Binding, Internalization, and Degradation of I-125-MultipotentialColony-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, k, and k,= 0.061 min(-1) and 0.0044 min(-1) for the rate constants of internalization of occupied and unoccupied receptors, respectively, k,, =0.008 min(-1) for the rate constant of degradation of internalized ligand, k,=2.9 XlO" M-' min-' for the rate constant of association and k,=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.