Cardenolide biosynthesis from acetate in Asclepias curassavica

Abstract

Cardenolides, free sterols, triacylglycerols, triterpenyl acetates, steryl- and triterpene esters were the major lipid classes in the stem of five- to eight-week-old Asclepias curassavica plants, in which 30% of the cardenolides occurred in the latex. Excised defoliated stems incorporated up to 6.7% of radioactive acetate into these lipids in a three day incorporation period. The label from [2- 14C]acetate proceeded mainly to the fatty acid moiety of the triterpene and steryl esters, the fatty acids of the triacylglycerols, the free sterols and the cardenolides. Optimal results were obtained with 10 cm stems from six-week-old plants. An increased supply of acetate (2.5–10 μmol stem −1) showed a saturated synthesis of cardenolides and triacylglycerols and caused an enhanced production of triterpenyl acetates. The highest yield of [ 14C]cardenolides was obtained after uptake of 5 μmol of acetate per stem. The cardenolide composition of the incubated stem tissue increased on the absorption of 10 μmol of acetate and major changes in organic acid composition were measured after uptake of 20 μmol of acetate. [ 13C]Uscharidin was biosynthesized from 0.5 mmol of [1,2- 13C]acetate by 100 stem tips. 13C NMR spectroscopy showed an unequal relative enrichment in both the genin and the dideoxyhexosulose moiety. The construction of the butenolide ring by the condensation of a pregnane derivative with one molecule acetate as observed for 5β-cardenolides is not confirmed by these-NMR data.