Incorporation of 14C‐labelled polyunsaturated fatty acids by juvenile turbot, Scophthalmus maximus (L.) in vivo

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Abstract
The ability of juvenile turbot, Scophthalmus maximus (L.), to elongate and desaturate various polyunsaturated fatty acids (PUFA) was examined in relation to their lipid composition. Triacylglycerols were the most abundant lipid class present in the fish and phosphatidylcholine was the predominant phospholipid. In all lipid classes examined the levels of (n‐3) PUFA exceeded that of (n‐6) PUFA. 18C PUFA were minor components in comparison with 20:5(n‐3) and 22:6(n‐3). 20:4(n‐6) was present in highest concentration in phosphatidylinositol in which it accounted for 16.9% of the fatty acids. When the fish were injected with either 14C‐labelled 18:2(n‐6), 18:3(n‐3), 20:4(n‐6), 20:5(n‐3) or 22:6(n‐3) the highest percentage recovery of radioactivity (69%) in body lipid was observed with 22:6(n‐3). With all labelled substrates free fatty acids contained only a small proportion of the total recovered radioactivity whereas triacylglycerols were highly labelled. Phosphatidylcholine/sphingomyelin was the most highly labelled polar lipid fraction. With 14C‐20:4(n‐6) as injected substrate, 23.2% of the radioactivity recovered in total lipid was present in phosphatidylinositol in comparison with less than 6% with the other substrates. Only small proportions of radioactivity from 14C‐18:2(n‐6) and 14C‐18:3(n‐3) were recovered in the 20 and 22C fatty acids of triacylglycerols and total polar lipid. With 14C‐20:5(n‐3) as substrate, 27 and 33% of the total radioactivity recovered in the fatty acids of triacylglycerols and polar lipids respectively was present in 22C fatty acids. The corresponding values for l4C‐20:4(n‐6) as substrate were 19 and 18%. The results confirm the limited capacity of turbot to convert 18C PUFA to longer chain PUFA but demonstrate their ability to synthesize 22C PUFA from 20C PUFA. They also suggest a small but specific requirement for 20:4(n‐6).

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