Platelet-activating factor stimulates receptor-mediated formation of reactive oxygen intermediates in human monocytes

Stimulation of production of reactive oxygen intermediates (ROI) was examined in human peripheral blood monocytes by luminol-dependent chemiluminescence. The dose-response curve characterizing the dependence of ROI production on the concentration of platelet-activating factor (PAF) showed that stimulation occurred within a concentration range of 2 x 10(-9)M to 5 x 10(-6)M. Transformation of the dose-response curve to an Eadie-Hofstee plot indicated that the process is characterized by two Km values. The Km value corresponding to the high-affinity branch of the curve is 1.3 +/- 0.14 nM. In the same cells, the dissociation constant for the [3H]PAF/receptor complex was determined. The Kd value was 0.8 +/- 0.1 nM, which agreed quite well with the high-affinity Km value obtained in the Eadie-Hofstee plot. The data indicate that stimulation of ROI generation is mediated through PAF binding at specific receptor sites at nanomolar PAF concentrations. Along with the specific receptor-mediated ROI generation, a nonspecific effect of PAF at a high concentration was demonstrated.