Geometry of conjugated double bonds of CLA isomers in a commercial mixture and in their hepatic 20:4 metabolites

Abstract

Rats were fed a fat-free diet for 2 wk. After this period, while maintaining the animals on the same diet, the rats were given intragastrically 180 mg per day of a mixture of conjugated linoleic acids (CLA) as triacylglycerols. Gas chromatography-mass spectrometry (GC-MS) analyses of this mixture, as well as hydrazine reduction and GC-MS and GC- Fourier transform infrared analyses of the resulting monoenes, revealed the presence of two major isomers, the 9c, 11t- and the 10t, 12c-18:2 accompanied by smaller amounts of the 8t, 10c and the 11c, 13t-18:2 isomers. Minor quantities of cis, cis and trans, trans conjugated isomers also were detected. The total fatty acid methyl esters from the liver lipids were fractionated by reversed-phase high-performance liquid chromatography, and the fraction containing the 20:4 isomers was further fractionated by silver nitrate thin-layer chromatography. A band containing two 20:4 conjugated isomers was submitted to hydrazine reduction and the resulting monoenes analyzed by GC-MS as dimethyl-oxazoline derivatives. The two conjugated isomers were tentatively identified as 5c, 8c, 11c, 13t-20:4 and 5c, 8c, 12t, 14c-20:4. These could be formed by desaturation and elongation of the 9c, 11t- and 10t, 12c-18:2 present in the commercial CLA mixture.