PURIFICATION AND CHARACTERIZATION OF STRICTOSIDINE SYNTHASE FROM A SUSPENSION-CULTURE OF CINCHONA-ROBUSTA

Four isoforms of strictosidine synthase (EC 4.3.3.2), which catalyses the stereospecific condensation of secologanin and tryptamine, were pu rified to homogeneity from a suspension culture of Cinchona robusta. T he M(r)s are 35 000, 33 000, 35 000 and 33 000, and the isoelectric po ints are 6.5, 6.5, 7.5 and 7.5, respectively. The K(m) values of the f our proteins for tryptamine are in the micromolar range. The enzymes a re inhibited by the product strictosidine and by quinoline alkaloids. The K(i) of quinine for one of the main isoforms is ca 7 muM. No subst rate inhibition was observed. In contrast with strictosidine synthase from Catharanthus roseus (Apocynaceae), all four isoforms of Cinchona robusta accepted 5-methoxytryptamine as a substrate. The enzyme does n ot catalyse the condensation of corynantheal with tryptamine derivativ es. All four isoforms are glycosylated.