The role of platelet-activating factor (PAF) as a mediator appeared in rather primitive organisms like protozoans and was maintained in more evolved organisms. No reports exist for the presence of PAF or PAF analogues—or even compounds that exhibit PAF-like activity—in cyanobacteria, even though they belong to a a group of organisms at a low evolutionary level where the content of alkylacyl forms of ether lipids is expected to be high. In addition, cyanobacteria serve as a rich source of novel bioactive metabolites. In the present study the total lipids of a strain of Scytonema julianum, a filamentous cyanobacterium isolated from a Greek cave, were separated into neutral lipids and phospholipids, the latter being further fractionated by HPLC. Each phospholipid fraction was tested in vitro for its ability to inhibit PAF-, arachidonic acid- and ADP-induced washed-rabbit-platelet aggregation and/or to cause platelet aggregation. Two types of phospholipids causing platelet aggregation were detected and shown to be an acetylsphingomyelin and an acylacetylglycerol phosphoacetylated glycolipid. The existence of the sphingomyelin analogues is very important, since ceramides, cerebrosides and related lipids are intracellular second messengers. The identification of the phosphoglycoglycerolipid demonstrates a new type of lipid in cyanobacteria, namely one that exhibits a biological activity very similar to that of PAF. Its presence reinforces the concept that PAF is a member of a large family of lipid mediators, apparently having different physiological roles in prokaryotic and eukaryotic organisms. In addition, Scytonema julianum contains a phosphatidylcholine (C16:0/18:2), even though bacteria in general seldom contain choline-containing phosphoacylglycerols.
Abbreviations used: AA, arachidonic acid; ESI, electrospray ionization; IC50, concentration giving half-maximal inhibition; MS/MS, tandem MS; PAF, platelet-activating factor; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PL, phospholipids and phosphoglycolipids; SM, sphingomyelin.