Linolenic acid biosynthesis via glycerolipid molecular species in pea and spinach leaves

Abstract

Only molecular species of the eukaryotic type (18:3/18:3; 18:2/18:3; 18:2/18:2, 16:0/18:3, etc…) are present in the phosphatidylcholine (PC) of pea or spinach leaves. In pea leaves, total monogalactosyldiacylglycerol (MGDG) is mostly represented by two eukaryotic molecular species: 18:3/18:3 (79 mol%) and 18:2/18:3 (9%). In spinach leaves, the prokaryotic molecular species 18:3/16:3 forms 39 mol% of total MGDG; however the eukaryotic species 18:3/18:3 (60 mol%) is dominant. At the end of a 30 min pulse with 14C-oleate, four molecular species of PC were intensively labelled in pea leaves: 18:3/ 14C-18: 1; 18:2/ 14C-18: 1; 16:0/ 14C-18: 1 and 18: 1/ 14C-18: 1. During the following 48 hr chase period, clear precursor-product relationships could be observed between 14C-18: 1 PC and 14C-18:2 PC molecular species on one side and between 14C-18:2 PC and 14C-18:3 PC molecular species on the other side. In spinach leaves the same 14C-18: 1 PC molecular species were labelled more slowly and desaturation was not observed beyond 14C-18:2 PC molecular species. In MGDG from pea leaves, labelled linolenic acid accumulated steadily in 18:3/18:3 MGDG, after a lag time of 1 hr. In MGDG from spinach leaves labelled 18:3/18:2 MGDG was a precursor for labelled 18:3/18:3 MGDG. It is concluded that linolenic acid is synthesized via PC molecular species in the eukaryotic pathway of pea leaves and via MGDG molecular species in the eukaryotic pathway of spinach leaves.