It has been postulated that the potent chemotactic properties of the complement-derived anaphylatoxin C5a are due, at least in part, to the interaction of the peptide with a specific PMN receptor. We have examined the nature of this receptor by two techniques. In the first case, the receptor has been directly demonstrated by examining the cellular binding of 125I-labeled human C5a. Kinetic studies showed that uptake of the labeled peptide was essentially complete within 1–3 min. The C5a binding data obtained under equilibrium conditions, over the concentration range of of 10-11 to 10-7 M, indicated the ED50 for specific binding was 2–6 × 10-9 M. This value is in close agreement with an ED50 of 2–8 × 10-9 M determined for PMN chemotaxis. A total of 1–3 × 105 binding sites per cell was estimated from the uptake results. Because C5a-induced release of lysosomal enzymes from cytochalasin B-treated PMNs closely parallels the binding of C5a, enzymatic release provides direct measure of the C5a-cellular interaction.

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