In vitro desaturation and elongation of rumenic acid by rat liver microsomes

Various nutritional studies on CLA, a mixture of isomers of linoleic acid, have reported the occurrence of conjugated long-chain PUFA after feeding experimental animals with rumenic acid, 9c,11t–18∶2, the major CLA isomer, probably as a result of successive desaturation and chain elongation. In the present work, in vitro studies were carried out to obtain information on the conversion of rumenic acid. Experiments were first focused on the in vitro Δ6-desaturation of rumenic acid, the regulatory step in the biosynthesis of long-chain n−6 PUFA. The conversion of rumenic acid was compared to that of linoleic acid (9c,12c–18∶2). Isolated rat liver microsomes were incubated with radiolabeled 9c,12c–18∶2 and 9c,11t–18∶2 under desaturation conditions. The data indicated that [1-¹⁴C]9c,11t–18∶2 was a poorer substrate for Δ6-desaturase than [1-¹⁴C]-9c,12c–18∶2. Next, in vitro elongation of 6c,9c,11t–18∶3 and 6c,9c,12c–18∶3 (γ-linolenic acid) was investigated in rat liver microsomes. Under elongation conditions, [1-¹⁴C]6c,9c,11t–18∶3 was 1.5-fold better converted into [3-¹⁴C]8c,11c,13t–20∶3 than [1-¹⁴C]6c,9c,12c–18∶3 into [3-¹⁴C]8c,11c,14c–20∶3. Finally, in vitro Δ5-desaturation of 8c,11c,13t–20∶3 compared to 8c,11c,14c–20∶3 was investigated. The conversion level of [1-¹⁴C]8c,11c,13t–20∶3 into [1-¹⁴C]5c,8c,11c,13t–20∶4 was 10 times lower than that of [1-¹⁴C]8c,11c,14c–20∶3 into [1-¹⁴C]5c,8c,11c,14c–20∶4 at low substrate concentrations and 4 times lower at the saturating substrate level, suggesting that conjugated 20∶3 is a poor substrate for the Δ5-desaturase.