Separation and characterisation of three 2-oxoglutarate-dependent dioxygenases from Cucurbita maxima L. endosperm involved in gibberellin biosynthesis

Three enzymes of the gibberellin (GA) biosynthetic pathway, a 7-oxidase, a 20-oxidase and a 3β-hydroxylase, were partially purified from Cucurbita maxima endosperm by ammonium sulfate precipitation, gel-filtration and anion-exchange chromatography. The enzyme activities, which were assayed by the oxidation of GA12-aldehyde to GA12, of GA12 to GA15 (and GA24) and of GA15 to GA37, respectively, were completely separated from each other. The apparent molecular masses as estimated by gel-filtration high-performance liquid chromatography were 34.5 kDa for the 7-oxidase, 44.5 kDa for the 20-oxidase and 58 kDa for the 3β-hydroxylase. The Michaelis-Menten constants (Km) were 8.6 μM, 0.15μM and 8.7 μM for the respective substrates. All three enzymes had properties typical of 2-oxoglutarate dependent dioxygenases. 2-Oxoglutarate was essential for activity and served as a co-substrate, giving Km values of 6.1 μM, 91 μM and 41 μM with the 7-oxidase, 20-oxidase and 3β-hydroxylase, respectively. Furthermore, 2 oxo[5-14C]glutarate was oxidised stoichiometrically to [14C]succinate when the GA-substrates were oxidised to their respective products, and the 1∶1 ratio was maintained under different oxygen concentrations. Approximately equimolar amounts of 14CO2 were released from 2-oxo[1-14C]glutarate when GA12 was oxidised to GA15/24 by the 20-oxidase. A crude enzyme preparation containing all three enzyme activities (and a 2β-hydroxylase) converted GA12-aldehyde to [18O2]GA4 and [18O5]GA43 under 18O2, showing that all O-atoms incorporated after GA12-aldehyde originate from O2. Accordingly, the reaction rates were near zero under anaerobic conditions, although very low concentrations of O2 sufficed to sustain the reactions. Both Fe2+ and dithiothreitol stimulated the enzyme activities strongly, but if they were added together, catalase was needed to prevent inhibition. The pH dependence showed two opposite trends; the 7-oxidase was most active at pH 6 and below, whereas the other enzymes were maximally active above pH 6.5.