Biosynthesis of eicosapentaenoic acid in the sea urchin Psammechinus miliaris

The sea urchin Psammechinus miliaris (Gmelin) (Echinodermata: Echinoidea) was shown by using a deuterated tracer (D 5 −18∶3n−3) and quantitation by negative chemical ionization gas chromatography‐mass spectrometry to convert 18∶3n−3 to 20∶5n−3. The rate of conversion was very slow, corresponding to 0.09 μg/g tissue/mg 18∶3n−3 eaten over 14 d. Deuterated arachidonic acid (D 8 −20∶4n−6) was also included in the diet to give a measure of the relative amounts of diet eaten by the different animals. The recovery of this fatty acid in tissue lipids was 33.7% compared with only 0.95% recovery of D 5 −18∶3n−3 and its anabolites, indicating that the majority of the D 5 ‐tracer was catabolized. Considerable elongation of D 5 −18∶3n−3 into 20∶3n−3 and a trace of 22∶3n−3 was found, and these were accompanied by minor amounts of the intermediates 18∶4n−3 and 20∶4n−3. No deuterated 22∶6n−3 was found.