Structures of Steroidal Saponins from the Tubers of Brodiaea californica and Their Inhibitory Activity on Tumor Promoter-Induced Phospholipid Metabolism

Phytochemical examination of the fresh tubers of Brodiaea californica resulted in the isolation of four new steroidal saponins. Their structures were determined, by extensive spectral analysis including two-dimensional (2D) NMR spectroscopy and acid-catalyzed hydrolysis, to be (25S)-spirost-5-ene-1β, 3β-diol [(25S)-ruscogenin] 1-O-{O-β-D-glucopyranosyl-(1→3)-O-α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranoside} (1), (25S)-ruscogenin 1-O-{O-β-D-glucopyranosyl-(1→3)-O-α-L-rhamnopyranosyl-(1→2)-O-[β-D-xylopyranosyl-(1→3)]-β-D-glucopyranoside} (2), the C-20 and C-22 isomer of 2 (3) and the 6'-O-acetyl derivative of 2 (4), respectively. The conformations of the tetrasaccharide moiety of 2 and 4 were inspected through molecular mechanics and molecular dynamics calculation studies, showing that the acetyl group attached to C-6 of the inner glucose was near the C-21 methyl of the aglycon in the calculated preferred conformation of 4, which must cause the downfield shift of 21-Me by 0.07 ppm in comparing the 1H-NMR of 4 with that of 2. The inhibitory activity of the isolated saponins on 12-O-tetradecanoylphorbor-13-acetate (TPA)-stimulated 32P-incorporation into phospholipids of HeLa cells was evaluated to identify new antitumor-promoter compounds.