Component of the seco-iridoid and derivatives monoterpenoid indole alkaloids (MIAs, e.g. vincristine, quinine, and strychnine) biosynthesis pathway. Catalyzes the oxidation of 19E-geissoschizine to produce a short-lived MIA unstable intermediate which can be spontaneously converted into akuammicine or oxidized by Redox1 and Redox2 to produce stemmadenine and 16S/R-deshydroxymethylstemmadenine (16S/R-DHS). (19S)-geissoschizine + O2 + reduced [NADPH--hemoprotein reductase] = akuammicine + formate + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] (19S)-geissoschizine + O2 + reduced [NADPH--hemoprotein reductase] = 15alpha-stemmadenine intermediate I + 2 H2O + oxidized [NADPH--hemoprotein reductase] Alkaloid biosynthesis. Expressed in leaf epidermis (PubMed:24710322, PubMed:30256480). Also present in the leaf internal phloem-associated parenchyma (IPAP) inside the mesophyll (PubMed:24710322). Reduced levels of catharanthine, vindoline and ajmalicine, but accumulation of their precursor 19E-geissoschizine and its derivatives pericyclivine and perivine. Belongs to the cytochrome P450 family.