INFLUENCE OF MG2-HORMONE-BINDING PROTEIN IN MAMMALIAN SERA( ON DETECTION OF SOMATOGENIC AND LACTOGENIC COMPONENTS OF GROWTH)

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 ser a (rabbit, horse, dog, pig and cat), based on the capacity of bovine ( b) and ovine prolactin (PRL) to displace I-125-labelled human growth h ormone (hGH) binding and on direct I-125-bPRL binding studies. In this study, we demonstrate the high degree of Mg2+ dependence of the bindi ng of the classically lactogenic hGH and bPRL, but not that of the som atogenic bGH to various mammalian sera (types I-IV). Serum GH-BP was a ssayed using a previously described and validated charcoal-separation assay. I-125-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+. T he Mg2+ effect was concentration-dependent between 3.7 mM and 70 mM, c ausing a significant and proportional increase in I-125-hGH binding to serum. Like I-125-hGH, I-125-bPRL binding to type-III sera was also M g2+-dependent. In contrast, I-125-bGH binding to all types of Serum GH -BP was not affected by Mg2+ concentrations of up to 35 mM. while 70 m M Mg2+, slightly, but significantly, reduced (by approx. 15%) bGH bind ing to rabbit serum. In keeping with the Mg2+-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 I-125- hGH binding to rabbit, dog, horse and human sera, while the effects of the somatogens bGH and rabbit Gfi were shifted to the right. Scatchar d analysis of hGH displacement curves with sera from various species y ielded linear plots and revealed that Mg2+ significantly increased (2. 3-3.0-fold) the affinity constants. but not the binding capacities. Th ese results demonstrate the ability of changes in Mg2+ concentration t o determine the degree of differential recognition of somatogens versu s lactogens by serum GH-BP. It remains to be determined whether such b ivalent cation effects may account, at least in part, for the growth r etardation seen in Zn2+ or Mg2+ ion deficiencies.