Binding, internalization, and degradation of 125I-multipotential colony-stimulating factor (interleukin-3) by FDCP-1 cells.

The kinetic parameters involved in determining the steady-state interaction of Multi-CSF with FDCP-1 cells at 37 degrees C have been determined by kinetic analysis under steady-state conditions and by curve-fitting the rate of approach to steady-state conditions. The two methods are in substantial agreement and yield values of Vr = 28 receptors/cell/min for the rate of appearance of receptors at the cell surface, ke and kt = 0.061 min-1 and 0.0044 min-1 for the rate constants of internalization of occupied and unoccupied receptors, respectively, kh = 0.008 min-1 for the rate constant of degradation of internalized ligand, ka = 2.9 X 10(8) M-1 min-1 for the rate constant of association and kd = 0.11 min-1 for the rate constant of dissociation of ligand with receptor. Analysis of steady-state conditions indicated that Multi-CSF caused substantial down-regulation of surface receptors and that considerably more Multi-CSF was inside the cell than at the cell surface. The implications of these results for utilization rates of Multi-CSF by FDCP-1 cells and the relationship of receptor occupancy to biological activity are discussed.