Chemical constituents from Elytropappus rhinocerotis and Rhoicissus tridentata : structural and activity studies.

Abstract

Traditional medicines are used by approximately 80% of South African population for their primary health care needs, but the chemistry and biological activity of many medicinal plants have not yet been investigated. This study focused on the isolation and structural elucidation of natural products, as well as developing a high-performance liquid chromatography (HPLC) method to fingerprint the crude extract of Elytropappus rhinocerotis (L.f.) Less. E. rhinocerotis is well known in traditional medicine for the treatment of colic, wind, diarrhoea, indigestion, dyspepsia, gastric ulcers and stomach cancer. This study was also conducted to isolate, elucidate structures and evaluate the uterotonic activity of natural products from Rhoicissus tridentata (L.f.) Wild & Drumm. subsp. cuneifolia, a medicinal plant used by many South African women to induce labour and to tone the uterus during pregnancy. From the ethyl acetate extract of the aerial parts of E. rhinocerotis, 6,7- dimethoxycoumarin, 5,6,4'-trihydroxyflavone, 5,7-dihydroxy-4'-methoxyflavone, 5,7- dihydroxy-6,4'-dimethoxyflavone, kaempferol-3-methyl ether, (+)-13-epi-labdanolic acid, (+)-labdanolic acid, (+)-labdanolic acid methyl ester, and (+)-labdanediol were isolated. These compounds are reported for the first time from E. rhinocerotis. The isolated flavonoids may justify the traditional use of this plant in the treatment of cancer, while the labdane diterpenes have shown anti-inflammatory activities in other studies. A HPLC method to fingerprint the crude extract from the aerial parts of E. rhinocerotis was successfully developed and minor variations were observed in the chemical compositions of E. rhinocerotis plants collected from different geographic locations. From the acetone fraction of the methanol extracts of the root of R. tridentata, catechin, quercetrin, morin 3-O-α-L-rhamnopyranoside, trans-resveratrol glucoside, an inseparable mixture of asiatic acid and arjunolic acid, β-sitosterol, and linoleic acid were isolated and characterised. Except for catechin and β-sitosterol, these compounds are reported for the first time from Rhoicissus and the occurrence of morin 3-O-α-Lrhamnopyranoside is reported for the first time from the family Vitaceae. The uterotonic activity of the crude methanol extract of the root of R. tridentata as well as the activity of the isolated pure compounds was evaluated using the isolated uterine smooth muscle strips obtained from stilboestrol-primed Sprague-Dawley rats. The mixture of asiatic acid and arjunolic acid showed a response of approximately 13% in the force of uterine muscle contractility at 1.23 μg/mL while β-sitosterol demonstrated a change of 40% in the force of uterine muscle contractility at a concentration of 57.1 μg/mL. Hence it was concluded that the mixture of asiatic acid and arjunolic acid is the most uteroactive component in the extract of R. tridentata. Morin 3-O-α-Lrhamnopyranoside and trans-resveratrol glucoside caused a relaxation in the contractions of the uterine smooth muscle. Both compounds showed a higher inhibition in the force of contractions when compared to the rate of contractions. These findings confirmed that R. tridentata possesses both oxytocic and tocolytic activities at different dosages. Catechin and quercetrin were cytotoxic to the uterine smooth muscle tissue.