Partial purification and specificity of triacylglycerol: Sterol acyltransferase from Sinapis alba

Triacylglycerol: sterol acyltransferase is present in roots of Sinapis alba seedlings. The enzyme is located predominantly in the cell membrane structures sedimenting at 300–16 000 g but can be solubilized by acetone treatment and buffer extraction. During gel filtration on Sephadex G-100 the acyltransferase activity was separated into two peaks corresponding to MW 1.8 × 10¹⁴ and MW ? 10⁵, respectively. A number of natural 3β-hydroxysterols can be esterified by the solubilized acyltransferase. The rate of esterification is much higher for sterols containing a planar ring system. The number and position of double bonds, as well as the structure of the side chain at C- 17 of the sterol molecule, are of secondary importance. Triacylglycerols containing fatty acids C, C₆-C₂₂ can be utilized as acyl donors. Among triacylglycerols containing saturated fatty acids, tripalmitoylglycerol (C_16:0) is the best acyl donor. For triacylglycerols containing C₁₈-fatty acids the following sequence was observed: trioleoylglycerol (C_18:1) > trilinoleoylglycerol (C_18:2) > trilinolenoylglycerol (C_18:3) > tristearoylglycerol (C_18:0).