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In this study, the in vitro activities of a natural sesquiterpene, a-cyperotundone, isolated from the root bark of Maytenus retusa and a cobalt(II)-complex of a natural occurring prenyl hydroxynaphthoquinone (lapachol) were evaluated against the trophozoite stage of Acanthamoeba castellanii Neff using a previously developed colorimetric 96-well microtiter plate assay, based on the oxido-reduction of Alamar Blue. The obtained activities showed that these two compounds were able to inhibit the in vitro growth of the amoebae at relatively low concentrations. Further identification of the molecular targets of these products and their effects on acanthamoebae should be determined to evaluate their possible therapeutic use. Acanthamoeba species are opportunistic causal agents of disseminated infections (mostly cutaneous and nasopharyngeal infections), such as a sight-threatening ulceration of the cornea called amoebic keratitis and a fatal Acanthamoeba Granulomatous Encephalitis (AGE) (Marciano-Cabral and Cabral, 2003; Clarke and Niederkorn, 2006; Khan, 2006). Present therapeutic measures for Acanthamoeba keratitis rely on topical applications of antimicrobials including a combination of propamidine isothionate and neomycin or chlorhexidine. The length of these treatments makes the process arduous. Furthermore, as present treatments are poorly effective against cystic stages of the protozoan, residual infection often remains even after treatment. No treatment against AGE has been established although therapeutic measures have been used with apparent effect as an adjunct to surgery (Schuster and Visvesvara, 2004; Khan, 2003, 2006; Martín-Navarro et al., 2008). Terpenes represent one of the largest and most diverse classes of secondary metabolites, with over 55,000 members isolated to date. The terpene cyclase enzymes used in nature convert simple, linear hydrocarbon phosphates into an exotic array of chiral, carbocyclic skeletons. Further oxidation and rearrangement results in an almost endless number of conceivable structures. The enormous structural diversity presented by this class of natural products ensures a broad range of biological properties—ranging from anticancer and anti-malarial activities to tumour promotion and ionchannel binding (Paduch et al., 2007; Patocˇka, 2003). The marked structural differences of terpenes also largely thwart the development of any truly general strategies for their synthetic construction (Maimone and Baran, 2007). Furthermore, several sesquiterpenes have shown to be promising for the treatment of infections due to parasitic protozoa such as Plasmodium, Leishmania and Trypanosoma (Karioti et al., 2007; Schmidt et al., 2009). In this work, the in vitro activity of a natural sesquiterpene and a synthetic cobalt( II)–lapachol complex against the trophozoite stage of Acanthamoeba castellanii Neff is reported. The first molecule is a patchoulane-type sesquiterpene, a-cypertundone (Neraldi et al., 1965), isolated from the root bark of Maytenus retusa. The second molecule tested is a cobalt(II)-complex of lapachol. Lapachol is a prenyl hydroxynaphthoquinone with interesting biological properties such as antitumoural and antibiotic (Ravelo et al., 2004). Unfortunately, it has serious negative side effects(de Sandoval et al., 1996). However, and due to some properties such as a good chelating ketoenol function and a prenyl residue making it lipophilic, pharmacologists are still interested in testing lapachol complexes with metal cations. In this work, the activity of a previously synthesized (Hernández- Molina et al., 2007) complex of Co(II) with lapacholate anions was tested. To our knowledge, the present study demonstrates for the first time the in vitro activity of a-cyperotundone and the Co(II)–lapachol complex against A. castellanii Neff. The obtained activities showed that these molecules inhibited the in vitro growth of the amoebae at relatively low concentrations, and thus highlights their future therapeutic use. |
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