Enzymic glucosylation of gibberellins

Starting from the well-known conversion of exogenously applied free gibberellic acid (GA₃) to its 3(O)-glucoside by intact immature fruits of runner beans (Phaseolus coccineus L.), a protein fraction has been prepared from this plant material possessing glucosylating activity towards GAs. This glucosyltransferase is located in the pericarp only and utilizes preferably UDP-glucose as a sugar donor. The product formed enzymically from GA₃ and UDP-glucose could be identified by derivatization and comparison with the authentic compound to be GA₃-3(O)-glucoside. Among 15 native or chemically modified GAs, the enzyme glucosylates only GA₃ and to a lower extent GA₇ and GA₃₀, indicating a high enzyme specificity with regard to the A ring of gibberellins. The physiological significance of the enzymic GA₃-3(O)-glucoside formation inPhaseolus coccineus is not clear, since this glucoside is not known to be endogenous in this plant. The enzyme preparation did not glucosylate substances of phenolic structure, such as hydroquinone, aesculetin, and quercetin. Glucosylation of GA₃ was achieved also by enzyme preparations fromVigna sinensis and from cell suspension cultures ofDigitalis purpurea. A number of other plant materials showed no activity.