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

Abstract

The storage triacylglycerols of meadowfoam (L. alba) seeds are composed essentially of C20 and C22 fatty acids, which contain an unusual .DELTA.5 double bond. When [1-14C]acetate was incubated with developing seed slices, 14C-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 [14C]acetate and [14C]malonate was used preferentially for the elongation of endogenous oleate to produce cis-[14C]11-eicosenoate, cis-13-[14C]docosenoate and cis-5,cis-13-[14C]docosadienoate and for the elongation of endogenous palmitate to produce the remaining C20 and C22 acyl species. The .DELTA.5 desaturation of the performed acyl chain and chain elongation of oleate and palmitate were demonstrated in vivo by incubation of the appropriate 1-14C-labeled free fatty acids. Using [1-14C]acyl-CoA thioesters as substrates, these enzyme activities were also demonstrated in vitro with a cell-free homogenate.