Heterologous receptor population for a chemotactic factor F-Met-Leu-Phe on the human neutrophil. Effect of pH and temperature.

Association kinetics of a chemotactic factor [3H]FMLP to its specific receptors on human neutrophils and the modulation of kinetic constants by pH and temperature were studied. The half-time for [3H]FMLP association to its specific receptors as calculated by a least-square fit equation was 27 min at 4 degrees C, 13 min at 24 degrees C, and 5 min at 37 degrees C. The Scatchard plot of [3H]FMLP binding to its specific receptors was found to be nonlinear, suggesting a heterogeneity of [3H]FMLP receptors and/or functional negative cooperativity. The computer analysis of the data showed two populations of FMLP receptors on human neutrophils with Kd (dissociation constant) values of 0.17 +/- SD 0.1 nM and 0.94 +/- SD 0.1 nM at 4 degrees C, and 0.17 +/- SD 0.02 nM and 1.1 +/- SD 0.2 nM at 37 degrees C. The optimum [3H]FMLP binding at 37 degrees C was observed at pH 7.5. There was essentially no difference in Kd values at pH 6.0, 7.5, or 8.5. Our results suggest the presence of two populations of [3H]FMLP receptors on human neutrophils in support of previous observations of other investigators. Our data also demonstrate that association constants between [3H]FMLP and its specific receptors on human neutrophils are not affected by changes in pH or temperature; maximal binding, however, is dependent on pH.