Distribution and biosynthesis of stearidonic acid in leaves of Borago officinalis

An octadecatetraenoic acid was present as a major fatty acid component of the leaf lipids of borage. Gas chromatography-mass spectrometry of its picolinyl esters gave unsaturation centres at carbons Delta(6), Delta(9), Delta(12), and Delta(15) and confirmed its identity as stearidonic acid (SDA; octadecatetraenoic acid, C18:4, Delta(6,9,12,15). The chloroplast galactolipids, monogalactosyldiacyglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were particularly rich in SDA. SDA was absent,however, from the plastid phospholipid, phosphatidylglycerol (PG). Stereochemical analysis of the fatty acids in leaf MGDG and phosphatidylcholine (PC) showed that both SDA and gamma-linolenic acid (GLA) were almost exclusively located at carbon sn-2 of these complex lipids. In time-course studies with excised seed cotyledons induced to green by light treatment, SDA appeared in the galactolipids before its detection in PC suggesting that its major site of synthesis in the leaf was prokaryotic and largely located in the chloroplasts. Borage seed microsomes, which have high Delta(6) desaturase (Delta(6)des) activity, catalysed the synthesis of SDA from exogenously supplied alpha-linolenic acid (ALA). Linseed cotyledons which have an active Delta(15)des, on the other hand, could not convert exogenously supplied GLA to SDA. These observations suggest that SDA is formed from ALA via Delta(6)des activity at carbon sn-2 of MGDG and not from GLA and subsequent Delta(15)-desaturation. Copyright (C) 1996 Elsevier Science Ltd.