Isolation of acaricidal compounds from Acokanthera schimperi with activity against Rhipicephalus appendiculatus

Abstract

Tick borne diseases have severe consequences on the health of millions of cattle worldwide and cause serious economic losses. Synthetic drugs have been effective acaricideshowever they are expensive, show side effects and develop resistance.This has generated interest in the use ofplant based acaricides, which however seem to offer a reliable, cheap and cost effective methods. In this research, the acaricidal activityof Acokanthera schimperi secondarymetabolites againstRipicephalus. appendiculatus is reported.These secondary metabolites were active against the larvae of R. appendiculatus. The study aimed to isolate, purify and elucidate the structures of acaricidal compounds from Acokanthera schimperi. The collected leaves were air dried under a shade, ground into powder and exhaustively extracted with methanol and then suspended in water. Sequential extraction using hexane and then ethyl acetate was done. Methanol, hexane, ethyl acetate crudes and the water phase were screened for acaricidal activity after 48hours. Methanol crude extract registered (LC5042.26/ LC90 79.14 mg/ml), Hexane crude (LC50 36.49/ LC9058.34 mg/ml), Ethyl acetate crude (LC50 47.11/ LC90 69.48 mg/ml). The fractionation of ethyl acetate crude extract yielded fractions FA11, FA21(LC50 31.94 / LC90 66.93 mg/ml) and FA30 (LC50 29.85 /LC90 87.65 mg/ml). Fraction FA21 subjected tofurther, fractionation and purification led to fractions FA21a (LC50 5.88 /LC90 11.19 mg/ml), FA21b (LC505.88 /LC90 11.19 mg/ml), FA21b1 (LC50 4.53/LC90 6.92mg/ml), FA21b2 (LC50 2.96/ LC90 6.09mg/ml),FA21a (LC50 5.88/ LC90 11.19 mg/ml), FA21b1 (LC50 4.53/ LC90 6.92mg/ml) and FA21b2 (LC50 2.96/ LC90 6.09mg/ml). Fraction FA21b2 subjected to further HPLCpurification yielded two pure compounds (23) and(24).Separate acaricidal mortality tests could not be determined for the pure compounds (23) and (24) due to the minute quantities of each after HPLC fractionation and purification. However, the compound mixtureFA21b2had (LC50 2.96/ LC90 6.09mg/ml).These two new compounds were successfully identified through analysis of 1D and 2D nuclear magnetic resonance spectroscopy and mass spectrometry data, as well as comparing with literature data. The two newlyisolated compounds were8-hydroxy-2H -chromen -2- one (23) and (E)-methyl-4-hydroxyl -7- oxo-5- (2-oxo-2H-chromen -8-yloxy) oct-2- enoate(24).These findings show that active principles from A. schimperiare likely to provide new, biodegradable, environmentally friendly biological active constituents that will serve as an alternativeto presently less effective and high cost synthetic acaricides.