Abstract
The specificity of galactosyltransferase involved in the biosynthesis of tomatidine monogalactoside in tomato leaves was studied using a wide spectrum of 3-OH steroids (i.e. steroidal alkaloids of spirosolane and solanidane type, steroidal sapogenins, typical sterols, androstane, pregnane and cholesterol derivatives as well as triterpenic alcohols) as sugar acceptors. The highest activity was found with tomatidine, but some other structurally-related compounds such as solasodine, nuatigenin, isonuatigenin, hecogenin, tigogenin, diosgenin were also glycosylated, however, at lower rates. UDP-galactose appeared to be the best sugar donor. The enzyme preparation was also able to utilize UDP-glucose as a sugar source for tomatidine glucosylation, however, at distinctly lower rate. Kinetic data showed apparent K m values of 2.48 μM for UDP-galactose and 0.83 μM for tomatidine. The investigated galactosyltransferase was stimulated by 2-mercaptoethanol and inhibited in the presence of UDP, UTP, divalent metal ions such as Cu 2+, Zn 2+, Hg 2+, Triton X-100, high ionic strength and N-ethylmaleimide. Divalent metals ions such as Mg 2+, Ca 2+, Mn 2+ and chelating agents (EDTA, EGTA) had no significant effect on the enzyme activity.
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