Phosphatidyl choline: Donor of 18-carbon unsaturated fatty acids for glycerolipid biosynthesis.

Kinetics of radiocarbon inforporation into the phyosphatidyl choline of pumpkin leaf fed 1(-14)C-acetate at low light intensity were strongly suggestive of lipid bound fatty acids acting as substrates for desaturase enzymes. After pulse labeling in direct sunlight with the same precursor, phosphatidyl choline and phosphatidyl glycerol contained up to 90% of total glycerolipid radiocarbon at the shortest sampling times. Subsequent loss of radiocarbon from phyosphatidyl choline and a corresponding gain in other glycerolipids is taken to indicate a net flow of long chain fatty acids through phosphatidyl choline and into other lipids. It is proposed that there may be 2 separate synthetases in leaf tissue, one producing oleic and the other palmitic acids as their end products. Oleic acid is transferred almost exclusively to phosphatidyl choline where it is further desaturated to linoleic and linolenic acids before being made available for the biosynthesis of other lipids. Palmitic acid is transferred mainly to phosphatidyl glycerol, where it is desaturated to trans-delta3-hexadecenoic acid.