Phytochemistry and bioactivity investigations of three Kenyan croton species

Abstract

Three Kenyan Croton species, C. megalocarpoides Friis and Gilbert, C. alienus Pax and C. sylvaticus Hochst were investigated for their phytochemistry and biological activity relevancies. Anti-microbial activity evaluation was done on aqueous and methanol crude plant extracts to enable selection of most active parts. Documented procedures were used to profile the selected extracts for their phytochemical concentrations followed by fractionation using column chromatography. The phytochemicals obtained were identified using NMR spectroscopic techniques and subjected to various biological activity tests. Forty one compounds (fifteen of them new) were isolated. C. megalocarpoides roots produced twenty diterpenoids belonging to, ent-clerodane (thirteen, twelve new), abietane (three, one new) and ent-trachylobane (four known) series. Two known triterpenoids (lupeol and acetyl aleuritolic acid) and common phytosterols (stigmasterol and sitosterol) were also isolated. Two novel compounds (alienusolin, a 4α-deoxyphorbol ester and crotonimide C, a glutarimide alkaloid derivative) and nine known compounds (an alkaloid, six methylcyclohexane derivatives of crotepoxide, a triterpenoid and a phytosterol were isolated from C. alienus leaves and roots. From C. sylvaticus roots, seven diterpenoids belonging to clerodane (four, one new), halimane (two known) and labdane (one known) series and a phytosterol were isolated. Anti-microbial activity tests were done using different strains of bacteria and fungi. Candida albicans was the most susceptible micro-organism to the crude plant extracts. C. alienus and C. sylvaticus (root and stem bark aqueous extracts) were active at the lowest concentration tested (25 mg / mL). C. sylvaticus stem bark (methanol extract) was the only crude extract that inhibited the growth of a bacteria strain (Bacillus subtillis) at a concentration of 10 mg / mL. The compounds that were isolated and assayed from C. alienus and C. megalocarpoides were inactive to all microorganisms used (IC50 > 20µg / mL). C. alienus leaves (MeOH: DCM, 1:1 v / v extract) is the only crude extract that showed activity against Leishmania donovanii (IC50 = 80µg / mL). The compounds isolated from it were however inactive against the same, L. donovanii (IC50 and IC90 > 40µg / mL). All the crude extracts and compounds isolated and tested from C. alienus and C. megalocarpoides were inactive against D6 and W2 strains of Plasmodium falciparum (IC50 > 4760 ng / mL); VERO (IC50 > 4760 ng / mL) and Aedes aegypti and Anopheles gambiae larvae (LC50 and LC95 >100 ppm). The methanol extract of C. megalocarpoides and C. sylvaticus stem barks had a low total phenolic content (1.89 + 0.02% -1.14 + 0.01% w / w equivalent of gallic acid) and anti-oxidant activity