CHARACTERIZATION OF UDP-GLUCOSE - SOLANIDINE GLUCOSYLTRANSFERASE AND UDP-GALACTOSE - SOLANIDINE GALACTOSYLTRANSFERASE FROM POTATO-TUBER

Upon differential fractionation of potato tuber homogenate, the soluble enzyme uridine-5'-diphospho-glucose (UDP-glucose):solanidine glucosyltransferase remained for the main part in the supernatants, whereas the membrane-bound UDP-glucose:beta-sitosterol glucosyltransferase activity was found both in a 13 000-g and a 100 000-g pellet. The-soluble fraction glycosylated not only solanidine, but also the steroidal alkaloids solasodine and tomatidine, using either UDP-glucose or UDP-galactose as sugar donor. The K(m)-values for the sugar donors in the solanidine glycosylating reactions were similar: UDP-glucose, 22-mu-M and UDP-galactose, 29-mu-M. UDP-galactose was a competitive inhibitor in the solanidine glucosylating reaction (K(i) 1.1 mM), whereas UDP-glucose showed non-competitive inhibition in the galactosylation reaction (K(i) 21-mu-M). After separations on gelfiltration, ion-exchange or chromatofocusing columns, glucosylating and galactosylating activities were recovered in the same fractions, but with loss of most of the galactosyltransferase activity in the two latter types of separation. The apparent molecular weight of the solanidine glycosylating enzyme(s) was 40 000 and the isoelectric point 4.8.