Sterols and triterpenes from the non-polar antitubercular fraction of Abutilon indicum

Introduction:Abutilon indicum Sweet (Malvaceae) is used in Filipino folk medicine as demulcent, diuretic, sedative, aphrodisiac and antidiabetic remedy. It is also effective in the treatment of leprosy. It is also used as a laxative for patients having hemorrhoids and in the treatment of coughs, puerperal disease, urinary disorders, chronic dysentery and fever. Methods: Air-dried powdered leaves of A. indicum were initially extracted with DCM-methanol (1:1) to afford a crude extract which was fractionated on a silica gel column to afford four fractions. The first fraction which showed antimycobacterial activity was further chromatographed in a silica gel column to afford bioactive fractions from which four compounds were obtained. The crude extract, fractions and isolated compounds were screened for their inhibitory effect against Mycobacterium tuberculosis H₃₇Rv (ATCC) using the colorimetric Microplate Alamar Blue Assay (MABA) method. Results: The fractions obtained after silica gel chromatography of the crude DCMmethanol (1:1) revealed the first fraction (MIC=64 mg/mL) to exert the highest inhibition against M. tuberculosis H₃₇Rv. Further separation of this fraction afforded sub-fractions with moderately strong inhibitory activity against the test organism (MIC up to 64 mg/mL). Chromatographic purification of sub-fraction 1 afforded four compounds which were spectroscopically identified as β-amyrin 3-palmitate (1), squalene (2) and a 1:1 mixture of the sterols β-sitosterol (3) and stigmasterol (4). Evaluation of the antimycobacterial activity of 1–4 showed insignificant inhibitory activity against the test organism (MIC = >128 mg/mL). Conclusion: The present results demonstrate the potential of Abutilon indicum as a plant source of compounds that may exhibit promising antituberculosis activity. While the known compounds isolated for this plant did not show antimycobacterial activity, the obtained results are considered sufficient reason for further study to isolate the metabolites from A. indicum responsible for the antitubercular activity.

Key words:Abutilon indicum, Malvaceae, antitubercular, squalene, β-amyrin 3-palmitate, β-sitosterol, stigmasterol.