Unusual pattern of fatty acid biosynthesis. Evidence for C-19 desaturase activity in freshwater sponges.

The two long chain fatty acids common in marine demosponges, (5Z, 9Z)-5,9-hexacosadienoic (delta 5,9-26:2) and (5Z, 9Z, 19Z)-5,9,19-hexacosatrienoic (delta 5,9,19-26:3) acids), were identified also as the major phospholipid components in the freshwater sponge Ephydatia fluviatilis. Whereas the typical marine sponge Microciona prolifera biosynthesizes the delta 5,9,19-26:3 acid solely by homologation of exogenous palmitoleic acid (delta 9-16:1) and subsequent desaturation at positions 5 and 9, it was found that the freshwater sponge could further desaturate the delta 5,9-26:2 acid to the triene, indicating for the first time the existence of delta 19 -desaturase activity in a living organism.