THE 3-O-GLUCOSYLATION OF STEROIDAL SAPOGENINS AND ALKALOIDS IN EGGPLANT (SOLANUM-MELONGENA) - EVIDENCE FOR 2 SEPARATE GLUCOSYLTRANSFERASES

UDP-glucose:diosgenin and UDP-glucose:solasodine glucosyltransferase a re present in leaves, stems, roots and ripening seeds of garden eggpla nt (Solanum melongena). These two enzymes share several common propert ies: i) they occur mainly in the soluble proteins fraction; ii) they c o-purify during purification procedure involving ammonium sulphate pre cipitation, ion exchange chromatography on Q-Sepharose and gel filtrat ion on Sephadex G-100; iii) they exhibit native M-r values of ca 55 00 0; iv) they absolutely require reduced -SH groups; and v) they are str ongly inhibited by some UDP-glucose analogues such as UDP, UDP-2',3'-d ialdehyde and UDP-mannose. However, the above mentioned enzyme activit ies can be clearly distinguished using some other effecters. Low conce ntrations of several non-ionic detergents including Tween 80, Triton X -100 and Tyloxapol almost completely abolish glucosylation of diosgeni n but have a slight stimulatory effect on glucosylation of solasodine. Synthetic diosgenin 3-O-beta-D-glucopyranoside strongly inhibits gluc osylation of diosgenin and some related spirostanols but not that of s olasodine. Cholesterol is a potent competitive inhibitor of diosgenin glucosylation (K-i = 1. 15 mu M) but it has little effect on the gluco sylation rate of solasodine. The above data strongly suggest that 3-O- glucosylation of steroidal sapogenins and alkaloids in eggplant leaves is catalyzed by two similar though separate UDP-glucose-dependent glu cosyltransferases. (C) 1998 Elsevier Science Ltd. All rights reserved.