Cardenolide biosynthesis from malonate in Asclepias curassavica.
A variable cardenolide composition was found in the aerial parts of A. curassavica: voruscharin was the major compound in the latex and was probably absent in other tissues, in which uscharidin accounted for 15-20% of the cardenolides. Stem discs and excised stem tips of A. curassavica were incubated in a medium containing various 14C-labelled water-soluble substrates, and the production of radioactive cardenolides and sterols was measured after 48-72 h. Labelled sucrose and mevalonate were relatively poorly converted into cardenolides. Better incorporation rates were obtained when acetate or pyruvate was absorbed by excised stem tips, but quantitatively the highest yield in cardenolide production was obtained with malonate. Radioactivity from [14C]malonate, supplied by a wick to an intact growing shoot, was incorporated into the major cardenolides of both latex and stem tissue. [13C]Uscharidin was biosynthesized by stem tips from [13C]malonate. Relative enrichment sites determined by 13C NMR spectroscopy were at C-6′ (1.55%), C-1′ (0.37%), C-2′ (0.25%), C-5′ (0.21%), C-16 (0.51%), C-19 (0.37%), C-1 (0.25%), C-7 (0.19%) and C-5 (0.18%).