Structural Requirements of Flavonoids for Nitric Oxide Production Inhibitory Activity and Mechanism of Action Original Research Article

To clarify the structure–activity relationships of flavonoids for nitric oxide (NO) production inhibitory activity, we examined the inhibitory effects of 73 flavonoids on NO production in lipopolysaccharide-activated mouse peritoneal macrophages. Among those flavonoids, apigenin (IC50=7.7 μM), diosmetin (8.9 μM), and tetra-O-methylluteolin (2.4 μM), and hexa-O-methylmyricetin (7.4 μM) were found to show potent inhibitory activity, and the results suggested the following structural requirements of flavonoids: (1) the activities of flavones were stronger than those of corresponding flavonols; (2) the glycoside moiety reduced the activity; (3) the activities of flavones were stronger than those of corresponding flavanones; (4) the flavones and flavonols having the 4′-hydroxyl group showed stronger activities than those lacking the hydroxyl group at the B ring and having the 3′,4′-dihydroxyl group; (5) the flavonols having the 3′,4′-dihydroxyl group (catechol type) showed stronger activities than those having the 3′,4′,5′-trihydroxyl group (pyrogallol type); (6) the 5-hydroxyl group tended to enhance the activity; (7) methylation of the 3-, 5-, or 4′-hydroxyl group enhanced the activity; (8) the activities of isoflavones were weaker than those of corresponding flavones; (9) methylation of the 3-hydroxyl group reduced the cytotoxicity. In addition, potent NO production inhibitors were found to inhibit induction of inducible nitric oxide synthase (iNOS) without iNOS enzymatic inhibitory activity.