Flavonoid impairment of neutrophil response

Flavonoids are a class of phenolic plant pigments which impair the oxidative burst of neutrophils to an extent dependent on their hydrophobicity. The distribution of quercetin and of morin in nitrogen-cavitated neutrophils paralleled their respective hydrophobic characteristics and respiratory burst inhibition. While both flavonoids were localized primarily in the specific granule membrane of neutrophils, the amount of quercetin was considerably greater than that of morin. We here demonstrate inhibition of the initial stimulation response, depolarization of the membrane potential as monitored by fluorescence of the membrane probe diS-C3-(5), and of the respiratory burst, monitored by following the destruction of diS-C3-(5), a reaction mediated by the H2O2 produced in the burst. The flavonoids kaempferol, morin, quercetin, or fisetin were preincubated with human neutrophils at a concentration of 100 microM per 2 X 10(6) cells/ml for 2-3 min and subsequently stimulated with 1 microgram/ml of the tumor promoter phorbol myristate acetate (PMA) or with 60 micrograms/ml of immune complex. The effect of each compound differed, i.e. depolarization was enhanced by some and inhibited by others, while H2O2 generation was inhibited by each, supporting our previous findings that membrane potential depolarization and the respiratory burst are dissociable events. Concentration-response experiments, performed at flavonoid concentrations between 12.5 and 500 microM to determine the IC50 values of these compounds for depolarization and burst activation, indicated that none of the flavonoids affected the resting potential, while all perturbed the stimulus-coupled response, the direction and extent of the perturbation depending upon the stimulus, and the function assessed. These data show that the effects of flavonoids on human neutrophils are complex and suggest several sites of action depending upon the flavonoid's subcellular distribution and pathway of stimulation.