LAUSR

Abstract Since ancient times, essential oils from medicinal plants have supplied a huge repository of secondary metabolites. Genus Callistemon, Myrtaceae, was described to have nematicidal, antithrombotic, antistaphylococcal, antioxidant, and insecticidal activities. In this study we investigate the chemical composition of essential oils from the leaves and the flowers of C. macropunctatus (Dum.Cours.) Court and Callistemon subulatus (Cheel) from two geographical locations: Australia and Egypt. GC MS analysis identified a total of sixty-one volatile components and showed that the major constituents were 1,8-cineole, α-pinene, α-terpineol, limonene and α-phellandrene in the leaves and α-pinene, 1,8-cineole, α-phellandrene, limonene and α-terpineol in the flowers of Callistemon species. To discriminate the oil complexity within factors as location, plant parts and species, principal component analysis (PCA) and hierarchical cluster analysis (HCA) were employed. Furthermore, the cytotoxic effect of the oil samples was assayed against the human liver cancer cell line HepG2 to reveal a potent anticancer potential with half-maximal inhibitory concentration (IC50) values ranging from 1.76 μg/mL to 13.41 μg/mL. To gain insight into the cellular mechanism of the cytotoxic activity, Callistemon volatile oils’ activities on topoisomerases I-β and II-β were explored, which demonstrated significant enzyme inhibition with IC50 values as low as 819.6 ng/mL. Molecular in-silico docking study of the major oil components within the active binding site of the human topoisomerases I-β and II-β was conducted. α-terpineol showed promising alkyl interactions in the binding pockets of topoisomerase I B and II B, with a docking Gibb’s free energy values of -27.0913 and -26.6478 kcal/mol, respectively. Whereas it bounded to Arg503 and four nucleotide regions deoxyadenosine DA12, deoxythymidine DT9, deoxyguanosine DG13, deoxycytidine DC8 in the topoisomerase II-β enzyme pocket, it interacted with Topo I-β enzyme through hydrophobic alkyl and Pi-alkyl binding interactions with DA12, DG13 and Arg503. Furthermore, the oxygen in the hydroxyl group showed two additional non-covalent bonds: a conventional hydrogen bond with Asp533 and water hydrogen bond with a water molecule in the vicinity of the binding surface. This research highlights the future possibility of having Callistemon subulatus as a potential natural drug for treatment of liver cancer.