Phosphatidylcholine molecular species involved in Γ-linolenic acid biosynthesis in microsomes from borage seeds

Boraginaceae seeds are particularly rich in Γ -linolenic acid (6,9,12-octadecatrienoic acid, Γ -18:3). In microsomes, the analysis of phosphatidylcholine (PC) molecular species by HPLC led to identification of 15 different molecular species; among them 4 contained Γ -18:3, mostly at position 2 of sn -glycerol. Time courses of acylation and desaturation in PC molecular species were examined when ¹⁴C]oleoyl-CoA or ¹⁴C]linoleoyl-CoA was provided as substrates to isolated microsomes. With ¹⁴C]oleoyl-CoA or ¹⁴C]linoleoyl-CoA and in the absence of NADH, 3 main labelled PC molecular species were found: 18:2/¹⁴C]18:1, 16:0/¹⁴C]18:1 and 18:1/¹⁴C]18:1. When NADH was present in the incubation medium, the fatty acids were progressively desaturated by the Δ12- and Δ6-desaturases successively (with ¹⁴C]oleoyl-CoA as precursor) or by the Δ6-desaturase alone (with ¹⁴C]linoleoyl-CoA as precursor). In both types of experiments, 7 final desaturation products in microsomes were evidenced; among them, 3 contained radioactive Γ -18:3, i.e . 18:2/¹⁴C] Γ -18:3, 18:1/¹⁴C] Γ -18:3 and 16:0/¹⁴C] Γ -18:3. While the Δ12-desaturase had no specificity for position on the glycerol backbone, labelled Γ -linolenic acid was recovered exclusively in the sn -2 position.