In vivo desaturation of cis-delta 9-monounsaturated to cis-delta 9,12-diunsaturated alkenylether glycerolipids.

Plants convert lipid-bound cis-n-9 monoenoic to polyenoic fatty acid residues without involvement of corresponding CoA-thioesters. To provide additional evidence for this type of lipid-linked desaturation we incubated sn-1-O- and 2-O-(cis-9)octadecenylglycerol isomers with photoautotrophic cell cultures from tomato. After 14 days the fractions of phosphatidylcholine and monogalactosyldiacylglycerol were isolated and the incorporated glycerol ether backbones released by treatment with LiAlH4 (reduction of ester bonds) and short acid hydrolysis (cleavage of enol ether bonds). High performance liquid chromatography and mass spectroscopy of the products in appropriately derivatized form showed that the (cis-9)octadecenyl group in the sn-1 position of the phospholipid was nearly completely desaturated to a (cis-9,12)octadecadienyl residue having the same double bond arrangement as linoleic acid. In the galactolipid fraction the desaturation had progressed to octadecatrienyl residues. Similarly, the octadecenyl residue in the sn-2 position of the phospholipid was nearly completely desaturated to an octadecadienyl group. These results are unambiguous proof for lipid-linked desaturation by both microsomal and plastidial desaturase systems of plants.