Formation of (n-9) and (n-7)cis-monounsaturated fatty acids in seeds of higher plants

摘要

The relative abundance of (n-9) and (n-7) isomers in the monounsaturated fatty acids of seed lipids has been determined for selected plants in order to assess the biosynthetic reactions involved in their formation. Δ9 Desaturation of stearic acid to (n-9) octadecenoic acid is almost exclusively operative in the formation of monounsaturated fatty acids in the seeds ofHelianthus annuus, Glycine maxandBrassica napus, cv. Quinta and Erglu, in which chain elongation of monounsaturated fatty acids terminates at the level of an 18 carbon chain. Δ9 Desaturation of palmitic acid is a minor yet significant pathway in the seeds ofSinapis albaandBrassica napus, cv. Rapol and Tira, in which chain elongation of monounsaturated fatty acids occurs extensively beyond the 18 carbon chain. In each of these seeds, both (n-9) and (n-7) octadecenoic acids formed are subsequently elongated to icosenoic acids. However, elongation of the (n-7) isomer is terminated at the level of a 20 carbon chain, whereas the (n-9) icosenoic acid is selectively elongated to docosenoic acid and even up to tetracosenoic acid inSinapis alba. Δ9 Desaturation of palmitic acid followed by elongation to (n-7) octadecenoic acid occurs to a minor extent in the seeds ofTropaeolum majus. Only the (n-9) octadecenoic acid, and not its (n-7) isomer, is elongated to icosenoic and docosenoic aci