Triacylglycerols are synthesised and utilized by transacylation reactions in microsomal preparations of developing safflower (Carthamus tinctorius L) seeds

Microsomal membrane preparations from the immature cotyledons of safflower (Carthamus tinctorius) catalysed the interconversion of the neutral lipids, mono-, di-, and triacylglycerol. Membranes were incubated with neutral lipid substrates, C-14-labelled either in the acyl or glycerol moiety, and the incorporation of radioactivity into other complex lipids determined. It was clear that diacylglycerol gave rise to triacylglycerol and monoacylglycerol as well as phosphatidylcholine. Radioactivity from added [C-14] triacylglycerol was to a small extent transferred to diacylglycerol whereas added [C-14] monoacylglycerol was rapidly converted to diacylglycerols and triacylglycerols. The formation of triacylglycerol from diacylglycerol occurred in the absence of acyl-CoA and hence did not involve diacylglycerol acyltransferase (DAGAT) activity. Monoacylglycerol was not esterified by direct acylation from acyl-CoA. We propose that these reactions were catalyzed by a diacylglycerol: diacylglycerol transacylase which yielded triacylglycerol and monoacylglycerol, the reaction being freely reversible. The specific activity of the transacylase was some 25% of the diacylglycerol acyltransferase activity and, hence, during the net accumulation of oil, substantial newly formed triacylglycerol equilibrated with the diacylglycerol pool. In its turn the diacylglycerol rapidly interconverted with phosphatidylcholine, the major complex lipid substrate for Delta 12 desaturation. Hence, the oleate from triacylglycerols entering phosphatidylcholine via this route could be further desaturated to linoleate. A model is presented which reconciles these observations with our current understanding of fatty acid desaturation in phosphatidylcholine and oil assembly in oleaceous seeds.