Long Chain (C 20 and C 22 ) Fatty Acid Biosynthesis in Developing Seeds of Tropaeolum majus

Abstract

The storage triacylglycerols of nasturtium (Tropaeolum majus) seeds are composed principally of cis-11-eicosenoate and cis-13-docosenoate. To investigate the biosynthesis of these C(20) and C(22) fatty acids, developing seed tissue was incubated with various (14)C-labeled precursors. Incubation with [1-(14)C]acetate produced primarily cis-11-[1-(14)C]eicosenoate and cis-13-[1,3-(14)C]docosenoate in the triacylglycerol fraction, the odd-carbon [U-(14)C]oleate also formed from [(14)C] acetate was in the polar lipid fraction. Kinetic data showed that this oleate was not channeled into cis-11-eicosenoate nor cis-13-docosenoate over a 24-hour period. Under suitable conditions, nasturtium seed could also produce [(14)C]stearate, [(14)C]eicosenoate, and [(14)C]docosenoate from [1-(14)C]acetate. The results are discussed in terms of the number of pathways producing fatty acids. From pool size and other considerations, the results can be rationalized only in terms of different de novo systems for oleate biosythesis, one supplying oleate for incorporation into phospholipids and the other supplying oleate for chain elongation and subsequent esterification into triacylglycerols. Because of the probable heterogeneous nature of the seed tissue, it is not known if these two systems are operating in different cell types, in the same cell type at different stages of development, or in the same cell type concurrently.