Gas chromatography-combustion isotope ratio mass spectrometry (GC-C-IRMS) offers high sensitivity and accuracy. Therefore, this analytical method would be particularly suitable to elucidate the metabolic origin of low-abundant lipids in milk, such as the products of less-well known fatty acid desaturases.
In this regard, Δ13 -desaturase has recently been shown to catalyze the synthesis of trans-11 cis-13 conjugated linoleic acid (CLA) in goats, but we are not aware of any similar research in other ruminant species or identifying other products of the enzyme. To fill this gap, in a first trial, 5 lactating sheep received an intravenous injection of 200 mg of [1- 13C]trans-11 18:1, and in a second and independent trial, 2 g of [1- 13C]18:0 were intravenously injected to 6 sheep. Milk samples were collected before and after isotopic tracer injection (up to 96 and 72 h in trial 1 and 2, respectively), and fatty acid composition was analyzed by complementary GC-FID and GC-C-IRMS. Despite the high precision of the latter technique, no 13C enrichment in trans-11 cis-13 CLA was found in the first trial, but 18:0 was Δ13 -desaturated to cis-13 18:1 in Trial 2.
This metabolic pathway contributed to 51.4% of the cis-13 18:1 secreted in milk. Our results reveal, for the first time, Δ13 -desaturation of 18:0 and the activity of this enzyme in sheep, but it is not possible to conclude whether the lack of trans-11 18:1 desaturation was due to a specificity in this ruminant species, to the dose of the isotopic tracer in Trial 1 (10 times lower than in Trial 2), or to the very low content of trans-11 cis-13 CLA in milk (0.007% of total fatty acids). Additional studies would be required to provide further insight into this metabolic pathway. Acknowledgements: PIE202240I195 (CSIC).