Investigating the essential fatty acids in the common cuttlefish Sepia officinalis (Mollusca, Cephalopoda): Molecular cloning and functional characterisation of fatty acyl desaturase and elongase

Some polyunsaturated fatty acids (PUFA) play critical roles in physiology and are essential for normal growth and development of aquatic animals including cephalopods like the common cuttlefish Sepia officinalis. This is particularly true for early life stages when neuronal tissues are rapidly developing and accumulating PUFA. The specific fatty acids (FAs) that can satisfy the essential requirements of any animal species depend upon the enzymatic capability to convert dietary FA into those physiologically important FAs required for normal function. In order to identify the dietary essential FA for the common cuttlefish, we have performed the molecular cloning and functional characterisation of two key enzymes involved in PUFA biosynthesis in this species, namely a fatty acyl desaturase (Fad) and an elongation of very long-chain fatty acid (Elovl) protein. The gene product of the cuttlefish Fad exhibited ∆5-desaturase activity, enabling this species to potentially biosynthesise the physiologically essential FAs eicosapentaenoic (20:5n-3, EPA) and arachidonic (20:4n-3, ARA) acids from 20:4n-3 and 20:3n-6, respectively. However, the cuttlefish Fad did not show ∆6 or ∆8 activity and therefore the biosynthesis of EPA and ARA from C18 PUFA precursors could be limited, suggesting that both may be essential FAs for cuttlefish. Moreover, the cuttlefish Fad also lacked ∆4-desaturase activity suggesting that DHA biosynthesis was not possible and thus DHA is also an essential FA for this species. The cuttlefish ∆5 Fad was able to produce non-methylene-interrupted (NMI) FA, a group of PUFAs typically found in marine invertebrates. The cuttlefish Elovl was able to elongate C18 and C20 PUFA substrates, but showed no activity towards C22 PUFA. Overall the results obtained in the present study allowed the prediction of the biosynthetic pathways of PUFA, including NMI FA, in the common cuttlefish. It was concluded that EPA, ARA and DHA were likely to be essential dietary FA for this species as endogenous production from precursor FA appears to be limited