Glycerolipid synthesis in Avena leaves during greening of etiolated seedlings II. : α-Linolenic acid synthesis
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概要
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To determine the synthesis of a-linolenic acid esterified to galactolipids, etiolated leaves from Avena sativa L. were fed with [1 -^<14>C]acetate for the first 3 hr of greening, and the redistribution of ^<14>C incorporated into the fatty acid moieties of lipid classes was ex-amined during a 21-hr chase. Phosphatidylcholine (PC) was most heavily labeled, but lost a large portion of its ^<14>C during the chase. Galactolipids, such as monogalactosyldiglyceride (MGDG) and digalactosyldiglyceride (DGDG), were only slightly labeled at the start, but gradually gained ^<14>C during the chase. When greening Avena leaves were pulse-labeled with [2-^3H]glycerol and chased in the same manner, a similar incorporation and redistribution of ^3H were observed in the glycerol moieties of lipid classes. A [1-^<14>C]oleic acid-feeding experiment also showed the same redistribution of ^<14>C from PC to galactolipids and little change of the ^<14>C incorporated into phosphatidylglycerol and phosphatidylethanolamine. These results are evidence that galactolipids were synthesized from PC in greening Avena leaves. Time courses for the ^<14>C in the fatty acid moieties of lipid classes in both [1-^<14>C]-acetate- and [1-^<14>C]oleic acid-feeding experiments showed dominant labeling of oleoyl-PC during the early hours and the subsequent transfer of the label from oleoyl-PC to linoleoyl-PC, linoleoyl-MGDG and finally to a-linolenoyl-MGDG. From these results, the major pathway of a-linolenic acid synthesis in greening Avena leaves is proposed: oleate synthesized de novo is first acylated to PC then desaturated to linoleoyl-PC, which is in turn converted to MGDG and desaturated to a-linolenoyl-MGDG. Because of the slow incorporation of ^<14>C into a-linolenoyl-DGDG, in contrast to the rapid incorporation of ^<14>C into a-linolenoyl-MGDG, in the [1-^<14>C]acetate- and [1-^<14>C]linoleic acid-feeding experiments, the former lipid class seems to be formed by the galactosylation of the latter rather than by the desaturation of linoleoyl-DGDG. The involvement of PC-exchange protein in the transfer of linoleoyl-PC from ER to the plastid is discussed.
- 日本植物生理学会の論文
著者
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Yamada Mitsuhiro
Departmellt Of Biology C:ollegc Of Gencral Educatioll Universsity Of Tokyo
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Ohnishi Jun-ichi
Department Of Biochemistry And Molecular Biology Faculty Of Science Saitama University
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