Luteolin is a naturally occurring flavone that reportedly has anti-inflammatory 
effects. Because most luteolin is conjugated following intestinal absorption, 
free luteolin is likely present at low levels in the body. Therefore, luteolin 
metabolites are presumably responsible for luteolin bioactivity. Here we 
confirmed that luteolin glucuronides, especially luteolin-3'-O-glucuronide, are 
the major metabolites found in plasma after oral administration of luteolin 
(aglycone) or luteolin glucoside (luteolin-7-O-glucoside) to rats. 
Luteolin-4'-O-glucuronide and luteolin-7-O-glucuronide were also detectable 
together with luteolin-3'-O-glucuronide in the liver, kidney, and small 
intestine. Next, we prepared these luteolin glucuronides and compared the 
anti-inflammatory effects of luteolin and luteolin glucuronides on gene 
expression in lipopolysaccharide-treated RAW264.7 cells. Luteolin glucuronides, 
especially luteolin-7-O-glucuronide, reduced expression of inflammatory genes in 
the cells, although their effects were weaker than those of luteolin. These 
results indicate that the active compound responsible for the anti-inflammatory 
effect of luteolin in vivo would be luteolin glucuronide and/or residual 
luteolin.