Biosynthesis of C20 and C22 Fatty Acids by Developing Seeds of Limnanthes alba

The storage triacylglycerols of meadowfoam (Limnanthes alba) seeds are composed essentially of C(20) and C(22) fatty acids, which contain an unusual Delta5 double bond. When [1-(14)C]acetate was incubated with developing seed slices, (14)C-labeled fatty acids were synthesized with a distribution similar to the endogenous fatty acid profile. The major labeled product was cis-5-eicosenoate, with smaller amounts of palmitate, stearate, oleate, cis-5-octadecenoate, eicosanoate, cis-11-eicosenoate, docosanoate, cis-5-docosenoate, cis-13-docosenoate, and cis-5,cis-13-docosadienoate. The label from [(14)C]acetate and [(14)C]malonate was used preferentially for the elongation of endogenous oleate to produce cis-[(14)C]11-eicosenoate, cis-13-[(14)C]docosenoate, and cis-5,cis-13-[(14)C]docosadienoate and for the elongation of endogenous palmitate to produce the remaining C(20) and C(22) acyl species. The Delta5 desaturation of the preformed acyl chain and chain elongation of oleate and palmitate were demonstrated in vivo by incubation of the appropriate 1-(14)C-labeled free fatty acids. Using [1-(14)C]acyl-CoA thioesters as substrates, these enzyme activities were also demonstrated in vitro with a cell-free homogenate.