Ivan L. Stoilov

Biosynthetic studies of marine lipids 12.: Biosynthesis in marine sponges of sterois possessing the δ5,7-nucleus typical of fungi and the 24-alkyl side chain characteristic of plants

Abstract The biosynthesis of sterols with structural patterns common to plant and yeast sterols was studied by incorporation of eleven radiolabeled sterol precursors and mevalonate. The capability of sponges to convert Δ 5 - into Δ 5,7 -sterols, which was as efficient as the conversion of Δ 7 - into Δ 5,7 -sterols, was demonstrated by double labelling experiments. By examining epimeric pairs, stereospecific conversion of codisterol and clerosterol into the corresponding 7-dehydrosterols was established. 24β-24-Alkyl sterols with homo-conjugated 22,25-diene side chains are proposed as actual intermediates in the formation of Δ 22 -24β-alkylsterols. An alternative pathway via oxidation at C-22 or C-23 with subsequent dehydration of an intermediate 22(R or S)- or C-23(R or S)-hydroxysterol is probably not operative due to lack of incorporation of any of the tritium labeled alcohols. These biosynthetic features, though unexpected for animals, nevertheless are attributed to the sponge because of the absence of symbiotic fungi and algae as shown by electron microscopy.

Fatty acids as biological markers for bacterial symbionts in sponges

Analyses of fatty acids with carbon numbers between C12 and C22 are reported for five Great Barrier Reef sponges. These analyses indicate that phototrophic cyanobacterial symbionts (blue-green algae) present in three of the sponges are chemically distinct, whereas the other two sponges do not contain cyanobacterial symbionts. All the sponges contain other, nonphototrophic bacteria. The fatty acid analyses indicate that the nonphototrophic bacterial populations present in the different sponges are distinct in both their chemical compositions and their abundances. Nonphototrophic bacteria are estimated to account for between 60 and 350 μg/g (extractable fatty acids:tissue wet weight), whereas cyanobacteria account for between 10 and 910 μg/g. One sponge (Pseudaxinyssa sp.) contains a relatively large amount of the isoprenoid acid, 4,8,12-trimethyltridecanoic acid; this acid is presumed to be derived from phytol, a degradation product of chlorophyll. This sponge also contains relatively large amounts of the nonmethylene interrupted fatty acid, octadeca-5,9-dienoic acid. Analyses of interior and cyanobacteria-rich surface tissues of this sponge indicate that these two acids are probably not associated with the symbiotic cyanobacteria.

Biosynthetic studes of marine lipids 7 : Experimental demonstration of a double alkylation at c-28 in the biosynthesis of 24-isopropylcholesterols in a sponge

Abstract The biosynthesis of 24-isopropyl sterols comprising 99% of the sterols of the Great Barrier Reef sponge Pseudaxinyssa sp. was studied. The major biosynthetic pathway was established by incorporation of 24-methylenecholesterol-[28-14C], fucosterol-[22-T], isofucosterol-(22-T], (24R)-24-isopropenylcholesterol-(29-14C], (24S)-24-isopropenylcholesterol-[29-14C], DL-mevalonate-(2-14C] and L-methionine-[methyl-14C]. Very efficient side chain branching through double alkylation at C28 was demonstrated in contrast to relatively poor de novo biosynthesis. A stereoselective mode for hydrogen migration is observed and a regioselective mode is proposed for this migration.

Cell membrane localization of sterols with conventional and unusual side chains in two marine demonsponges

Subcellular fractionation by differential centrifugation was performed on two previously unstudied marine sponges that predominantly contain either conventional (Reniera sp.) or unconventional (Pseudaxinyssa sp.) sterols. Direct evidence for the presence of unconventional sterols with C24 alkylated side chains in the cellular membranes ofPseudaxinyssa sp. is provided, but the presence of unconventional sterols in sponge membranes is shown not to be a universal feature of the Porifera. Possible structural and functional roles of unconventional lipid molecules in sponge cell membranes are discussed.

Biosynthetic studies of marine lipids 8 : Course of the stereoselective alkylation and regioselective hydrogen migration in the biosynthesis of the sponge sterol 24(s)-24 isopropenylcholesterol

Abstract The detailed mechanism of the formation of (24S)-24-isopropenylcholesterol, the key intermediate in the biosynthesis of 24-isopropylcholesterol, was studied in a Pseudaxinyssa sponge species. A stereoselective aIkylation at C28 of (iso)fucosterol was demonstrated involving a regioselective hydrogen migration.

Biosynthetic studies of marine lipids 10. Double side chain extension in the triply alkylated sponge sterol xestosterol

Abstract Xestosterol, a triply alkylated marine sponge sterol, with a symmetrically extended side chain is shown to be efficiently biosynthesized by the sponge Xestospongia testudinaria via a nonstereoselective SAM biomethylation and 1,2-hydrogen shift. De novo biosynthesis via mevalonate, rarely demonstrated in sponges, has also been demonstrated.

Minor and trace sterols in marine invertebrates 55. the isolation, structure elucidation and synthesis of ergosta-5,24 (28),25-trien-3β-ol

The isolation, structure determination and synthesis of ergosta-5, 24(28), 25-trien-3 beta-ol, as well as the synthesis of its 28-14C analog--a possible biosynthetic precursor of several marine sterols--is described.

Biosynthetic studies of marine lipids 18. Biosynthesis of 3β-(hydroxymethyl)-A-nor-5α-cholest-15-ene in the marine sponge phakellia aruensis

Abstract Through appropriate labeling experiments, it was shown that the unusual sterol 3β-(hydroxymethyl)-A-nor-5α-cholest-15-ene ( 1 ) is generated by ring contraction of dietary cholesterol followed by the introduction of the 15–16 double bond, rather than by ring contraction of 15-dehydrocholesterol ( 4 ).