Elongation of (n−9) and (n−7)cis‐monounsaturated and saturated fatty acids in seeds ofsinapis alba

Lipids in developing seeds ofSinapis alba contain appreciable proportions of (n−7)octadecenoic (vaccenic) acid besides its (n−9) isomer (oleic acid), whereas the constituent very long chain (>C18) monounsaturated fatty acids of these lipids are overwhelmingly composed of the (n−9) isomers. Cotyledons of developingSinapis alba seed use [1‐14C]acetate, [1‐14C]malonate or [1,3‐14C]malonyl‐CoA for de novo synthesis of palmitic, stearic and oleic acids and for elongation of preformed oleic, vaccenic and stearic acids to their higher (n−9), (n−7) and saturated homologs, respectively. Moreover, elongation of preformed (n−7)palmitoleic acid to vaccenic acid is observed. Stepwise C2‐additions to preformed oleoyl‐CoA by acetyl‐CoA or malonyl‐CoA yielding (n−9)icosenoyl‐CoA, (n−9)docosenoyl‐CoA and (n−9)tetracosenoyl‐CoA are by far the most predominant reactions catalyzed by the elongase system, which seems to have a preference for oleoyl‐CoA over vaccenoyl‐CoA as the primer. The pattern of14C‐labeling of the very long chain fatty acids formed from either acetate or malonate shows a close analogy in the mode of elongation of monounsaturated and saturated fatty acids.