Properties of Vinorine Synthase — the Rauwolfia Enzyme Involved in the Formation of the Ajmaline Skeleton

Abstract

Abstract Vinorine synthase, a key enzyme in the formation of the Rauwolfia alkaloid ajmaline and its derivatives, has been isolated from Rauwolfia serpentina cell suspension cultures. The new enzy­me has been 160-fold purified and characterized in detail. The synthase catalyses a single step in the biosynthesis of ajmalan alkaloids by the acetyl-coenzyme A and 16-epi-vellosimine dependent formation of vinorine, which shows the basic ajmaline skeleton. Besides a characteristic substrate specificity the major properties of the enzyme are a relatively high pH optimum (pH 8.5), a temperature optimum of 35 °C, an isoelectric point of pH 4.4, and a relative molecular weight of 31000 ± 8%. The apparent K m values for 16-epi-vellosimine and acetyl-coenzyme A were 19.4 μm and 64 μm resp. for the formation of vinorine. Kinetic data of the catalysed reaction indicate that 17-deacetylvinorine is not involved as a free intermediate during the vinorine biosyn­thesis. Vinorine was proved by in vivo feeding experiments to be the biosynthetic precursor of the alkaloids in the final stages of the ajmaline pathway, vomilenine (21-hydroxyvinorine), 17-O-acetylnorajmaline and 17-O-acetylajmaline.