Solubilization of a particulate UDP-glucose: Sterol β-glucosyl-transferase in developing cotton fibers and seeds and characterization of steryl 6-acyl- d-glucosides

Abstract

A particulate enzyme fraction from developing cotton fibers catalyzed the transfer of [ 14C]glucose from UDP-[ 14C]glucose to endogenous sterol acceptors, and in turn catalyzed the esterification of the steryl glucosides with fatty acids from an endogenous acyl donor. Analysis of the products by gas-liquid chromatography and mass spectrometry revealed that β-sitosterol was the predominant sterol moiety, while campesterol, cholesterol, and stigmasterol were present in smaller amounts. Glucose was the only sugar present, and it appeared to have the β-configuration. Palmitate and oleate were the major acyl components of the esterified steryl glucoside, and these fatty acid moieties appeared to be linked to the 6-position of glucose as indicated by both periodate cleavage and permethylation studies of the esterified steryl glucoside. When the UDP-glucose: sterol glucosyl transferase was solubilized with Triton X-100 and partially purified, it demonstrated an absolute requirement for added sterol acceptor and could utilize both cholesterol and stigmasterol almost as efficiently as β-sitosterol. However, it was specific for UDP-glucose and showed only slight activity with ADP-glucose, GDP-glucose, CDP-glucose, TDP-glucose, UDP-galactose, and UDP-mannose. The reaction rate was stimulated three-fold by the addition of plant lecithins, while egg lecithin had no effect. Intracellular localization studies demonstrated that the membrane vesicles containing the UDP-glucose:sterol glucosyl transferase had a sucrose density gradient profile which was similar to that of several other enzymes thought to be involved in the synthesis of cell wall polysaccharides.