Analysis of the binding of a hemopoietic growth factor, P-cell-stimulating factor, to a cell surface receptor using quantitative absorption of bioactivity.

Quantitative absorption of biological activity was used to study the interaction of persisting (P)-cell-stimulating factor (PSF), a T-cell-derived lymphokine, with PSF-dependent lines of hemopoietic cells (P cells). It was shown that suspension of P cells in medium containing PSF at 4 degrees C resulted in a diminution of PSF activity in the medium. Similar results were obtained with homogeneous, pure PSF or crude supernatants from cells secreting PSF. This diminution was specific and involved saturable, reversible binding of PSF to the cells rather than degradation of PSF or the release of an inhibitor. Calculations based on the measurement of PSF activity remaining after absorption and estimates of the specific activity of PSF indicated that there were approximately 1000 receptors/cell and that PSF bound with an equilibrium dissociation constant (Kd) of 5 X 10(-12) M. Increased amounts of PSF were absorbed at 37 degrees C; however, in the presence of metabolic inhibitors, the amount of PSF activity removed was equivalent to that seen at 4 degrees C.