Biosynthesis of withanolides in Acnistus breviflorus

Administration of [2?¹⁴C]mevalonolactone to excised leaves of Acnistus breviflorus produced labelled withaferin A and jaborosalactone A. The former was degraded leading to the isolation of glyceric acid from C-25–C-27 of the withanolide. These carbons represented only 2 % of the total radioactivity of withaferin A. The relative radioactivity of these carbons indicated that C-26 is directly derived from C-2 of mevalonolactone suggesting that the 25-pro-R-methyl group of cholesterol or any other sterol intermediate had been oxidized to form the lactone ring of the withanolide. The total radioactivity value found for C-25–C-27 was much lower than the expected 20 % of the total value for the withanolide indicating that the side chain of the sterol precursor had been partially cleaved during the biosynthetic process.