Water-Soluble Specific Growth Hormone Binding Sites from Cultured Human Lymphocytes: Preparation and Partial Characterization

Abstract

Cultured human lymphocytes (IM-9 cell line) have specific binding sites or receptors for human growth hormone (hGH). Under appropriate conditions, this specific binding material is spontaneously released into the incubation medium and is solubilized without the use of detergents. This water-soluble preparation binds [125I]iodo-hGH with the same specifictiy as receptors on the intact cell. Unlabeled hGH, but not porcine growth hormone or insulin, competes with labeled hGH for binding to the soluble preparation. Growth hormone preparations of varying purity compete for binding to the soluble binding preparation in the same rank order as they compete for binding to the intact IM-9 lymphocyte. [125I]iodo-hGH incubated with, but not bound to, the soluble preparation is partially degraded, while the [125I]iodo-hGH that is bound to the soluble preparation is protected from degradation, and its ability to rebind to fresh cells is enhanced. The [125I]iodo-hGH-soluble binding preparation complex can be dissociated by the addition of large quantities of unlabeled hGH or by lowering the pH, and [125I]iodo-hGH in both instances remains intact and undegraded. The soluble binding preparation did not sediment when centrifuged at 200,000 X g for 4 hours and was not retained on 0.20 micron Millipore filters. The soluble binding preparation was not retarded on Sephadex G-200. Binding activity was abolished by tryptic digestion. These studies demonstrate that hGH-binding sites, like previously reported insulin binding sites, can be spontaneously solubilized from cultured human lymphocytes, without the use of detergents; these soluble binding preparations are of high molecular weight and are, at least in part, protein in nature.