Study of platelet-activating factor (PAF) carrier protein in human serum.

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The behavior of platelet-activating factor(PAF)produced in stimulated human polymorphonuclear leukocytes(PMN)was investigated in the presence of serum under conditions close to those existing in vivo. When the cells were stimulated in the presence of the serum obtained from a PAF acetylhydrolase(PAF-AH)-deficient Japanese subject, over 60% of synthesized PAF was detected in the extracellular medium by bioassay, scintillation proximity RIA and selected ion monitoring/gas chromatography/mass spectrography analysis. The release of PAF from PMN after stimulation with FMLP and A23187 was also observed in the presence of normal serum treat ed with acid to inactivate PAF-AH. The heterogeneity of the molecular species of extracellu. 1ar PAF was similar to that of intracellular PAF produced in- stimulated PMN in the presence of PAF-Aft-deficient serum, ruling out the possibility that a specific molecular species of PAF was preferentially released from the cells in the presence of the serum. As … More these data suggested the occurrence of PAF-releasing factor(s)in the serum, an attempt was made to partially purify this factor from PAF-AH-deficient serum and acid-treated normal serum by ammonium sulfate fractionation and column chromatography with DEAE-Cellulofine and Sepharose CL-6B. The molecular mass of PAF-releasing factor revealed on a TSK gel G3000 SW HPLC column was 24OkDa, which was differnt from that of albumin. The binding assay, newly developed for this study, revealed that the PAF-binding activity of PAF-releasing factor is stronger than that of albumin, and that the PAF-releasing factor forms a complex with PAF at low concentration (10^9 M). PAF bound to this factor was difficult to be hydrolyzed by serum PAF-AH. On the other hand, the PAF/PAF-releasing factor complex had aggregatory activity toward washed rabbit platelets. These observations suggest that certain protein(s)releases and carries the PAF newly synthesized by PMN in blood plasma/serum. Thus it appears that PAF functions as an autacoid in vivo, along with other mediators. Less