Demonstration of a formyl peptide receptor on lung macrophages. Correlation of binding properties with chemotaxis and release of superoxide anion

We have shown previously that the formylated peptides are uniquely potent chemo-attractants for guinea pig alveolar macrophages and that these synthetic peptides also stimulate alveolar macrophage oxygen metabolism, leading to the extracellular release of superoxide anion and hydrogen peroxide. To extend these observations, we identified the formylated peptide receptor on intact guinea pig alveolar macrophages, taking advantage of the availability of the radiolabeled peptide, N-formyl-methionyl-leucyl-[³H]phenylalanine(³H-FMLP). At 4°C, specific binding was rapid (half-time less than 1 min), reaching an equilibrium within 5 min. Specifically bound ³H-FMLP was saturable and rapidly reversible (80 to 90% displaced in 4 to 5 min) after the addition of an excess (thousandfold) of cold FMLP. A Scatchard analysis of the binding data at 4° C gave an equilibrium constant (K(d)) equal to 2.4 x 10^-8 M and about 5 x 10⁴ binding sites per cell. A linear Scatchard and Hill plot (coefficient = 1.06) of the binding data over a wide concentration range of free ³H-FMLP at 4°C suggested the presence of a single population of high affinity binding sites and the absence of cooperativity. Specificity of the formylated peptide binding sites was demonstrated by (1) inhibition of bound ³H-FMLP and of stimulated O-₂ release by two competitive antagonists for the formylated peptide receptor (carbobenzoxyphenylalanyl-methionine and Boc-phe-leu-phe-leu-phe) and (2) 50% inhibition of bound ³H-FMLP by two related peptides (N-formyl-methionyl-phenylalanine and N-formyl-norleucyl-phenylalanine) at concentrations that correspond to their K(d) and in rank order of biologic activity in stimulating O₂ release and chemotaxis. The K(d) for FMLP was in good agreement with the concentration required to stimulate half maximal response for chemotaxis (~1 x 10^-8 M) and O-₂ release (3 x 10^-8 M) induced by FMLP. Also, the rate of binding of ³H-FMLP to intact alveolar macrophages was compatible with the rapid metabolic responses induced by these peptides. These studies provide evidence that the biologic responses (i.e., chemotaxis, O-₂ release) induced by the formylated peptides are related to the binding of these peptides to specific receptors on the alveolar macrophage surface. Thus, the formylated peptides, which are similar in structure to the oligopeptides of bacterial metabolism, may be useful probes in elucidating how the alveolar macrophage participates in the lung's defense against airborn particles and microorganisms.