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

Abstract

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 [ 14 C]oleoyl-CoA or [ 14 C]linoleoyl-CoA was provided as substrates to isolated microsomes. With [ 14 C]oleoyl-CoA or [ 14 C]linoleoyl-CoA and in the absence of NADH, 3 main labelled PC molecular species were found: 18:2/[ 14 C]18:1, 16:0/[ 14 C]18:1 and 18:1/ [ 14 C]18:1. When NADH was present in the incubation medium, the fatty acids were progressively desaturated by the Δ12- and Δ6-desaturases successively (with [ 14 C]oleoyl-CoA as precursor) or by the Δ6-desaturase alone (with [ 14 C]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/[ 14 C]γ-18:3, 18:1/[ 14 C]γ-18:3 and 16:0/[ 14 C]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.