Characterisation of Enzymes Determining Fatty Acid Chain Length in Developing Seeds of Limnanthes douglasii

Total fatty acid composition was determined in Limnanthes douglasii seeds during different stages of development and acyl specificities of acyl-ACP thioesterase and acyl-CoA elongase activities were investigated in extracts of the developing seeds. It was concluded that L. douglasii developing seeds possess acylACP thioesterase(s) with high activity towards saturated acyl-ACPs of a chain length OfC14-C18 as well as oleoyl (18: 1Δ9)-ACP. L. douglasii acyl-CoA elongase(s) efficiently elongated 14: 0-CoA to 20: 0 fatty acids but not to 22: 0 whereas 18:1Δ9-CoA was efficiently elongated to 22: 1Δ13 (erucic acid). The results obtained suggest that 20:1Δ5, the dominating very long chain fatty acid in L. douglasii seeds, is synthesised from mainly 14:0 fatty acids produced in the plastid, which are elongated to 20:0-CoA in the cytosolic compartment and then further desaturated at the Δ5 position. The erucic acid present in L. douglasii is synthesised from oleic acid exported from the plastid, which is elongated to 22 carbons by an acyl-CoA elongase enzyme that appears distinct from that responsible for the production of 20:0.