Superoxide-dependent and -independent nitrite formation from hydroxylamine: Inhibition by plant extracts

Reactive oxygen species such as OH·, peroxynitrite and the non-radical, hypochlorous acid, play outstanding roles in many diseases. The formation of OH· (Fenton-)-type radicals is catalyzed by enzymes such as xanthine oxidase (XOD) via one-electron reduction of molecular oxygen producing superoxide radical anions (O₂-). Subsequent transfer of one electron to hydrogen peroxide by Fe²⁺ or Cu⁺- ions yields OH-radicals measurable as ethene release from 1-keto-4 methylthiobutyrate (KMB). Xanthine oxidase or activated neutrophils are prominent sources of this strong oxidant produced at inflammatory sites. Many natural compounds such as salicylates or flavonoids interfere either with the production of these activated oxygen species or function as radical scavengers and thus as antioxidants. Extracts from willow-bark (Salix spec.) and also other species such as ash-tree (Fraxinus spec.) or poplar (Populus spec.) have been used as antiinflammatory drugs since a long time. In this communication we wish to report on model reactions to demonstrate a) the radical scavenging activities of such plant extracts inhibiting ethene release from KMB induced by Fenton-type oxidants and b) the inhibition of the formation of nitrogen monoxide (NO) from hydroxylamine including XOD either in the presence or absence of myoglobin (MYO) measurable as nitrite formation: In the absence of MYO, superoxide dismutase is an excellent inhibitor of nitrite formation but is inactive in its presence. Extracts from the willow-bark or the drug Phytodolor(R), however, are inhibitory both in the presence and absence of MYO. As active principle, the flavonoid rutin included in these extracts is likely to function as one inhibitor of the XOD-mediated reaction.