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Metabolism and Enzymology

Epoxidation in Vivo of Hyoscyamine to Scopolamine Does Not Involve a Dehydration Step

Research Center for Cell and Tissue Culture, Faculty of Agriculture, Kyoto University, Kyoto 606, Japan

Hyoscyamine is epoxidized to scopolamine via 6-hydroxyhyoscyamine in several solanaceous plants. 6,7-Dehydrohyoscyamine has been proposed to be an intermediate in the conversion of 6-hydroxyhyoscyamine to scopolamine on the basis of the observation that this unsaturated alkaloid is converted to scopolamine when fed to a Datura scion. To determine whether a dehydration step is involved in scopolamine biosynthesis, [6-¹⁸O]6-hydroxyhyoscyamine was prepared from l-hyoscyamine and ¹⁸O₂ using hyoscyamine 6-hydroxylase obtained from root cultures of Hyoscyamus niger L. When [6-¹⁸O]6-hydroxyhyoscyamine was fed to shoot cultures of Duboisia myoporoides R. B_R., the labeled alkaloid was converted to scopolamine which retained ¹⁸O in the epoxide oxygen. It is concluded that 6-hydroxyhyoscyamine is converted in vivo to scopolamine without a dehydration step.