Spinach Leaves Desaturate Exogenous [C]Palmitate to Hexadecatrienoate : Evidence that de Novo Glycerolipid Synthesis in Chloroplasts Can Utilize Free Fatty Acids Imported from Other Cellular Compartments.

Long-chain (14)C-fatty acids applied to the surface of expanding spinach leaves were incorporated into all major lipid classes. When applied in diethyleneglycol monomethyl ether solution, as done by previous workers, [(14)C]palmitic acid uptake was much lower than that of [(14)C] oleic acid. However, when applied in a thin film of liquid paraffin the rate of [(14)C] palmitic acid metabolism was rapid and virtually complete. Considerable radioactivity from [(14)C]palmitate incorporated into lipids following either application method gradually appeared in polyunsaturated C(16) fatty acids esterified to those molecular species of galactolipids previously thought to be made using only fatty acids synthesized and retained within the chloroplast. Evidence for the incorporation of radioactivity from exogenous [(14)C]oleate into those same molecular species of galactolipids was less compelling. The unexpected availability of fatty acids bound to extrachloroplastidal lipids for incorporation into galactolipids characteristically assembled entirely within the chloroplast emphasizes the need to reassess interrelations between the "prokaryotic" and "eukaryotic" pathways of galactolipid formation.