A novel platelet-activating protein derived from rat submandibular glands

Among the various naturally occurring substances which induce platelet aggregation and secretion, considerable attention has been focused recently on the phospholipid platelet-activating factor (PAF) derived from a variety of mammalian cells. In this report, we describe a platelet-activating protein, designated RS-PAP, which was isolated and partially purified from the submandibular salivary glands of adult male rats. This material caused a dose-related in vitro activation of washed, [3H]serotonin-labeled platelets from rats, rabbits and humans. The biologic activity of RS-PAP was compared with that of 1-0-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine (AGEPC), a well-characterized representative of the phospholipid platelet-activating factors, and collagen, a potent platelet-stimulating protein. The activation of washed rabbit platelets by RS-PAP required extracellular calcium for both aggregation and secretion of [3H]serotonin, and was not inhibited by indomethacin. Furthermore, RS-PAP was not inhibited by pretreatment with phenylmethylsulfonyl fluoride, iodoacetate, hirudin or an acetylhydrolase which specifically degrades the phospholipid AGEPC. RS-PAP was completely inactivated by heating at 56 degrees C for 20 minutes, by trypsinization and by a phospholipid extraction procedure. Gel filtration on Sephacryl S-200 indicated an approximate molecular weight of 22,000 - 25,000 daltons. Thus RS-PAP appears to be a new platelet-stimulating protein which activates a variety of mammalian platelets.