Two new compounds, kinanthraquinone C (1) and kinanthraquinone
D (2), were isolated along with two known compounds,
kinanthraquinone (3) and kinanthraquinone B (4), produced by the heterologous expression of the kiq biosynthetic gene cluster and its pathway-specific regulator, kiqA, in Streptomyces lividans TK23. The chemical structures of compounds 1 and 2 were determined using mass spectrometry and nuclear magnetic
resonance analyses. To examine a biosynthetic pathway of compounds 1 and 2, incubation experiments were conducted
using S. lividans TK23 to supply the
compounds 3 and 4. These experiments indicated
that compounds 3 and 4 were converted to
compounds 2 and 1, respectively, by the
endogenous enzymes of S. lividans TK23.
Compounds 2, 3, and 4 had antimalarial
activities at half-maximal inhibitory concentration values of 0.91,
1.2, and 15 μM, respectively, without cytotoxicity up to 30
μM.