Vinorine

Salutaridinol 7-0-acetyltransferase catalyzes the conversion of the phenanthrene alkaloid salutaridinol to salutaridinol-7-Oacetate, the immediate precursor of thebaine along the morphine biosynthetic pathway in P. somniferum (Fig. 10.7).26 Acetyl CoA-dependent acetyltransferases have an important role in plant alkaloid metabolism. They are involved in the synthesis of monoterpenoid indole alkaloids in medicinal plant species such as Rauwolfia serpentina. In this plant, the enzyme vinorine synthase transfers an acetyl group from acetyl CoA to 16-epi-vellosimine to form vinorine. This acetyl transfer is accompanied by a concomitant skeletal rearrangement from the sarpagan- to the ajmalan-type (reviewed in2). An acetyl CoA-dependent acetyltransferase also participates in vindoline biosynthesis in Catharanthus roseus, the source of the chemotherapeutic dimeric indole alkaloid vinblastine (reviewed in2). Acetyl CoA deacetylvindoline 4-O-acetyltransferase catalyzes the last step in vindoline biosynthesis. A cDNA encoding acetyl CoA deacetylvindoline 4-0-acetyltransferase was recently successfully isolated.27... 

Bayer, A., Ma, X.Y. and Stockigt, J. (2004) Acetyltransfer in natural product biosynthesis - functional cloning and molecular analysis of vinorine synthase. Bioorg. Med. Chem., 12,2787-95. 

Ma, X., Koepke, J., Panjikar, S., Fritzsch, G. and Stockigt, J. (2005) Crystal structure of vinorine synthase, the first representative of the BAHD superfamily. /. Biol. Chem., 280, 13576-83. 

In the next step of the ajmaline pathway, vinorine synthase transforms the sarpagan alkaloid epi-vellosamine to the ajmalan alkaloid vinorine (84). Vinorine synthase also has been purified from Rauwolfla cell culture, subjected to protein sequencing, and cloned from a cDNA library (85, 86). The enzyme, which seems to be an acetyl transferase homolog, has been expressed heterologously in E. coli. Crystallization and site-directed mutagenesis studies of this protein have led to a proposed mechanism (87). 

Vinorine hydroxylase hydroxylates vinorine to form vom-ilene (88). Vinorine hydroxylase seems to be a P450 enzyme that requires an NADPH-dependent reductase. This enzyme is labile and has not been cloned yet. Next, the indolenine bond is reduced by an NADPH-dependent reductase to yield 1,2-dihydrovomilenene. A second enzyme, 1,2-dihydrovomilenene reductase, then reduces this product to... 

Pfitzner A, Stoeckigt J. Biogenetic link between sarpagine and ajmaline type alkaloids. Tetrahedron Lett. 1983 24 5197-5200. Gerasimenko I, Ma X, Sheludko Y, Mentele R, Lottspeich E, Stockigt J. Purification and partial amino acid sequences of the enzyme vinorine synthase involved in a crucial step of ajmaline biosynthesis. Bioorg. Med. Chem. 2004 12 2781-2786. 

N(l)-Demethylseredamine32 (24a) has been identified as a constituent of Rauwolfia sumatrana Jack as well as sandwicine (24b), isosandwicine (24c) has been isolated from R. vomitoria,33 and vinorine (24d)34 and majoridine (24e)35 from Vinca species. Raucaffricine (24f)36 from R. caffra is the first of the ajmaline... 

Figure 16 Enzymatic synthesis of vinorine (17) from 49, catalyzed by vinorine synthase, depending on acetyl-CoA.

Figure 17 (a) Overall structure of vinorine synthase in ribbon diagram and in surface diagram, (b) Mechanism of VS and role of the catalytic residue His160. Taken from... 

The enzyme which follows the vinorine synthase was commonly named vinorine hydroxylase. VH is responsible for the generation of vomiienine (29) by selective hydroxylation of vinorine (17) at the C-21 position (Figure 18). The reaction is catalyzed exclusively by a microsomal protein from Rauvolfia cultured cells in the presence of the substrate vinorine, NADPH, and O2. The investigation of the taxonomical distribution of the enzyme indicated that vinorine hydroxylase is exclusively present in ajmahne-producing plant cells. It was also observed that the substrate specificity of VH was high. Alkaloids with vinorine-related structures such as fJ-tetra-phyllicine or its Z-configurated isomer were not hydroxylated... 

Figure 18 Reaction step catalyzed by membrane-bound vinorine hydroxylase from Rauvolfia cell cultures leading from 17 to the hydroxylated 29.

Whereas vinorine hydroxylase catalyzes the first reaction (hydroxylation) taking place on the periphery of the ajmalan skeleton at C-21, the second and the third steps represent two hydrogenations. These reactions are performed by the NADPH-dependent soluble reductases, vomilenine reduc-... 

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