cDNA cloning and characterization of human Δ5-desaturase involved in the biosynthesis of arachidonic acid
Two human expressed sequence tag (EST) cDNA sequences with identity with ∆ 5 - and ∆ 6 -desaturases from a filamentous fungus, Mortierella alpina , were identified from the LifeSeq ® database of Incyte Pharmaceuticals, Inc. (Palo Alto, CA, U.S.A.). An oligonucleotide complementary to the 3ʹ EST cDNA sequences was used to screen human liver cDNA using rapid amplification of cDNA ends (RACE)-PCR. The amplified DNA fragment had 98% identity with a putative open reading frame (ORF) predicted from a human genomic sequence, and encoded 444 amino acids. Expression of this ORF in mouse fibroblast cells demonstrated that the encoded protein was a ∆ 5 -desaturase, as determined by the conversion of dihomo-γ-linolenic acid (C 20:3, n -6 ) into arachidonic acid (C 20:4, n -6 ). The human ∆ 5 -desaturase contained a predicted N-terminal cytochrome b 5 -like domain, as well as three histidine-rich domains. A tissue expression profile revealed that this gene is highly expressed in fetal liver, fetal brain, adult brain and adrenal gland. A search of the existing databases led to localization of this ORF within a 14 kb interval flanked by the flap endonuclease-1 (FEN1) and vitelliform macular dystrophy (Best9s disease; VMD2) loci of chromosome 11q12.