Plant Physiology 100:836-845 (1992)
© 1992 American Society of Plant Biologists
Metabolism and Enzymology
Two Tropinone Reductases with Distinct Stereospecificities from Cultured Roots of Hyoscyamus niger1
Takashi Hashimoto,
Keiji Nakajima,
Godelieve Ongena2 and
Yasuyuki Yamada
Department of Agricultural Chemistry, Faculty of Agriculture, Kyoto University, Kyoto 606, Japan
Tropinone is an alkamine intermediate at the branch point of biosynthetic pathways leading to various tropane alkaloids. Two stereospecifically distinct NADPH-dependent oxidoreductases, TR-I and TR-II, which, respectively, reduce tropinone to 3 -hydroxytropane (tropine) and 3 -hydroxytropane ( -tropine), were detected mainly in the root of tropane alkaloid-producing plants but not in nonproducing cultured root. Both reductases were purified to near homogeneity from cultured root of Hyoscyamus niger and characterized. The TR-I reaction was reversible, whereas the TR-II reaction was essentially irreversible, reduction of the ketone being highly favored over oxidation of the alcohol -tropine. Marked differences were found between the two reductase in their affinities for tropinone substrate and in the effects of amino acid modification reagents. Some differences in substrate specificity were apparent. For example, N-propyl-4-piperidone was reduced by TR-II but not by TR-I. Conversely, 3-quinuclidinone and 8-thiabicyclo[3,2,1]octane-3-one were accepted as substrates by TR-I but hardly at all by TR-II. Both enzymes were shown to be class B oxidoreductases, which transfer the pro-S hydrogen of NAD(P)H to their substrates. Possible roles of these tropinone reductases in alkaloid biosynthesis are discussed.
2 Present address: Innogenetics NV, Industriepart Zwijnaarde 7, Box 4, B-9052 Ghent, Belgium.
1 This work was supported in part by a grant from the Ministry of Education, Science, and Culture of Japan (No. 2660090 to T.H.).
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