Biosynthetic intermediates and stereochemical aspects of the aldehyde biosynthesis in the marine diatom Thalassiosira rotula

Intermediates of the aldehyde biosynthesis in Thalassiosira rotula are investigated. Use of labeled precursors and cell preparations proves production of 2E,4Z-octadienal (1a) from 6Z,9Z,12Z-hexadecatrienoic acid (C16:3 x 4) through the lipoxygenase-dependent intermediate (9S)-9-hydroperoxyhexadeca-6,10,12-trienoic acid. On the contrary, synthesis of 2E,4Z,7Z-decatrienal (2a) involves mainly EPA (C20:5 x 3) by a 11R-lipoxygenase, as suggested by identification of chiral 11R-HEPE (12% e.e.) in the diatom extracts. Consistently with the necessity to have a rapid transport and metabolization of the intermediate hydroperoxides, we show that lipoxygenase and lyase activities are both found in the same subcellular fraction of the microalga.