Molecular cloning, expression and characterization of tropinone reductase II, an enzyme of the SDR family in Solanum tuberosum (L.)

Calystegines are nortropane alkaloids that are found in Solanaceae containing the classical tropane alkaloids hyoscyamine and scopolamine, and in other Solanaceae such as potato, Solanum tuberosum (L.). Calystegines are assumed to derive from the classical tropane alkaloid pathway. We isolated a cDNA from S. tuberosum with high homology to the pseudotropine-forming tropinone reductase (TRII), which presents as the first putative metabolite specific to calystegines. The equivalent amino acid sequence shows typical motifs of a short-chain dehydrogenase (SDR). The recombinant TRII protein expressed in Escherichia coli catalyzes pseudotropine formation from tropinone with a K-m, value, a pH optimum, substrate and co-substrate preferences similar to those reported for the TRII enzymes from other Solanaceae species. The gene is expressed in roots, tubers and aerial parts of potato. The distribution of the TRII transcript in comparison with the calystegine concentrations in the tissues suggests transport of calystegines or their precursors between potato organs.