Evidence for the presence of specific high affinity cytosolic binding sites for platelet-activating factor in human neutrophils

In this study evidence for the presence of specific cytosolic platelet-activating factor (PAF) binding sites has been presented. The equilibrium dissociation constant (Kd) as determined by Scatchard analysis was 4.29 x 10(-9) M for the cytosolic and 3.71 x 10(-9) M for the membrane fraction. The maximal number of binding sites estimated were 219 fmol/100 micrograms protein for the cytosol and 154 fmol/100 micrograms protein for the membrane, respectively. The specific receptor binding of [3H]PAF to cytosol could be displaced by two potent specific PAF receptor antagonists, BN 50739 and WEB 2086, the equilibration inhibition constants (Ki) being 1.27 x 10(-7) M and 5.7 x 10(-6) M, respectively. These data demonstrate clearly the role of intracellularly synthesized PAF as a second messenger in the receptor-mediated neutrophil activation.