Characterization of 2 beta (R)-17-O-acetylajmalan: acetylesterase--a specific enzyme involved in the biosynthesis of the Rauwolfia alkaloid ajmaline.

A novel enzyme was isolated, partially purified (217-fold) and characterized from cell suspension cultures of Rauwolfia serpentina Benth. The enzyme catalyzes one of the late biochemical reactions in the biosynthesis of ajmaline by hydrolysis of 17-O-acetylated alkaloids of the ajmalan group forming the appropriate deacetylated compounds. This esterase exhibits an unusually high substrate selectivity and exclusively accepts acetylated ajmaline derivatives with the naturally occurring 2 beta (R)-configuration. The properties of the enzyme were determined showing an optimum pH at 7.5, an isoelectric point of pH 4.9 and a relative molecular weight of 33 +/- 2 kDa. Inhibition studies of enzyme activity point to the necessity of SH-groups. The esterase seems not to be inhibited by ajmaline, the end product of the pathway. The highest enzyme activities were observed in leaves and cell suspension tissues of the tribe Rauwolfieae which are known to synthesize ajmaline and its congeners. The specific function of the esterase in the biosynthesis of the later alkaloids was established.