Peroxidase from Catharanthus roseus (L.) G. Don and the biosynthesis of alpha-3',4'-anhydrovinblastine: a specific role for a multifunctional enzyme.

We have characterized a basic peroxidase with alpha-3',4'-anhydrovinblastine (AVLB) synthase activity, which was purified from Catharanthus roseus leaves. This enzyme was the single peroxidase isoenzyme detected in C. roseus leaves, and the single AVLB synthase activity detected in C. roseus extracts. It was observed that the monomeric substrates of AVLB, vindoline and catharanthine, are both suitable electron donors for the oxidizing intermediates of the basic peroxidase, compounds I and II. Results also showed that the reaction proceeds by a radical-propagated mechanism. Substrate specificity studies of the enzyme revealed that it was also able to oxidize several common peroxidase substrates, indicating a broad range of substrate specificity that is characteristic of class III plant peroxidases. Cytochemical studies showed that the enzyme is localized in C. roseus mesophyll vacuoles, in individual spots at the inner surface of the tonoplast. This particular location suggests a meaningful spatial organization that led to the proposal of a metabolic channeling model for the peroxidase-mediated synthesis of AVLB. The importance of this type of mechanism in the regulation of peroxidase isoenzyme functions in vivo is discussed. In view of the results obtained it is concluded that the basic peroxidase present in C. roseus leaves fulfills all the requirements to be considered as an AVLB synthase, and it is proposed that this specific function of this multifunctional enzyme is determined by metabolic channeling resulting from specific protein-protein interactions.

Purification and characterization of alpha-3',4'-anhydrovinblastine synthase (peroxidase-like) from Catharanthus roseus (L.) G. Don.

An H2O2-dependent enzyme capable of coupling catharanthine and vindoline into alpha-3',4'-anhydrovinblastine (AVLB) was purified to apparent homogeneity from Catharanthus roseus leaves. The enzyme shows a specific AVLB synthase activity of 1.8 nkat/mg, and a molecular weight of 45.40 kDa (SDS-PAGE). In addition to AVLB synthase activity, the purified enzyme shows peroxidase activity, and the VIS spectrum of the protein presents maxima at 404, 501 and 633 nm, indicating that it is a high spin ferric heme protein, belonging to the plant peroxidase superfamily. Kinetic studies revealed that both catharanthine and vindoline were substrates of the enzyme, AVLB being the major coupling product.