TRIACYLGLYCEROLS ARE SYNTHESIZED AND UTILIZED BY TRANSACYLATION REACTIONS IN MICROSOMAL PREPARATIONS OF DEVELOPING SAFFLOWER (CARTHAMUS-TINCTORIUS L) SEEDS

Microsomal membrane preparations from the immature cotyledons of saffl ower (Carthamus tinctorius) catalysed the interconversion of the neutr al lipids, mono-, di-, and triacylglycerol. Membranes were incubated w ith neutral lipid substrates, C-14-labelled either in the acyl or glyc erol moiety, and the incorporation of radioactivity into other complex lipids determined. It was clear that diacylglycerol gave rise to tria cylglycerol and monoacylglycerol as well as phosphatidylcholine. Radio activity from added [C-14] triacylglycerol was to a small extent trans ferred to diacylglycerol whereas added [C-14] monoacylglycerol was rap idly 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 diacylg lycerol: diacylglycerol transacylase which yielded triacylglycerol and monoacylglycerol, the reaction being freely reversible. The specific activity of the transacylase was some 25% of the diacylglycerol acyltr ansferase activity and, hence, during the net accumulation of oil, sub stantial newly formed triacylglycerol equilibrated with the diacylglyc erol pool. In its turn the diacylglycerol rapidly interconverted with phosphatidylcholine, the major complex lipid substrate for Delta 12 de saturation. Hence, the oleate from triacylglycerols entering phosphati dylcholine via this route could be further desaturated to linoleate. A model is presented which reconciles these observations with our curre nt understanding of fatty acid desaturation in phosphatidylcholine and oil assembly in oleaceous seeds.