The dimeric Catharanthus alkaloid vincristine (1) is oxidized to the same ring fission product in incubations with either horseradish peroxidase or the human serum copper oxidase ceruloplasmin. Horseradish peroxidase-catalyzed oxidation of vincristine requires hydrogen peroxide, whereas ceruloplasmin-catalyzed oxidation of vincristine requires chlorpromazine as a "shuttle oxidant". Preparative-scale incubations allowed for the production, isolation, structural characterization, and biological evaluation of the metabolite. The metabolite was identified as the heterocyclic ring cleavage product N-formylcatharinine (5). N-Formylcatharinine was 118 times less active than vincristine in an in vitro test against a human T-cell leukemic cell line. Therefore, these enzyme-catalyzed reactions lead to bioinactivation of vincristine.