Biosynthesis and structure of glycosyl diglycerides, steryl glucosides, and acylated steryl glucosides

Abstract

A particulate enzyme fraction fromMycobacterium smegmatis catalyzed the transfer of¹⁴C-glucose from UDP-¹⁴C-glucose into neutral glycolipids. The two major radioactive components were purified by column chromatography on 0-diethylamino ethyl cellulose (acetate) and thin layer chromatography on silica gel in several solvents. The first product yielded a water-soluble component upon saponification, which had a hexoseglycerol ratio of 1∶1 with all of the hexose being identified as glucose. The second product yielded a water-soluble component upon saponification which contained hexose and glycerol in a 2∶1 ratio and, in addition to glucose, contained lesser amounts of mannose and galactose. Palmitate and oleate were the predominant fatty acids and were present in equimolar amounts. The products thus have been identified as monoglycosyldiglyceride and diglycosyldiglyceride. The diglycosyldiglyceride could also be labeled with¹⁴C-galactose when UDP-¹⁴C-galactose served as the donor, but the monoglycosyldiglyceride was only slightly labeled with¹⁴C-galactose. Membrane fractions from mung bean seedlings catalyzed the transfer of¹⁴C-glucose from UDP-¹⁴C-glucose into a neutral glycolipid which has been purified by thin layer chromatography and analyzed by combined gas liquid chromatographymass spectrometry. It was determined to be a steryl glucoside with the two major sterol components being β-sitosterol and stigmasterol linked to β-D-glucose. Particulate fractions from developing cotton fibers also catalyzed the formation of steryl glucosides and, in addition, they catalyzed the esterification of steryl glucosides at the 6 position of glucose with fatty acids (primarily palmitate and oleate) from an endogenous acyl donor. Both the glucosyl transferase and the acyltransferase have been solubilized with Triton X-100 and partially purified by chromatography on Sephadex G-200. The acyltransferase activity was reconstituted by the addition of the steryl glucoside and a phospholipid acyl donor.