Bio-Assay Guided Fractionation of Anti-Mosquito Limonoids from Turraea abyssinica and Turraea cornucopia

Abstract

Malaria, transmitted by mosquitoes of the genus Anopheles, is the most important parasitic disease worldwide. It affects 40% of the global population mostly within the tropical world. Yearly, over one million children under the age of five die in Africa as a result of malaria. The genus Turraea belongs to the family Meliaceae. Meliaceae is characterized by the presence of tetranortriterpenoids (limonoids), a group of compounds that exhibit a wide variety of biological properties including anti-insect, anti-protozoa, anti-bacterial and anti-fungal activities. In the present study, the root bark of T. abyssinica and T. cornucopia were extracted with methanol and partitioned between water and chloroform. The activity of crude methanol and chloroform extracts was tested against larvae and adults of Anopheles gambiae sensu stricto (Diptera: Culicidae). As larvicides, the methanol extract of T. cornucopia was the most active (LD50 202 ppm). The chloroform extract of T. cornucopia was the most active as an adulticide (LD50 302.1 ppm). Partitioning of the methanolic extracts between chloroform and water, followed by silica gel chromatography of the organic extract gave limonoids-rich fractions that had larvicidal activity higher than those of the corresponding crude extracts. Column chromatography of the chloroform fraction followed by semi-preparative HPLC yielded 1α-12α-diacetoxy-1,2-dihydro-7-deacetyl-3β-7α-dihydroxyazadiron,12α-acetoxy-7-deacetylazadiron and mzikonone. The structures of these compounds were elucidated using spectroscopic methods (IR UV, MS, 1H-, 13C- NMR, gradient COSY, and gradient HMBC experiments). Stereo chemical assignments were made by gradient NOE spectroscopy. These plants offer a significant potential for mosquito control as larvicides and adulticides.