Conversion of monogalactosyldiacylglycerols to triacylglycerols in ozone-fumigated spinach leaves.

Molecular species and fatty acid distribution of triacylglycerol (TG) accumulated in spinach (Spinacia oleracea L.) leaves fumigated with ozone (0.5 microliter per liter) were compared with those of monogalactosyldiacylglycerol (MGDG). Analysis of positional distribution of the fatty acids in MGDG and the accumulated TG by the enzymatic digestion method showed that hexadecatrienoate (16:3) was restricted to sn-2 position of the glycerol backbone in both MGDG and TG, whereas alpha-linolenate (18:3) was preferentially located at sn-1 position in MGDG, and sn-1 and/or sn-3 positions in TG, suggesting that 1,2-diacylglycerol moieties of MGDG are the direct precursor of TG in ozonefumigated leaves. Further analysis of TG molecular species by argentation chromatography and mass spectrometry showed that TG increased with ozone fumigation consisted of approximately an equal molar ratio of sn-1,3-18:3-2-16:3 and sn-1,2,3-18:3. Because the molecular species of MGDG in spinach leaves is composed of a similar molar ratio of sn-1-18:3-2-16:3 and sn-1,2-18:3, we concluded that MGDG was converted to 1,2-diacylglycerol and acylated with 18:3 to TG in ozone-fumigated spinach leaves.