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The leaves of Catharanthus roseus (L.) G. Don produce the first natural drugs used in cancer therapy - the dimeric terpenoid indole alkaloids vinblastine and vincristine. The study of C. roseus further revealed two other terpenoid indole alkaloids with important pharmacological activity: ajmalicine, used as an antihypertensive, and serpentine, used as sedative. The biosynthetic pathway of the medicinal alkaloids has been investigated in much detail and a number of steps are now well characterized at the enzyme and gene level and, recently, several regulatory genes have also been isolated and characterized. Since early studies of the biosynthesis of vinblastine, during the 1970s and 1980s, the dimerization reaction has attracted much attention due to its possible regulatory importance and potential application for the semi synthetic production of the dimeric alkaloids. After initial, inconclusive work suggesting the involvement of peroxidase-like enzymes, the search for the dimerization enzyme in leaf tissue detected a single dimerization activity credited to the single class III plant peroxidase present in the leaves of the plant - the basic isoenzyme CRPRX1. The enzyme was purified to homogeneity, the respective cDNA and genomic sequences were characterized, and a channeling mechanism was proposed for the peroxidase-mediated-vacuolar synthesis of the first dimeric alkaloid intermediate, α-3′,4′-anhydrovinblastine. On the other hand, the oxidation of ajmalicine into serpentine has been attributed to basic peroxidase isoenzymes localized in the vacuole of C. roseus cells. An overview of the work implying class III plant peroxidases in the biosynthesis of terpenoid indole alkaloids in C. roseus is presented here.Abbreviations: CRPRX1 -Catharanthus roseus peroxidase 1; DAB - diaminobenzidine; IEF - isoelectric focusing; UV - ultraviolet.