The trans-10,cis-15 18:2: a missing intermediate of trans-10 shifted rumen biohydrogenation pathway?

The "trans-10 shifted" biohydrogenation pathway is frequently established in the rumen when high starch diets are fed to ruminants, resulting in the accumulation of trans-10 18:1 in ruminant products. It has been proposed that the "trans-10 shifted" biohydrogenation pathway of α-linolenic acid generates two intermediates, the trans-10,cis-15 18:2 and trans-10,cis-12,cis-15 18:3, although none of these have been found in the rumen. We analyzed digestive contents and meat samples from two trials, where animals were fed: a compound feed diet supplemented with 8% oil blend containing linseed oil (samples A); and a forage based diet supplemented with 6% linseed oil (samples B). The use of the new SLB-IL111 chromatographic column allowed the detection of two different 18:2 isomers in each sample trial, which could not be resolved when the CP-Sil 88 column is used. The two 18:2 isomers were characterized by mass spectrometry using 4,4-dimethyloxazoline derivatives. However, because they were subject to higher temperatures and present different chromatographic properties compared with the fatty acid methyl esters, we also used the "covalent adduct chemical ionization" technique to confirm the identity of both 18:2 isomers. We detected and identified the 10,15-18:2 in samples A and the 11,15-18:2 in samples B. The geometry of both isomers was tentatively assigned as trans,cis taking in account their elution order and biologic plausibility. As far as we know, this is the first time that the trans-10,cis-15 18:2 has been found in ruminant digestive contents and meat samples associated with the "trans-10 shifted" biohydrogenation pathway of α-linolenic acid.