regulation of highly
unsaturated fatty acid (hUFA) biosynthesis in fish: characterisation and
expression of genes
D.R. Tocher, X. Zheng, M.J. Leaver, J.G. Bell, G.
Krey, A.J. Teale
Fatty acid desaturases and
elongases are critical enzymes in the pathways for the biosynthesis of
the long-chain C20
and C22 highly unsaturated fatty acids (HUFA), arachidonate
(20:4n-6), eicosapentaenoate (20:5n-3) and docosahexaenoate
(22:6n-3) from shorter chain C,8 polyunsaturated
fatty acids (PUFA) such as linoleic (18:2n-6) and -linolenic
(18:3n-3) acids. Recently, a variety of fatty acid desaturases and
elongases have been cloned from a range of freshwater and marine
teleosts, including Atlantic salmon. We describe the cloning of the cDNAs,
characterisation of the protein sequences, genomic gene structures and
tissue distributions.
Functional characterisation by heterologous
expression in the yeast Saccharomyces cerevisiae has
shown that fish desaturases are usually monofunctional displaying
either 6
or 5
activities although some can be bifunctional, displaying both activities
in the one gene product, as in zebrafish. Elongases generally have broad
substrate specificity for PUFA with a range of chain lengths from C18
to C22. The expression of both desaturase
and elongase genes have been demonstrated to be under nutritional regulation
being
up regulated in the livers of fish fed diets containing vegetable oils rich
in C18
PUFAs
and devoid of C20/22
HUFA. Genes for peroxisome
proliferator activated receptors (PPARs), nuclear hormone receptors
known to be involved in the regulation of HUFA synthesis, have also
been cloned from fish species. Possible mechanisms involved in the
regulation of the HUFA biosynthetic pathway are discussed.
(Institute of Aquaculture,
University of Stirling, Stirling FK9 4LA, Scotland, U.K., e-mail: d.r.tocher@stir.ac.uk)