Cloning, expression, and fatty acid regulation of the human Δ-5 desaturase

Arachidonic (20:4(n-6)), eicosapentaenoic (20:5(n-3)), and docosahexaenoic (22:6(n-3)) acids are major components of brain and retina phospholipids, substrates for eicosanoid production, and regulators of nuclear transcription factors. One of the two rate-limiting steps in the production of these polyenoic fatty acids is the desaturation of 20:3(n-6) and 20:4(n-3) by Δ-5 desaturase. This report describes the cloning and expression of the human Δ-5 desaturase, and it compares the structural characteristics and nutritional regulation of the Δ-5 and Δ-6 desaturases. The open reading frame of the human Δ-5 desaturase encodes a 444-amino acid peptide which is identical in size to the Δ-6 desaturase and which shares 61% identity with the human Δ-6 desaturase. The Δ-5 desaturase contains two membrane-spanning domains, three histidine-rich regions, and a cytochrome b₅ domain that all align perfectly with the same domains located in the Δ-6 desaturase. Expression of the open reading frame in Chinese hamster ovary cells instilled the ability to convert 20:3(n-6) to 20:4(n-6). Northern analysis revealed that many human tissues including skeletal muscle, lung, placenta, kidney, and pancreas expressed Δ-5 desaturase mRNA, but Δ-5 desaturase was most abundant in the liver, brain, and heart. However, in all tissues, the abundance of Δ-5 desaturase mRNA was much lower than that observed for the Δ-6 desaturase. When rats were fed a diet containing 10% safflower oil or menhaden fish oil, the level of hepatic mRNA for Δ-5 and Δ-6 desaturase was only 25% of that found in the liver of rats fed a fat- free diet or a diet containing triolein. Finally, a BLAST and Genemap search of the human genome revealed that the Δ-5 and Δ-6 desaturase genes reside in reverse orientation on chromosome 11 and that they are separated by <11,000 base pairs.