We recently classified the growth-hormone (GH)-binding protein (GH-BP) in a wide range of mammalian [including human (h)] sera and reported the existence of a major lactogenic component in GH-BP of type-III sera (rabbit, horse, dog, pig and cat), based on the capacity of bovine (b) and ovine prolactin (PRL) to displace 125I-labelled human growth hormone (hGH) binding and on direct 125I-bPRL binding studies. In this study, we demonstrate the high degree of Mg2+ dependence of the binding of the classically lactogenic hGH and bPRL, but not that of the somatogenic bGH to various mammalian sera (types I-IV). Serum GH-BP was assayed using a previously described and validated charcoal-separation assay. 125I-hGH binding to rat, ovine, bovine, rabbit, horse, dog and human sera was enhanced 1.5-2.5-fold in the presence of 70 mM Mg2+. The Mg2+ effect was concentration-dependent between 3.7 mM and 70 mM, causing a significant and proportional increase in 125I-hGH binding to serum. Like 125I-hGH, 125I-bPRL binding to type-III sera was also Mg(2+)-dependent. In contrast, 125I-bGH binding to all types of serum GH-BP was not affected by Mg2+ concentrations of up to 35 mM, while 70 mM Mg2+ slightly, but significantly, reduced (by approx. 15%) bGH binding to rabbit serum. In keeping with the Mg(2+)-dependent stimulation of lactogenic hormone binding to GH-BP, 70 mM Mg2+ caused a shift to the left in the displacement curves of hGH and bPRL competing with 125I-hGH binding to rabbit, dog, horse and human sera, while the effects of the somatogens bGH and rabbit GH were shifted to the right. Scatchard analysis of hGH displacement curves with sera from various species yielded linear plots and revealed that Mg2+ significantly increased (2.3-3.0-fold) the affinity constants, but not the binding capacities. These results demonstrate the ability of changes in Mg2+ concentration to determine the degree of differential recognition of somatogens versus lactogens by serum GH-BP. It remains to be determined whether such bivalent cation effects may account, at least in part, for the growth retardation seen in Zn2+ or Mg2+ ion deficiencies.

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