Homogeneous Strictosidine Synthase Isoenzymes from Cell Suspension Cultures ofCatharanthus roseus

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Abstract
Four separable isoforms of Strictosidine synthase, which catalyze condensation of tryptamine with secologanin to form Strictosidine, were purified to homogeneity from cultured cells of Catharanthus roseus and from leaves of C. roseus plants. These enzymes are distinguished by their isoelectric points as well as by their catalytic properties. The specific activity of the main form (isoform III) is 104 nkat/mg, Kcat = 3.2 s-1. The relative molecular mass is 31 kDa as estimated by gel filtration, and 41.5 kDa as estimated by SDS gel electrophoresis. The pH-optimum is observed at pH 6.7. The apparent KM-value for tryptamine is 1.9 mM (isoform III). Secologanin shows a positive cooperativity with an nH of 2.2 (isoform III). Polyclonal antibodies raised against isoform III show cross reactivity against all four isoforms. Furthermore, these antibodies also react with Strictosidine synthases from cell cultures of other species of the family Apocynaceae but not with those of the family Rubiaceae.