Intracellular localization and some properties of UDPG: sterol glucosyltransferase from Calendula officinalis.

UDPG: sterol glucosyltransferase is localized in the 2-week-old C. officinalis seedling in the membrane structures, separated from chloroplasts and mitochondria, and consisting probably of fragments of the Golgi apparatus. A minor part of the enzyme activity is associated with the microsomal fraction. A number of synthetic detergents stimulate the activity of the membrane-bound enzyme causing its solubilization. The enzyme preparation purified about 70-fold is strongly inhibited by HgCl2 and p-chloromercuribenzoate; it is markedly stimulated by mercaptoethanol and dithiothreitol, and to a lesser extent by Mg2+ and Ca2+ as well as by some chelating and reducing agents. UMP stimulates and UDP and UTP markedly inhibit the enzyme activity. The enzyme does not act on 4-methylsterols although it utilizes a number of 4-demethylsterols. It seems that the presence of a double bond in ring B enhances the affinity of the substrate for the enzyme. Delta-5-Sterols are utilized at a higher rate than delta-7-sterols. Saturated sterols and delta-25-sterols are poor substrates.