Biological activities of sesquiterpene lactones from Helianthus annuus: antimicrobial and cytotoxic properties; influence on DNA, RNA, and protein synthesis.
Sesquiterpene lactones, produced in light and capable of inhibiting auxin-induced elongation growth of coleoptile and hypocotyl segments, were isolated from young leaves of Helianthus annuus (Spring and Hager, Planta in press, 1982). These compounds have an antibiotic effect on gram-negative and gram-positive bacteria as well as on some fungi. The minimal inhibiting concentration (MIC) of compound II (15-hydroxy-3-dehydrodesoxyfruticin, Fig. 1), for example, is 15 micrograms/ml for Bacillus brevis, and 95 micrograms/ml for the fungus Eremothecium ashbyi. In addition, cytotoxic effects on mouse myeloma cells (NS-1) were also shown. Compound II causes a 50% inhibition of cell proliferation (ED50) at a concentration of 170 nM, compound I (niveusin C, Fig. 1) at 220 nM. The LD50-values were 0.15 micrograms II/ml and 1.24 micrograms I/ml, respectively. By measuring 14C-labelled thymidine, uridine and leucine incorporation into murine cells of the ascitic form of Ehrlich carcinoma (EAC) it could be shown that compounds I and II inhibit DNA and RNA synthesis, but do not affect the translation processes involved in protein synthesis. Furthermore, it could be shown that the exocyclic methylene group in the molecules of I and II plays an important role in triggering the described inhibitory effects.