Isolation and characterization of 5-O-desmethyl kinanthraquinone B generated through O-methyltransferase gene disruption

5-O-desmethyl kinanthraquinone B, a new kinanthraquinone analogue, was isolated from a kiqT1 disruptant. Its structure was elucidated by high-resolution electrospray ionisation mass spectrometry and nuclear magnetic resonance spectrometry and subsequently compared with the spectroscopic data of kinanthraquinone B. The newly isolated analogue exhibited antibacterial activity against Staphylococcus aureus, antimalarial activity against Plasmodium falciparum, and cytotoxicity toward HL-60 cells, with IC₅₀ values of 3.7, 5.5 and 11 μg/mL, respectively. Comparison of the biological activities of 5-O-desmethyl kinanthraquinone B and kinanthraquinone B indicated that methylation of the C-5 hydroxyl group reduced antibacterial activity and cytotoxicity with little effect on antimalarial activity.