Involvement of Phospholipids in Polyunsaturated Fatty Acid Synthesis in Developing Soybean Cotyledons 1

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Developing soybean (cv. Dare) cotyledons harvested at 30 days after flowering were pulse-labeled with [1-14C]oleoyl-CoA. The metabolic interrelation of radiolabeled unsaturated fatty acids between the major glycerolipid classes was determined at various time intervals. At chase time zero, [14C]oleic acid accounted for 99.2% of the total glycerolipid radioactivity, and phospholipids contained 92% of the total incorporated radioactivity. With time, phospholipids were metabolized in triacylglycerol biosynthesis and radioactivity was detected in polyunsaturated fatty acids. The hypothesis that phospholipids were metabolic intermediates in polyunsaturated fatty acid biosynthesis was tested by comparing the theoretical and the actual amount of radiolabeled oleic acid that was associated with triacylglycerol as a function of time. The radioactive oleic acid found in triacylglycerol at various intervals was derived from phospholipids via a diacylglycerol intermediate. Assuming no phospholipid desaturation, the potential or theoretical amounts of [14C]oleic acid that could be transferred to triacylglycerol from phospholipids was defined by a system of differential equations. The results demonstrated that the decline in [14C]oleic acid from phospholipid after long chase intervals was equal to the total amount of radioactive unsaturated fatty acids found in neutral lipids. The difference between the theoretical and actual amounts of [14C]oleic acid present in triacylglycerol after long time intervals was equal to the amount of radioactivity present in polyunsaturated fatty acids. Based upon those findings in soybeans, the desaturation of oleic acid associated with phospholipids was highly probable.

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