Biosynthesis of Tropane Alkaloids

Summary

Roots of various solanaceous plants grow well in vitro and produce large amounts of tropane alkaloids. These root cultures provide excellent materials for tracer experiments and for the extraction of the alkaloid biosynthetic enzymes. Our enzymic and tracer studies on the early biosynthesis of tropane alkaloids indicated that N-methylputrescine is synthesized mostly from symmetrical putrescine in our root cultures; these results are contrary to the previous asymmetrical biosynthetic scheme.

We found an enzymic activity that hydroxylates hyoscyamine at the C-6 position in cell-free extracts from various root cultures. The enzyme hyoscyamine 6β-hydroxylase (EC 1.14.11.-) requires l-hyoscyamine, 2-oxoglutarate, ferrous ions and molecular oxygen for activity. Ascorbate and catalase stimulate the hydroxylation. The purified enzyme epoxidized 6,7-dehydrohyoscyamine, a hypothetical precursor of scopolamine, to scopolamine. Several lines of evidence indicate that a single 2-oxoglutarate-dependent dioxygenase catalyses both the hydroxylation and the epoxidation reactions.

Shoot cultures of alkaloid-producing plants are unique in that they do not accumulate alkaloids, yet they will convert added hyoscyamine to scopolamine. When [6-¹⁸O]6β-hydroxyhyoscyamine was fed to Duboisia shoot cultures, the labelled alkaloid was converted to scopolamine which retained ¹⁸O in the epoxide oxygen. Therefore, 6β-hydroxyhyoscyamine is converted in vivo to scopolamine without a dehydration step. Some other interesting reactions in the tropane alkaloid biosynthesis are also presented.