Characteristics of solubilized human-somatotropin-binding protein from the liver of pregnant rabbits

Abstract

A specific binding site for somatotropin was solubilized by 1% (v/v) Triton X-100 from a crude particulate membrane fraction of pregnant rabbit liver, partially purified and characterized. The solubilized binding site retained many of the characteristics observed in the original particulate fraction, indicating that extraction of the binding site with Triton X-100 does not cause any major changes in its properties. The binding of human 125I-labelled-somatotropin to the solubilized binding site is a saturable and reversible process, depending on temperature, incubation time, pH and ionic environment. Analysis of the kinetic data revealed a finite number of binding sites with an affinity constant of 0.32 x 10(10)M-1. The binding activity for human 125I-labelled-somatotropin was adsorbed to a concanavalin-A-Sepharose column and was dissociated from the column with alpha-methyl-D-glucoside, suggesting that the binding protein may be a glycoprotein. Using affinity chromatography on concanavalin-A-Sepharose, ion-exchange chromatography on DEAE-cellulose and gel filtration on Sepharose 6B, the binding protein was purified 1000-4000-fold from the original liver homogenate. When the partially purified preparation was chromatographed on Sepharose 6B, the binding protein eluted as a molecule with an apparent molecular weight of 200000, with a Stokes' radius of 4.9 nm. Sucrose-density-gradient centrifugation of the preparation showed that the sedimentation coefficient of the binding protein was 7.2S. Isoelectric focusing experiments revealed that a major part of the protein has an acidic pI (4.2-4.5). Exposure of the protein to trypsin decreased the binding activity for human 125I-labelled-somatotropin or bovine 125I-labelled-somatotropin, whereas ribonuclease, deoxyribonuclease, phospholipase C or neuraminidase had little or no effect.

Analysis of hepatic growth hormone binding sites of pregnant rabbit crosslinked to 125I‐labelled human growth hormone

“…When peak B fractions of Sepharose 6B column were pooled and applied to a concanavalin ASepharose column, -80% of the radioactivity was adsorbed to the column, and a major part of the adsorbed counts could be dissociated by elution with 0.3 M cY-methyl-D-glucoside (not shown). The behavior of t251-hGH crosslinked receptor in both Sepharose 6B and concanavalin A-Sepharose columns is quite similar to that of its non-crosslinked counterpart [3], indicating that crosslinking using DSS does not cause gross changes in the Mr of the receptor or in its properties as a glycoprotein.…”

“…4: (A) which barely entered the gel, may represent a high M,> 250 000) aggregate of the 125 I-hGH:receptor complex; (C) representing non-crosslinked 1251-hGH; (B) migrating to the position corresponding to M, 80 000 + 3000 (the mean + SD of 10 expt.) and was largely elimi- hepatic GH receptor of pregnant rabbits has been estimated to be 300 000 [2] or 200 000 [3]. The receptor may be composed of M, 59 000 subunit non-covalently associated with a non-identical component.…”

“…The t*sI-hGH crosslinked membranes were solubilized with 1% (v/v) Triton X-100 as in [3]. About 60% of the total membrane proteins and Membrane fractions were incubated with i251-hGH in the presence or absence of unlabelled hGH (10 pg/ml), washed, and then treated with 1 mM DSS for 30 min at 20°C.…”

“…Crude membrane fractions were prepared from the livers of pregnant rabbits (New Zealand White, 25-29 days pregnant) as in [ 1,3]. The fractions were stored at -20°C until use.…”

Prolonging the Action of Protein and Peptide Drugs by a Novel Approach of Reversible Polyethylene Glycol Modification

“…After 10 min PEG 40,000 -SH (10 mg) was added, and the reaction mixture was stirred for 1 h and dialyzed overnight against water. An hGH displacement assay was performed as described elsewhere (21).…”