Phytochemical and Biological Studies on Ravenia spectabilis and Erythrina variegata

Abstract

The methanolic extract of the leaves of Ravenia spectabilis Lindl. (Family: Rutaceae) and stem bark of Erythrina variegata L. (Family: Fabaceae) were investigated for the isolation of secondary metabolites and evaluation of bioactivities. A total of twenty one compounds were isolated from these two plants, among them six appeared to be new. The structures of the compounds were elucidated mainly by spectroscopic studies including 1H NMR, 13C NMR, HSQC, HMBC, 1H-1H COSY and NOSEY experiments and the molecular weights were determined by ESI mass spectrometry. Among the new compounds, four were alkaloids and two were very unusual C34 terpenoids. These are 3,5-diprenylindole, 3-prenyl-5-(2-keto-but-3-enyl)indole, 3-prenyl-indole-5-carbaldehyde, iso-oligophyline, ravespanol and ravespanone, all of which were isolated from Ravenia spectabilis. The known compounds isolated from this plant include ravenoline, γ-fagarine, arborinine, atanine, oligophyline, ravenine, methyl linoleate and β-sitosterol. Phytochemical investigation of Erythrina variegata afforded seven known compounds namely scandenone, alpinumisoflavone, lupeol, stigmast-4-en-3-one stigmasta-4,22-dien-3-one, stigmasterol and 3β,28-dihydroxyolean-12-ene. Different fractions of the crude methanolic extract of the investigated plants and some pure compounds, isolated in this study, were screened for their cytotoxic, antimicrobial, thrombolytic and antioxidant activities by standard methods. The new compounds 3,5-diprenylindole, 3-prenyl-5-(2-keto-but-3-enyl)indole and 3-prenyl-indole-5-carbaldehyde were investigated for cytotoxicity using the MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide] colorimetric assay method. Among the three compounds, 3,5-diprenylindole was found to be most cytotoxic to human pancreatic adenocarcinoma cell lines with IC50 value of 9.5 ± 2.2 μM, moderately cytotoxic to human cervical and lung cancer cell lines with IC50 values of 11.3 ± 1.3 μM and 13.5 ± 1.66 μM respectively and weakly cytotoxic to non-tumour cell line (WI-38 ) with IC50 value of 68.5 ± 3.5 μM as compared to the standard (0.19 ± 0.12 to 6.3 ± 0.3 μM). The rest two compounds showed very poor cytotoxicity (IC50 >50 μM) against the four cell lines tested. In vitro antimicrobial activity was measured by disc diffusion method against ten gram positive and gram negative bacterial strains using kanamycin as the standard. Among the samples tested, the pet ether fraction of Ravenia spectabilis and the carbontetrachloride fraction of Erythrina variegata demonstrated the highest antimicrobial activity against Bacillus subtilis and Bacillus cereus respectively with zone of inhibition of 20.5 ± 0.74 mm and 19.5 ± 1.18 mm as compared to the standard (34.0 ± 0.50 mm and 24.30 ± 0.44 mm). Ravenoline isolated from R. spectabilis showed moderate inhibition against Vibrio cholerae (17.2 ± 0.41 mm). Mild to moderate thrombolytic activities were observed by arborinine and different fractions of the crude extract with clot lysis ranging from 30.43 ± 1.03 to 57.78 ± 0.24 % as compared to the standard streptokinase with clot lysis of 74.34 ± 0.73 % for Ravenia extract and 76.54 ± 0.90% for Erythrina extract. The antioxidant activity was evaluated by DPPH radical scavenging method using butylated hydroxytoluene as the standard. Among the crude extract tested, the chloroform and aqueous extract of E. variegata exhibited moderate antioxidant activities with IC50 values of 67.59 ± 1.87 μg/ml and 75.02 ± 2.62 μg/ml respectively as compared to the standard 23.09 ± 1.37 μg/ml. The pure compounds arborinine and ravenoline showed very poor antioxidant activity.