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 commerical CLA mixture.