IDENTIFICATION AND INITIAL CHARACTERIZATION OF SERUM GROWTH-HORMONE BINDING-PROTEIN IN THE TURTLE CHRYSEMYS DORBIGNI

Proteins that bind growth hormone (GHBP) have been identified in the b lood of many mammalian and avian species, but not in reptilian species . We carried out binding studies with the serum of turtles using chrom atographic techniques as well as the dextran-charcoal separation metho d. As in other species, we found at feast two different GHBPs: one wit h high MW and law affinity and the other with lower MW and higher affi nity. The high affinity GHBP was partially purified using gelfiltratio n 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 p rotein in the purified material). When the high affinity GHBP was char acterized, 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 fo r turtle higher affinity GHBP. The specificity of this high affinity G HBP is somatogenic, since bovine GH competes as well as human GH for I -125-hGH bound to binding protein, while ovine PRL competes only parti ally and with low affinity. Unrelated hormones, as insulin and glucago n, can not displace the I-125-hGH bound to turtle GHBP. A very importa nt seasonal variation in turtle GHBP activity was observed: maximum bi nding was found in November (springtime), followed by a continuous dec line over March and May.