The biosynthesis of triacylglycerols in microsomal preparations of developing cotyledons of sunflower (Helianthus annuus L.)

Abstract

The synthesis of triacylglycerols was investigated in microsomes (microsomal fractions) prepared from the developing cotyledons of sunflower (H. annuus). Particular emphasis was placed on the mechanisms involved in controlling the C18-unsaturated-fatty-acid content of the oils. The microsomes were capable of the following: the transfer of oleate from acyl-CoA to position 2 of sn-phosphatidylcholine for its subsequent desaturation and the return of the polyunsaturated products to the acyl-CoA pool by further acyl exchange; the acylation of sn-glycerol 3-phosphate with acyl-CoA to yield phosphatidic acid, which was further utilized in diacyl- and tri-acylglycerol synthesis; and the equilibrium of a diacylglycerol pool with phosphatidylcholine. The acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine coupled to the equilibration of diacylglycerol and phosphatidylcholine brings about the continuous enrichment of the glycerol backbone with C18 polyunsaturated fatty acids for triacylglycerol production. Similar reactions operate in another oilseed plant, safflower (Carthamus tinctorius L.). The microsomes of avocado (Persea americana) mesocarp, which synthesize triacylglycerol via the the Kennedy (1961) pathway, were deficient in acyl exchange and the diacylglycerol .tautm. phosphatidylcholine interconversion. The results provide a working model that helps to explain the relationship between C18-unsaturated-fatty-acid synthesis and triacylglycerol production in oilseeds.