Specificity and some other properties of cytosolic and membranous udpglc: 3β-hydroxysteroid glucosyltransferases from Solanum tuberosum leaves

Abstract

Our earlier studies have suggested that potato plant leaves contain two UDPGlc-dependent glucosyltransferases: the membranous enzyme with high affinity to sitosterol and cytosolic enzyme with high affinity to solanidine. Some properties of both these glucosyltransferases have been compared and their specificity for various 3β-hydroxysteroids representing different types of structure examined in detail. Both enzymes displayed the ability to glucosylate the examined steroid substrates. The membranous enzyme glucosylated them in the following sequence: typical plant sterols > androstane and pregnane derivatives > steroid alkaloids of the spirosolane type and steroid sapogenins > steroid alkaloids of the solanidane type. The cytosolic enzyme glucosylated the steroid substrates in the order: steroid alkaloids of the solanidane type > steroid alkaloids of the spirosolane type > steroid sapogenins > sterols, pregnane and androstane derivatives. It was found that glucosylation of steroid compounds by the membranous enzyme was stimulated by 0.1% Triton X-100, whereas glucosylation catalysed by the cytosolic enzyme was inhibited in the presence of this detergent. Both enzymes displayed a similar optimum pH (6.5–7.0) and did not require divalent ions. The cytosolic enzyme, in contrast to the membranous one, was somewhat stimulated by 2-mercaptoethanol and inhibited in the presence of high ionic strength. The results afforded additional evidence that potato plant leaves contain, apart from the membranous UDPGlc:sterol glucosyltransferase resembling that occurring in many other higher plants, the enzyme preferentially glucosylating solanidine, which may be involved in initiation of the synthesis of the sugar chains in glycoalkaloids of the α-chaconine series.