Identification, purification, and characterization of monoacylglycerol acyltransferase from developing peanut cotyledons

Abstract

Biosynthesis of diacylglycerols in plants occurs mainly through the acylation of lysophosphatidic acid in the microsomal membranes. Here we describe the first identification of diacylglycerol biosynthetic activity in the soluble fraction of developing oilseeds. This activity was NaF-insensitive and acyl-CoA-dependent. Diacylglycerol formation was catalyzed by monoacylglycerol (MAG) acyltransferase (EC 2.3.1.22) that transferred an acyl moiety from acyl-CoA to MAG. The enzyme was purified by successive chromatographic separations on octyl-Sepharose, blue-Sepharose, Superdex-75, and palmitoyl-CoA-agarose to apparent homogeneity from developing peanut (Arachis hypogaea) cotyledons. The enzyme was purified to 6,608-fold with the final specific activity of 15.86 nmol min⁻¹ mg⁻¹. The purified enzyme was electrophoretically homogeneous, and its molecular mass was 43,000 daltons. The purified MAG acyltransferase was specific for MAG and did not utilize any other acyl acceptor such as glycerol, glycerol-3-phosphate, lysophosphatidic acid, and lysophosphatidylcholine. The K_m values for 1-palmitoylglycerol and 1-oleoylglycerol were 16.39 and 5.65 μm, respectively. The K_m values for 2-monoacylglycerols were 2- to 4-fold higher than that of the corresponding 1-monoacylglycerol. The apparentKm values for palmitoyl-, stearoyl-, and oleoyl-CoAs were 17.54, 25.66, and 9.35 μm, respectively. Fatty acids, phospholipids, and sphingosine at low concentrations stimulated the enzyme activity. The identification of MAG acyltransferase in oilseeds suggests the presence of a regulatory link between signal transduction and synthesis of complex lipids in plants.