RÉSUMÉ
Isosteviol (ent-16-ketobeyeran-19-oic acid) is a hydrolysis product of stevioside, which is a natural sweetener produced in the leaves of Stevia rebaudiana (Bertoni) Bertoni. In this report, we prepared isosteviol and related compounds from stevioside by microbial transformation and chemical conversion and assayed the inhibitory activities toward DNA metabolic enzymes and human cancer cell growth. Among twelve compounds obtained, only isosteviol (compound 3) potently inhibited both mammalian DNA polymerases (pols) and human DNA topoisomerase II (topo II), and IC50 value for pol alpha was 64.0 microM. This compound had no inhibitory effect on higher plant (cauliflower) pols, prokaryotic pols, human topo I, and DNA metabolic enzymes such as human telomerase, T7 RNA polymerase, and bovine deoxyribonuclease I. With pol alpha, isosteviol acted non-competitively with the DNA template-primer and nucleotide substrate. Isosteviol prevented the growth of human cancer cells, with LD50 values of 84-167 microM, and 500 microg of the compound caused a marked reduction in TPA (12-O-tetradecanoylphorbol-13-acetate)-induced inflammation (inhibitory effect, 53.0%). The relationship between the structure of stevioside-based compounds and these activities were discussed.
Sujet(s)
Anti-inflammatoires non stéroïdiens/pharmacologie , Antinéoplasiques d'origine végétale/pharmacologie , Diterpènes de type kaurane/pharmacologie , Antienzymes/pharmacologie , Inhibiteurs de la synthèse d'acide nucléique , Inhibiteurs des topoisomérases , Animaux , Anti-inflammatoires non stéroïdiens/composition chimique , Antinéoplasiques d'origine végétale/composition chimique , Brassica/enzymologie , Bovins , Survie cellulaire/effets des médicaments et des substances chimiques , Diterpènes de type kaurane/composition chimique , Relation dose-effet des médicaments , Antienzymes/composition chimique , Humains , Cinétique , Souris , Rats , Relation structure-activité , Thymus (glande)/enzymologie , Inhibiteurs de la topoisomérase-I , Inhibiteurs de la topoisomérase-IIRÉSUMÉ
Microbial transformation of isosteviol (2), a beyerane-type diterpenoid obtained from stevioside (1) by acid hydrolysis, yielded 7beta-hydroxyisosteviol (3), 11beta-hydroxyisosteviol (5), and 12beta-hydroxyisosteviol (6) by the fungus Aspergillus niger, 17-hydroxyisosteviol (7) by the fungus Glomerella cingulata, and 3 and 7-oxoisosteviol (4) by the fungus Mortierella elongate. The five metabolites, 3-7, along with 1 and 2 were evaluated for their inhibitory effects on Epstein-Barr virus early antigen (EBV-EA) activation induced by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) in Raji cells as a primary screening test for inhibitors of tumor promoters. All the diterpenes tested showed potent inhibitory effects, with the five metabolites 3-7 exhibiting more potent effects.
