Lobatamunsolides A–C, Norlignans from the Roots of Pueraria lobata and their Nitric Oxide Inhibitory Activities in Macrophages

Phytochemical investigation of the methanol (MeOH) extract of Pueraria lobata roots, known as “kudzu”, combined with liquid chromatography/mass spectrometry (LC/MS)-based analysis, resulted in the identification of four norlignans (1–4), including three new norlignans, lobatamunsolides A–C (1–3), and five known isoflavonoids (5–9). The structures of the new compounds were elucidated by a combination of spectroscopic methods, including 1D and 2D nuclear magnetic resonance (NMR) and high resolution (HR)-electrospray ionization mass spectrometry (ESIMS), and their absolute configurations were determined by chemical reaction and quantum chemical electronic circular dichroism (ECD) calculations. The isolated compounds (1–9) were evaluated for their inhibitory effects on nitric oxide (NO) production in lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages. Compound 9 displayed the strongest NO inhibitory effect and compound 2 showed a weak effect. The potential mechanism of the effect of compound 9 was investigated by analysis of its molecular docking on the active site of inducible nitric oxide synthase (iNOS), which showed the potential interactions of compound 9 with key amino acid residues and the heme cofactor of iNOS. The mechanism as the inhibition of transcriptional iNOS protein expression was confirmed by western blotting experiments.