Glucosylation and galactosylation of diosgenin and solasodine by soluble glycosyltransferase(s) from Solanum melongena leaves

Abstract

When the crude, lipid-depleted cytosolic fraction (105 000 g supernatant) from Solanum melongena leaves is incubated with UDP- [ 14C]glucose and diosgenin (a steroidal sapogenin of spirostane type) or solasodine (a steroidal alkaloid of spirosolane type), the main reaction product consists of diosgenin or solasodine monoglucoside. However, a small amount (usually less than 5%) of the corresponding monogalactoside is also synthesized. With UDP- [ 14C]galactose as sugar source, almost equal amounts of monoglucoside and monogalactoside derivatives are formed. These results can be explained by the presence of highly active UDP-glucose 4-epimerase in the investigated enzyme preparation. It was found that this epimerase can be almost completely blocked by an addition of 0.1 mM UDP-xylose to the reaction mixture. The use of this inhibitor allowed comparison of the specificities of glucosylation and galactosylation for some potential steroid acceptors. It was shown that the specificity patterns of both these reactions are very similar, which suggests that a single enzyme catalyses sugar transfer from both UDPGlc and UDPGal. Good substrates for both the glucosyltransferase and galactosyltransferase reactions consist of, apart from diosgenin and solasodine, some other spirostane-type sapogenins structurally related to diosgenin, i.e. tigogenin, yamogenin and hecogenin, as well as tomatidine which is a spirosolane-type steroidal alkaloid closely related to solasodine. In contrast to spirosolane alkaloids, steroidal alkaloids of the solanidane type, i.e. solanidine or demmissidine, are very poor substrates. Sterols are neither glucosylated nor galactosylated by the cytosolic enzyme(s) from S. melongena.