Biosynthesis of Ephedrine Initiated by Pyridoxal Phosphate‐Dependent Formation of Cathinone

Ephedra alkaloids possess some of the most basic structures of alkaloids. Despite their importance for human use and their commercial relevance, the biosynthesis of ephedra alkaloids has remained enigmatic. The predominant biosynthetic pathway in the literature proposes a thiamin-dependent carboligation followed by a transaminase, although no candidate enzymes have yet been identified in ephedra alkaloid producers. In this work, an alternative pathway in plants to ephedra alkaloids via (S)-cathinone is investigated that circumvents the formation of 1-phenylpropane-1,2-dione as an intermediate and is in full agreement with previous biosynthetic studies. This alternative pathway involves the pyridoxal phosphate (PLP)-dependent carboligation of -benzoyl-CoA- and L-alanine in a single step. The PLP-dependent formation of labeled and unlabeled (S)-cathinone is detected in the plant lysate of young stem tissue of various Ephedra species that contain Ephedra alkaloids, as well as in young leaf tissue of Catha edulis. The incorporation of labeled nitrogen from L-alanine into (S)-cathinone supports the hypothesis that an α-oxoamine synthase (AOS) catalyzes the formation of (S)-cathinone, bypassing the dione as an intermediate. These results demonstrate the involvement of a PLP-dependent AOS as a pivotal step in the biosynthesis of ephedra alkaloids.