- Abstract Ephedrine and pseudoephedrine are phenylpropylamino alkaloids widely used in modern medicine. Some Ephedra species such as E. sinica Stapf (Ephedraceae), a widely used Chinese medicinal plant (Chinese name: Ma Huang ), accumulate ephedrine alkaloids as active constituents. Other Ephedra species, such as E. foeminea Forssk. (syn. E. campylopoda C.A. Mey) lack ephedrine alkaloids and their postulated metabolic precursors 1-phenylpropane-1,2-dione and ( S )-cathinone. Solid-phase microextraction analysis of freshly picked young E. sinica and E. foeminea stems revealed the presence of increased benzaldehyde levels in E. foeminea , whereas 1-phenylpropane-1,2-dione was detected only in E. sinica . Soluble protein preparations from E. sinica and E. foeminea stems catalyzed the conversion of benzaldehyde and pyruvate to ( R )-phenylacetylcarbinol, ( S )-phenylacetylcarbinol, ( R )-2-hydroxypropiophenone ( S )-2-hydroxypropiophenone and 1-phenylpropane-1,2-dione. The activity, termed benzaldehyde carboxyligase (BCL) required the presence of magnesium and thiamine pyrophosphate and was 40 times higher in E. sinica as compared to E. foeminea. The distribution patterns of BCL activity in E. sinica tissues correlates well with the distribution pattern of the ephedrine alkaloids. ( S )-Cathinone reductase enzymatic activities generating (1 R ,2 S )-norephedrine and (1 S ,1 R )-norephedrine were significantly higher in E. sinica relative to the levels displayed by E. foeminea . Surprisingly, (1 R ,2 S )-norephedrine N -methyltransferase activity which is a downstream enzyme in ephedrine biosynthesis was significantly higher in E. foeminea than in E. sinica . Our studies further support that benzaldehyde is the metabolic precursor to phenylpropylamino alkaloids in E. sinica .