Isolation and characterization of limonoid glucosyltransferase from pummelo albedo tissue

An enzyme that catalyses the transfer of a glucose unit from UDPG to limonoids was isolated from the albedo tissue of pummelo fruit (Citrus grandis Osbeck) in electrophoretically homogeneous form. The enzyme was purified to 180-fold by a combination of (NH4)2SO4 fractionation, ion exchange chromatography on DEAE-cellulose and DEAE-Toyopearl. SDS-PAGE showed a molecular weight of 55 kDa for the enzyme. The purified enzyme, limonoid glucosyltransferase, displayed an optimum activity at pH 7.8 and 37 °C with apparent Km values of 65 and 200 μM for limonin and UDPG, respectively. Mn2+ and Co2+ stimulated the enzyme activity by 33 and 30%, respectively, while EDTA completely inhibited it. Cu2+, Hg2+ and diethyl pyrocarbonate also inhibited the enzyme, indicating a possible role of histidine in catalysis. The enzyme was stable at 4 °C for 6 months in Tris–HCl buffer, pH 7.5.