Component of iboga and aspidosperma monoterpenoid indole alkaloids (MIAs, e.g. tabersonine and catharanthine) biosynthesis pathway from 19E-geissoschizine. Catalyzes the second 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 + NADP(+) = 15alpha-stemmadenine intermediate II + 2 H(+) + NADPH Alkaloid biosynthesis. Monomer. Expressed in leaf epidermis. Reduced accumulation of catharanthine and vindoline, but accumulation of 16S- and 16R-deshydroxymethylstemmadenine (16S/16R-DHS) and, to some extent, of akuammicine. Belongs to the aldo/keto reductase family.