Biochemical and molecular studies of the polyunsaturated fatty acid desaturation pathway in fish

Abstract

Fish have an absolute dietary requirement for certain polyunsaturated fatty acids (PUFA) termed “essential fatty acids” (EFA) that include members of both the n-6 and n-3 series typified by linoleic acid, 18:2n-6, and α-linolenic acid, 18:3n-3. However, the biologically active forms of EFA are generally the C20 and C22 metabolites of 18:2n-6 and 18:3n-3, viz. 20:4n-6, 20:5n-3 and 22:6n-3. Some fish species can convert C18 PUFA to the C20 and C22 PUFA through a series of alternating desaturation and chain elongation reactions mediated by microsomal systems containing elongases and Δ6 and Δ5 fatty acid desaturases. In species that cannot perform these conversions, the C20 and C22 PUFA themselves are dietary EFA and their C18 homologues do not satisfy EFA requirements. The extent to which the foregoing statements apply quantitatively to a given fish species varies widely. Therefore, a vital area in lipid nutrition in fish is the provision of sufficient amounts of the correct EFA to satisfy the requirements for normal growth and development, requirements that can vary quantitatively during the life of the fish and are particularly important factors in larval marine fish. This paper reviews the work on defining and characterising the fatty acid desaturation and elongation pathway in fish. Biochemical studies have been advanced by the use of cell cultures which have elucidated key parts of the pathway. Thus, the presence of the so-called Sprecher shunt, where 22:6n-3 is produced from 20:5n-3 through two successive elongations and a Δ6 desaturase followed by peroxisomal chain shortening, was demonstrated in trout. Similarly, the block in the pathway in marine and/or piscivorous fish could be due to either a deficiency of C18-20 elongase or Δ5 desaturase and this varies between different marine species. Recent work has focussed on the molecular biology of the pathway with the cloning of fatty acid desaturases and elongases from a variety of fish species. Zebrafish have been used as a model species and a unique desaturase possessing both Δ6 and Δ5 activity along with an elongase with very high C18-20 activity have been cloned and characterised. Understanding this pathway is of increased importance due to the current dependence of salmonid and marine fish aquaculture on fish oil, the supply of which is becoming increasingly limited and unsustainable, necessitating the use in fish feeds of sustainable plant oils, rich in C18 PUFA, but devoid of C20 and C22 PUFA.