The free sterols and phospholipids of the demospongeAplysina fistularis were isolated and analyzed. The free sterols consisted mainly of the unusual 26-methylated sterols aplysterol (53%) and 24(28)-dehydroaplysterol (7%) together with 7 commonly occurring sterods. The major phospholipids were phosphatidylcholine, phosphatidylglycerol, phosphatidylinositol, phosphatidylethanolamine, phosphatidylserine and diphosphatidylglycerol. The major fatty acyl components of the phospholipids consisted of 85% C14-C20 acids, including the unprecedented 2,6,10-trimethyl-5-tetradecenoic acid and 11-methyloctadecanoic acid. The remaining 15% were C27-C30 demospongic acids, including 2 novel acids tentatively assigned the structures 5,9,23-octacosatrienoic acid and 5,9,23-nonacosatrienoic acid, and 3 novel acids proven to be 5,9,21-octacosatrienoic acid, Z,Z-20-methyl-5,9-hexacosadienoic acid and Z,Z-22-methyl-5,9-octacosadienoic acid. The biosyntheses of the novel demospongic acids are proposed to occur by chain elongation of monoenoic or branched precursors followed by desaturation. The large quantities of typically bacterial phospholipids and fatty acids found implied the presence of bacteria in the sponge, in agreement with microscopic studies. Analysis of the phospholipid-bound fatty acids in a sponge cell-enriched fraction indicated that the demospongic acids, including the 2 branched structures, were the major acids of the sponge cells. The presence inA. fistularis of demospongic acids containing membrane disordering groups-methyl branches or double bonds-on the ω7 carbon is proposed to be due to the need by the sponge for membranes possessing fluidity near the middle of the phospholipid bilayer. It is also proposed that the C26 methyl group of aplysterol causes disordering of the phospholipid bilayer in the same region, and thus also evolved in response to this need.

The fatty acyl components of the phospholipids from the sponge Petrosia ficiformis consisted predominantly of branched, especially iso and anteiso acids. The two major components of the complex mixture are the hitherto unknown Z,Z-25-methyl-5,9-hexacosadienoic and Z,Z-24:methyl-5,9hexacosadienoic acids. Other unknown acids are: 7,13,16-docosatrienoic acid, 15-methyldocosanoic acid, 15-methyltricosanoic acid and 24-methyl-5,9-pentacosadienoic acid. Short branched-chain fatty acids, presumably of bacterial origin, are considered to be the possible bioprecursors of these novel phospholipid constituents. The major phospholipids were PE, PC, PG, PS and PI. The distribution of fatty acids among the phospholipid classes was also studied.

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