Uptake and destruction of 125I‐CSF‐1 by peritoneal exudate macrophages

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Abstract
The binding and uptake of the colony‐stimulating factor CSF‐1 by peritoneal exudate macrophages (PEM) from lipopolysaccharide insensitive C3H/HeJ mice was examined at 2°C, and at 37°C. At 2°C, 125I‐CSF‐1 was bound irreversibly to the cell surface. At 37°C, 90% of the cell surface associated 125I‐CSF‐1 was rapidly internalized and subsequently degraded and the remaining 10% dissociated as intact 125I‐CSF‐1. Thus classical equilibrium or steady state methods could not be used to quantitatively analyze ligand–cell interactions at either temperature, and alternative approaches were developed. At 2°C, the equilibrium constant (Kd ⩽ 10−13M) was derived from estimates of the rate constants for the binding (kon ≃ 8 × 105M−1s−1) and dissociation (koff ⩽ 2 × 10−7s−1) reactions. At 37°C, the processes of dissociation and internalization of bound ligand were kinetically competitive, and the data was formally treated as a system of competing first order reactions, yielding first order rate constants for dissociation, koff = 0.7 min−1 (t1/2 = 10 min) and internalization, kin = 0.07 min−1 (t1/2 = 1 min). Approximately 15 min after internalization, low‐molecular weight 125I‐labeled degradation products began to appear in the medium. Release of this degraded 125I‐CSF‐1 was kinetically first order over three half‐lives (Kd = 4.3 × 10−2 min−1, t1/2 = 16 min). Thus CSF‐1 binds to a single class of receptors on PEM, is internalized with a single rate limiting step, and is rapidly destroyed without segregation into more slowly degrading intracellular compartments.