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The metabolism of platelet activating factor in platelets and plasma of various animals.

Metabolism of platelet-activating factor (PAF) in rabbit plasma or in rabbit platelets was studied. [C3H3]-Labeled PAF was degraded into lysoPAF and choline in plasma. An agonist of PAF, NT071 was not degraded in the plasma. Albumin protects the degradation of PAF in plasma deprived of albumin but not the degradation of lysoPAF. These findings indicate that PAF may be metabolized in plasma by acetylhydrolase and then by lysophospholipase D. PAF was converted to phosphatidylcholine (PC) in washed rabbit platelets. The radioactivities in PC was recovered in the fraction of lysoPC after mild alkaline treatment, suggesting that the product is 1-alkyl-2-acyl-glycerophosphocholine. The binding of PAF and lysoPAF to rabbit platelets, rabbit erythrocytes and liposomal membranes were next examined. The binding of PAF to various membranes was inhibited by albumin. Albumin also suppressed the activation of platelets by PAF. A monomeric form of PAF, which is free from albumin, may react with target cell membrane and also be degraded by catabolic enzymes. The binding of lysoPAF to platelets, erythrocytes and liposomes was more effectively inhibited by albumin than that of PAF. The affinity of PAF to lipid bilayers may be higher than that of lysoPAF.