Cloning, expression, and fatty acid regulation of the human Delta-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, s ubstrates for eicosanoid production, and regulators of nuclear transcriptio n factors. One of the two rate-limiting steps in the production of these po lyenoic fatty acids is the desaturation of 20:3(n-6) and 20:4(n-3) by Delta -5 desaturase, This report describes the cloning and expression of the huma n Delta-5 desaturase, and it compares the structural characteristics and nu tritional regulation of the Delta-5 and Delta-6 desaturases, The open readi ng frame of the human Delta-5 desaturase encodes a 444-amino acid peptide w hich is identical in size to the Delta-6 desaturase and which shares 61% id entity with the human Delta-6 desaturase, The Delta-5 desaturase contains t wo membrane-spanning domains, three histidine-rich regions, and a cytochrom e b(5) domain that all align perfectly with the same domains located in the Delta-6 desaturase. Expression of the open reading frame in Chinese hamste r ovary cells instilled the ability:to convert 20:3(n-6) to 20:4(n-6), Nort hern analysis revealed that many human tissues including skeletal muscle, l ung, placenta, kidney, and pancreas expressed Delta-5 desaturase mRNA, but Delta-5 desaturase was most abundant in the liver, brain, and heart. Howeve r, in all tissues, the abundance of Delta-5 desaturase mRNA was much lower than that observed for the Delta-6 desaturase. When rats were fed a diet co ntaining 10% safflower oil or menhaden fish oil, the level of hepatic mRNA for Delta-5 and Delta-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 Delta-5 and Delta -6 desaturase genes reside in reverse orientation on chromosome 11 and that they are separated by <11,000 base pairs.