Biochemical and Metabolic Insights into Hyoscyamine Dehydrogenase

Hyoscyamine, a member of the class of compounds known as tropane alkaloids (TAs), is clinically used as an anticholinergic drug. Previous research has predicted that hyoscyamine is produced via the reduction of hyoscyamine aldehyde. In this study, we identified a root-expressed gene from Atropa belladonna, named AbHDH, which encodes a hyoscyamine dehydrogenase involved in the formation of hyoscyamine. Enzymatic assays indicated that AbHDH was able to not only reduce hyoscyamine aldehyde to produce hyoscyamine but also oxidize hyoscyamine to form hyoscyamine aldehyde under different conditions. To elucidate its catalytic mechanism, the crystal structure of AbHDH at a 2.4-Å resolution was also determined. Overexpression of AbHDH significantly enhanced the biosynthesis of hyoscyamine and the production of anisodamine and scopolamine in root cultures of A. belladonna. However, suppression of AbHDH using RNAi technology did not reduce the production of hyoscyamine, anisodamine, or scopolamine, though it did increase the accumulation of hyoscyamine aldehyde. In summary, this study provided timely biochemical and metabolic insights into hyoscyamine dehydrogenase, pointing to an alternative, promising way to produce pharmaceutical TAs via metabolic engineering in planta.