Component of iboga and aspidosperma monoterpenoid indole alkaloids (MIAs, e.g. tabersonine and catharanthine) biosynthesis pathway from 19E-geissoschizine. Catalyzes the first oxidation step of the unstable intermediate product resulting from the reaction triggered by the geissoschizine oxidase (GO) in the stemmadenine biosynthesis process from 19E-geissoschizine. 15alpha-stemmadenine intermediate I + H2O + NADPH = (16S)-deshydroxymethyl-stemmadenine + formate + NADP(+) 15alpha-stemmadenine intermediate I + H2O + NADPH = (16R)-deshydroxymethyl-stemmadenine + formate + NADP(+) 15alpha-stemmadenine intermediate II + NADP(+) = 15alpha-stemmadenine intermediate I + NADPH Binds 2 Zn(2+) ions per subunit. Alkaloid biosynthesis. Expressed in leaf epidermis. Reduced accumulation of catharanthine and vindoline, but accumulation of akuammicine and short-lived MIA. Belongs to the zinc-containing alcohol dehydrogenase family.