Trans-vaccenate is Δ13-desaturated by FADS3 in rodents

Fatty acid desaturases play critical roles in regulating the biosynthesis of unsaturated fatty acids in all biological kingdoms. As opposed to plants, mammals are so far characterized by the absence of desaturases introducing additional double bonds at the methyl-end site of fatty acids. However, the function of the mammalian fatty acid desaturase 3 (FADS3) gene remains unknown. This gene is located within the FADS cluster and presents a high nucleotide sequence homology with FADS1 (Delta 5-desaturase) and FADS2 (Delta 6-desaturase). Here, we show that rat FADS3 displays no common Delta 5-, Delta 6- or Delta 9-desaturase activity but is able to catalyze the unexpected Delta 13-desaturation of trans-vaccenate. Although there is no standard for complete conclusive identification, structural characterization strongly suggests that the Delta 11,13-conjugated linoleic acid (CLA) produced by FADS3 from trans-vaccenate is the trans 11,cis13-CLA isomer. In rat hepatocytes, knockdown of FADS3 expression specifically reduces trans-vaccenate Delta 13-desaturation. Evidence is presented that FADS3 is the first methyl-end fatty acid desaturase functionally characterized in mammals.