PHOSPHOLIPID-COMPOSITION OF THE GRANULAR AMEBOCYTE FROM THE HORSESHOE-CRAB, LIMULUS-POLYPHEMUS

The phospholipid composition was determined for the amebocyte of the p rimitive arthropod Limulus polyphemus. The total fatty acid compositio n of the cells' lipids was analyzed by gas chromatography/mass spectro metry (GC/MS) of fatty acid methyl esters (FAME). The FAME analysis re vealed high levels of 20-carbon polyunsaturated fatty acids (PUFA), es pecially arachidonic (20:4n-6) and eicosapentaenoic (20:5n-3) acids. A lmost 20% of the total lipid profile was comprised of dimethyl acetals of 16- to 20-carbon chain lengths, indicative of plasmalogens in the phospholipid pool. Phospholipids, analyzed by high-pressure liquid chr omatography, included phosphatidylethanolamine (PE), phosphatidylcholi ne (PC), phosphatidylserine (PS), phosphatidylinositol (PI), sphingomy elin (SPH), and cardiolipin (CL). PE and PC levels predominated at 42. 2 and 36.3%, respectively. Smaller amounts of PS (9.0%) and PI (6.2%) were present, as well as low levels of SPH (4.6%), CL (1.6%), and trac e amounts of lysophosphatidylcholine. The major phospholipid species, PE, PC, PS and PI, were collected and their molecular species were exa mined by electrospray-ionization mass spectrometry. The molecular spec ies within the phospholipid classes reflected the high levels of PUFA seen in the total lipid profile. PI was mainly composed of 18:0a/20:4. Over half of the PS consisted of 18:0d18:1 and 18:0a/20:4. The major PE species were 20:1p/20:5, 20:1p/20:4, 18:0p/20:5, and 18:0p/20:4. PC had the largest distribution of molecular species, and its most abund ant species were 16:0e/20:5, 16:0e/20:4, and 16:0p/20:4. The presence of 16:0e/20:4 is the first documentation of a specific precursor to pl atelet-activating factor in an invertebrate hemocyte. Note: at the sn- 1 position: [a = 1 = O-acyl, e = 1-O-alkylether, and p = 1-O-alk-1'-en yl (plasmalogen)].