Effects of trans n-6 fatty acids on the fatty acid profile of tissues and liver microsomal desaturation in the rat

Abstract

18:2Δ 9c,12t and 18:2 Δ9t,12c are present in our diet, as result of heat treatment of vegetable oils. A nutritional study was carried out in order to obtain more precise information on the conversion of these two isomers into long chain polyunsaturated fatty acids (PUFA) by rat tissues. This in vivo study performed using rat fed with small quantities of mono trans linoleic acid isomers (0.6% of total energy) showed that 18:2 Δ9c,12t was converted into 20:4 Δ5c,8c,11c,14t while 18:2 Δ9t,12c was only slightly converted into 20:4 Δ5c,8c,11t,14c. Furthermore 18:2 Δ9t,12c was preferentially elongated into 20:2 Δ11t,14c. Each C20 metabolite of these mono trans 18:2 isomers was isolated as methyl ester by semi-preparative high-performance liquid chromatography (HPLC) followed by silver nitrate thin layer chromatography (AgNO₃-TLC).The structure of the components was identified using partial hydrazine reduction, AgNO₃-TLC of the resulting monoenes and gas-liquid chromatography coupled with mass spectrometry (GC-MS) of the 4,4-dimethyloxazoline (DMOX) derivatives. Fourier-transform-infrared spectroscopy (GC-FTIR) confirmed the frans geometry. Gas-liquid chromatography (GC) analyses showed that 18:2 Δ9c,12t and 18:2 Δ9t,12c were present in different tissue lipids (liver, heart, testes, brain and adipose tissue), and without any modification in the amount of 20:4n-6. 20:4 Δ5c, 8c,11c,14t was incorporated in different rat tissues except in brain. Furthermore, its incorporation followed that of its structural analogue, 20:3n-9 in liver phospholipid classes (phosphatidylethanolamine, phosphatidylinositol and phosphatidylcholine). Finally, an in vitro study carried out with rat liver microsomes showed that dietary trans 18:2 isomers could inhibit the Δ6- desaturation of 18:2n-6 to 18:3n-6 and the Δ5-desaturation of 20:3n-6 to 20:4n-6.