbit much greater than horse = dog = pig > cat. Scatchard analysis of [I-125]bPRL binding revealed linear plots with similar affinity constants (K(a)) of 1.7-3.3 x 10(9) m-1. In keeping with the lactogenic nature of type III sera, [I-125]bPRL binding to these sera was most strongly displaced by bPRL and hGH. These data indicate that substantial differences exist in the blood transport of GH among various mammals. The functional significance and possible relationship of serum GH-BP/PRL-BP to membrane and cytosolic GH and PRL receptors are currently being investigated.