PLANT PHYSIOLOGY , Vol 105, Issue 4 1067-1073, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Purification of a Membrane-Bound UDP-Glucose:Sterol [beta]-D-Glucosyltransferase Based on Its Solubility in Diethyl Ether
D. C. Warnecke and E. Heinz
Institut fur Allgemeine Botanik, Universitat Hamburg, Ohnhorststrasse 18, 22609 Hamburg, Germany
Membrane-bound UDP-glucose:sterol [beta]-D-glucosyltransferase
(UDPG-SGTase) catalyzes the formation of steryl glucosides from UDP-glucose
and free sterols. This enzyme was purified from etiolated oat shoots (Avena
sativa L. cv Alfred) in five steps. UDPG-SGTase was solubilized from a
microsomal fraction with the detergent n-octyl-[beta]-D-thioglucopyranoside
and then extracted into diethyl ether. Subsequent removal of the organic
solvent, resolubilization with an aqueous buffer, and two column
chromatographic steps on Q-Sepharose and Blue Sepharose resulted in a
12,500-fold overall purification. Sodium dodecyl sulfate-polyacrylamide gel
electrophoresis of the final preparation revealed a 56-kD protein band, the
intensity of which correlated with enzyme activity in the respective
fractions. Polyclonal antibodies raised against this 56-kD protein did not
inhibit enzyme activity but specifically bound to the native UDPG-SGTase.
These results suggest that the 56-kD protein represents the UDPG-SGTase.
The purified enzyme was specific for UDP-glucose (Km = 34 [mu]M), for which
UDP was a competitive inhibitor (inhibitor constant = 47 [mu]M). In
contrast to the specificity with regard to the glycosyl donor, UDPG-SGTase
utilized all tested sterol acceptors, such as [beta]-sitosterol,
cholesterol, stigmasterol, and ergosterol.