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Aspects of Glycerolipid Metabolism in Developing Soybean Cotyledons¹

The metabolism of glycerolipids (GL) labeled with palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2), or linolenic (18:3) acids was studied with unsliced soybean [Glycine max (L.) Merr. cv. Dare] cotyledons harvested at 30 days after flowering. An inhibitor of lipoxygenase, n-propylgallate, was required in the incubation media to prevent the peroxidation of the exogenous 18:2 and 18:3 substrates by the soybean tissues. Phospholipids (TPL) accounted for greater than 75% of the glycerolipid radioactivity incorporated from each of the acyl-substrates. The distribution of radioactivity in phospholipid molecular species was determined by assaying the diacylglycerol (DG) molecular species prepared from the phospholipid fraction by phospholipase C hydrolysis. The predominate phospholipid molecular species synthesized from the respective substrates were: SD, [1-¹⁴C] 16:0 or [1-¹⁴C]18:0; SM and MD, [1-¹⁴C]18:1; SD and DD, [1-¹⁴C]18:2; and DT and TT, [1-¹⁴C]18:3; where S= 16:0 or 18:0, M = 18:1, D = 18:2, and T = 18:3. The predominate triacylglycerol (TG) molecular species synthesized from the respective substrates were: S₂M, S₂D, and SD₂, [1-¹⁴C]16:0 or [1-¹⁴C]18:0; M₂D and MD₂, [1-¹⁴C]18:1; SD₂, D₃, and D₂T, [1-¹⁴C]18:2; and DT₂ and T₃, [1-¹⁴C]18:3. Stereospecific analysis of the radioactivity present in phospholipid, DG, and TG indicated a preference for unsaturated fatty acid incorporation at the sn-2 position of each class. The observed acylincorporation patterns supported the precursory interrelation of phospholipids in soybean TG biosynthesis, and also established the nature of soybean TG molecular species biosynthesis from specific acyl-substrates. The method described has general application for preparing specifically labeled DG molecular species with high specific radioactivity and for determining the biochemical nature of TG biosynthesis in experimental soybean lines selected for altered fatty acid composition.

Key Words: Glycine max (L.) Merr. • n-Propylgallate • Fatty acid • Incorporation • Molecular species

² Research plant physiologist, USDA, SEA, AR and associate professor of crop science, NCSU, Raleigh, NC 27650.

Received for publication July 25, 1980.