Substrate specificity of partially purified UDP-glucose: nuatigenin glucosyltransferase from oat leaves

Starting from the cytosol fraction of oat leaves an enzymes catalyzing the transfer of D-glucosyl moiety from UDP-Glc to nuatigenin (22S, 25S-epoxy-furost-5-ene-3β,26-diol) has been partially purified by ammonium sulphate fractionation, acetone precipitation and gel filtration on Sephadex G-100. The specificity of this glucosyltransferase was studied using a wide range of 3-OH steroids, i.e. steroid sapogenins, sterols, steroidal alkaloids andd androstane or pregnane derivatives as sugar acceptors. The highest activity was found with nuatigenin but some other structurally-related sapogenins such as isonuatigenin, chlorogenin, diosgenin or tigogenin were also glucosylated, however, at distinctly lower rates. The enzyme could glucosylate also tomatidine and solanidine as well as some 3β-OH derivatives of androstane or pregnane such as pregnenolone, androstenolone and androstanol. Typical phytosterols were very poor acceptors (5% less of the activity as compared to nuatigenin). UDP-Glc was a much better sugar source than TDP-Glc, ADP-Glc or UDP-Gal. Our results strongly suggest that the physiological function of the enzyme is its participation in the initiation of sugar chain formation during the biosynthesis of oat saponins-avenacosides A and B.