CONCENTRATION OF FREE GROWTH HORMONE-BINDING PROTEIN IN THE SERUM OF MICE IS NOT REGULATED BY GROWTH-HORMONE

Ames dwarf mice that do not express growth hormone (GH) or prolactin ( PRL) genes were used to study the effects of GH deficiency on the pres ence and the characteristics of GH-binding protein (GHBP) in serum. Ch romatographic techniques were used to alloy characterization of biolog ical rather than immunological activity of GHBP. Two GH-binding fracti ons were found in dwarf mice serum, one with low affinity and high cap acity (GHBP(I)) and one with high affinity, low capacity and lower mol ecular mass (GHBP(II)). Serum concentration of the high-affinity GHBP was 0.73 +/- 0.03 mu M with a K-d of 6.3 +/- 1.7 nM. Since Ames dwarfm ice have no GH in the circulation, all the GHBP is free. Interestingly , the concentration of GHBP in dwarf mice was similar to the levels of free GHBP measured in normal mice from the same line. Moreover, this value (0.7 nM) closely resembles the concentration of free GHBP in the serum of transgenic mice overexpressing GH, in which peripheral GH le vels are grossly elevated. These observations can be interpreted as ev idence that the levels of free GHBP in mouse serum are independent of GH concentration, and that GH influences only the levels of bound GHBP in peripheral circulation.