Biosynthesis of eicosapentaenoic acid in the sea urchin Psammechinus miliaris

The sea urchin Psammechinus miliaris (Gmelin) (Echinodermata: Echinoidea) w as shown by using a deuterated tracer (D-5-18:3n-3) and quantitation by neg ative chemical ionization gas chromatography-mass spectrometry to convert 1 8:3n-3 to 20:5n-3. The rate of conversion was very slow, corresponding to 0 .09 mug/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 1 8:3n-3 and its anabolites, indicating that the majority of the D-5 tracer w as catabolized. Considerable elongation of D-5-18:3n-3 into 20:3n-3 and a t race of 22:3n-3 was found, and these were accompanied by minor amounts of t he intermediates 18:4n-3 and 20:4n-3. No deuterated 22:6n-3 was found.