Effect of pH on Binding and Dissociation of Colony-Stimulating Factor

Abstract

Abstract

¹²⁵I-labeled colony-stimulating factor (CSF) binds to granulocytic and monocytic cells in the bone marrow in an irreversible manner. Addition of a 1000-fold excess of unlabeled CSF does not displace the bound material. The present studies showed that brief exposures to pH 2.7-5.0 caused a marked release of the bound material. Such treatments were nontoxic to the marrow cells as judged by trypan blue dye exclusion, assay of colony-forming cells, and by analysis of rebinding of fresh ¹²⁵I-CSF to the acid-treated cells. The CSF released from marrow cells by low pH revealed two peaks of radioactivity on SDS-acrylamide gel. The first peak (67,500 Da) corresponded to native CSF; a second peak of 53,500 Da was observed. Despite this apparent mild degradation of CSF, the released material showed greater binding to marrow and greater precipitation by anti-CSF than the native ¹²⁵I-CSF. Further studies showed that acid treatment of marrow cells led to stabilization of the CSF receptors. Pretreatment at pH 4.0 led to retention of binding sites after conversion of marrow cultures to pH 7.5 and incubation at 22–37°C. In contrast, cells that were not exposed to low pH lost receptors rapidly at these temperatures. The extent of preservation of the binding sites was related to the duration of acid exposure. These studies indicate that CSF is retained on the cell surface after binding at 0°C and that the CSF can be eluted by acid conditions. Similar treatment of the cells prior to binding appears to stabilize the cells and prevent the receptor loss that ordinarily occurs at 22–37°C.