Fisetin is a flavonoid present in strawberries; quercetin is abun-
dant in onions and broccoli, and luteolin can be found in celery,
green pepper, parsley and chamomile tea (Shimoi et al., 1998). In
this study, we observed that fisetin is a flavonoid that possesses
a high potency in arginase inhibition. Fisetin was the most potent
arginase inhibitor, with four and ten times higher potency than
quercetin and luteolin, respectively. Comparing the structures of
these flavonoids revealed that the hydroxyl group at position 3
contributed significantly to the inhibitory activity of arginase,
while the hydroxyl at position 5 did not.