STRUCTURAL ELUCIDATION OF OXYGENATED STORAGE LIPIDS IN CUCUMBER COTYLEDONS - IMPLICATION OF LIPID BODY LIPOXYGENASE IN LIPID MOBILIZATION DURING GERMINATION

At early stages of germination, a special lipoxygenase is expressed in cotyledons of cucumber and several other plants, This enzyme is local ized at the lipid storage organelles and oxygenates their storage tria cylglycerols, We have isolated this lipid body lipoxygenase from cucum ber seedlings and found that it is capable of oxygenating in vitro di- and trilinolein to the corresponding mono-, di-, and trihydroperoxy d erivatives. To investigate the in vivo activity of this enzyme during germination, lipid bodies were isolated from cucumber seedlings at dif ferent stages of germination, and the triacylglycerols were analyzed f or oxygenated derivatives by a combination of high pressure liquid chr omatography, gas chromatography/mass spectrometry, and nuclear magneti c resonance spectroscopy, We identified as major oxygenation products triacylglycerols that contained one, two, or three 13S-hydroperoxy-9(Z ),11(E)-octadecadienoic acid residues, During germination, the amount of oxygenated lipids increased strongly, reaching a maximum after 72 h and declining afterward. The highly specific pattern of hydroperoxy l ipids formed suggested the involvement of the lipid body lipoxygenase in their biosynthesis. These data suggest that this lipoxygenase may p lay an important role during the germination process of cucumber and o ther plants and support our previous hypothesis that the specific oxyg enation of the storage lipids may initiate their mobilization as a car bon and energy source for the growing seedling.