ABSTRACT
Synthetic chemicals employed in the control of insect pests pose environmental and health challenges. Plant materials have been and are still being investigated as sources of alternatives to these chemicals. This study investigated the insecticidal and cytotoxic properties of crude extracts, fractions and isolates of the seeds of Dennettia tripetala G. Baker (Pepper Fruit). The acetone and hexane extracts of the pulverized seeds of D, tripetala were subjected to a preliminary insecticidal (adulticidal) assay on Callosobruchus maculatus (Cowpea beetle). The acetone extract was subjected to phytochemical analyses and successively partitioned into hexane, dichloromethane (DCM) ethylacetate and aqueous fractions. Adulticidal assays employing Rhyzopertha dominica (Lesser grain borer) and Sitophilus oryzae (Rice weevil); and larvicidal assays using Aedes aegypti (Yellow fever mosquito) guided the isolation of insecticidal compounds. Cytotoxic activities were assayed on brine shrimp (Artemia salina L.) nauplii and 3T3 (mouse fibroblast) cells. The isolation of compounds was performed by GC-MS, column chromatography, qualitative TLC, and preparative HPLC; structural elucidation was by NMR, EIMS, ESI-MS and FT-IR spectroscopic techniques. The mechanism of insecticidal action was investigated employing in vitro anticholinesterase assay; and in silico docking analysis of the interaction of an active compound and identified analogs of this compound with modelled insect-like β-adrenergic octopamine receptor. The results of the preliminary insecticidal assay revealed potent insecticidal activities for both acetone and hexane extracts with the acetone extract possessing a significantly higher activity (P ≤ 0.05) at the least concentration of 1mg/g cowpea. Phytochemical analyses revealed the presence of volatile oils, steroids, terpenoids, alkaloids, glycosides, saponins, phenols, tannins and flavonoids. Phenols and flavonoids were in high concentrations, while tannins and alkaloids in low concentrations. The hexane fraction displayed maximum adulticidal and larvicidal activities in consonance with its acetone extract and thus prompted the isolation of 1-Nitro-2-phenylethane (NPE), Nerolidol, Linalool and β-Sitosterol. The compounds: Quercetin, Avicularin and Daucosterol were also isolated from the non-insecticidal ethylacetate fraction. Cytotoxicity on brine shrimp revealed high toxicity for the acetone extract expressed in its hexane and aqueous fractions at maximum concentration of 1000 μg/ml. However no toxicity was recorded for the extract and fractions on 3T3 cells at 30 µM, xxiii except for the DCM fraction which exhibited a mild toxicity; thus indicative of indirect cytotoxic impact for the extract and toxic fractions, and mild direct cytotoxic impact for the DCM fraction. The major compound 1-Nitro-2-phenylethane, displayed potent neuroinsecticidal effects while nerolidol was inactive as a fumigant; both compounds were inactive on A. aegypti larvae; however synergistic phenomena is plausible in the activity of subfractions containing these compounds. No cytotoxicity was observed for 1-Nitro-2- phenyethane and nerolidol. 1-Nitro-2-phenyethane and nerolidol showed no inhibitory activity on acetylcholinesterase at 0.5 mM. 1-Nitro-2-phenylethane, and its analogs; 1-Phenyl2-nitropropene and 2-Phenylethanol all displayed favourable conformational binding with the modelled octopamine receptor characterized by hydrophobic and hydrogen bond interactions. Data obtained in this study corroborate the application of D. tripetala in the protection of stored products; and are crucial to the commercialization of insecticidal compounds (and formulations) from the plant.
