Biological Activities of Extracts and Isolated Calotropin from Gomphocarpus purpurascens Leaves: A Computational and Experimental Study

Gomphocarpus purpurascens A.Richs. (sub-family: Asclepiadaceae, family: Apocynaceae) is an endemic plant in Ethiopia and Eritrea. The chromatographic fractionation of the chloroform:methanol (1:1) leaf extract resulted in the isolation of a cardenolide-type compound named calotropin. Its structure was confirmed based on the 1D-NMR (1H, 13C, and DEPT-135) spectral data along with reported work. This is the first report on the isolation of calotropin from the leaves of G. purpurascens. The n-hexane extract displayed better antibacterial activity against E. coli and P. aeruginosa (8.1 ± 0.0 to 10.8 ± 0.1 mm) up to 12,500 μg/mL concentration. Generally, all the extracts and the isolated compound, calotropin, showed better antibacterial activity against the P. aeruginosa strain than chloramphenicol (7.1 ± 0.2 to 7.3 ± 0.5 mm). The tested chloroform: methanol (1:1) extract indicated a weak antifungal activity against C. albicans compared to ketoconazole 10 µg/disc (17.67 ± 2.52 mm). On the contrary, all the extracts were found to have a good DPPH radical scavenging activity (IC50 values, 1.1-2.7 µg/mL) comparable to ascorbic acid (1.0 µg/mL). The isolated calotropin exhibited a stronger binding capacity (-10.3 kcal/mol) against the P. aeruginosa PqsA (5OE3) protein model than chloramphenicol (-7.0 kcal/mol). This compound was also found to violate Lipinski’s rules of five (with a molecular weight > 500 g/mol) and showed immunotoxicity and cytotoxicity properties. Given that this is a preliminary report, further biochemical investigations would be needed on this G. purpurascens plant to identify additional phytoconstituents with superior efficacy.