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), D elta(9), Delta(12), and Delta(15) and confirmed its identity as steari donic acid (SDA; octadecatetraenoic acid, C18:4, Delta(6,9,12,15). The chloroplast galactolipids, monogalactosyldiacyglycerol (MGDG) and dig alactosyldiacylglycerol (DGDG) were particularly rich in SDA. SDA was absent,however, from the plastid phospholipid, phosphatidylglycerol (P G). Stereochemical analysis of the fatty acids in leaf MGDG and phosph atidylcholine (PC) showed that both SDA and gamma-linolenic acid (GLA) were almost exclusively located at carbon sn-2 of these complex lipid s. In time-course studies with excised seed cotyledons induced to gree n by light treatment, SDA appeared in the galactolipids before its det ection in PC suggesting that its major site of synthesis in the leaf w as prokaryotic and largely located in the chloroplasts. Borage seed mi crosomes, which have high Delta(6) desaturase (Delta(6)des) activity, catalysed the synthesis of SDA from exogenously supplied alpha-linolen ic acid (ALA). Linseed cotyledons which have an active Delta(15)des, o n the other hand, could not convert exogenously supplied GLA to SDA. T hese 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.