Identification and Initial Characterization of Serum Growth Hormone Binding Protein in the Turtle Chrysemys dorbigni

Abstract

Proteins that bind growth hormone (GHBP) have been identified in the blood of many mammalian and avian species, but not in reptilian species. We carried out binding studies with the serum of turtles using chromatographic techniques as well as the dextran-charcoal separation method. As in other species, we found at least two different GHBPs: one with high MW and low affinity and the other with lower MW and higher affinity. The high affinity GHBP was partially purified using gel filtration and affinity chromatography, reaching a degree of purification of 11,000 times (0.17 nmol/g of serum protein in the serum vs 1900 nmol/g protein in the purified material). When the high affinity GHBP was characterized, it was found to have a dissociation constant (K_d: 2.6 ± 0.7 nM) similar to those described for mouse or rat, but lower than those for chicken, rabbit or man. The binding capacity (B_max) was 120 ± 43 fmoles/mg of protein, which can be also expressed as 1.08 ± 0.38 pmol/ml of serum. A preliminary MW estimation of 50–60 kDa was obtained for turtle higher affinity GHBP. The specificity of this high affinity GHBP is somatogenic, since bovine GH competes as well as human GH for ¹²⁵I-hGH bound to binding protein, while ovine PRL competes only partially and with low affinity. Unrelated hormones, as insulin and glucagon, can not displace the ¹²⁵I-hGH bound to turtle GHBP. A very important seasonal variation in turtle GHBP activity was observed: maximum binding was found in November (springtime), followed by a continuous decline over March and May.