Inhibition of chemiluminescence in human PMNs by monocyclic phenolic acids and flavonoids

Flavonoids are metabolized in vivo to monocyclic phenolic acids. We investigated whether 18 phenolic acids of the benzoic, phenylacetic, phenylpropanoic or cinnamic series-known or potential metabolites of flavonoids-inhibit reactive oxygen species (ROS) released by human polymorphonuclear neutrophils (PMNs). Chemiluminescence was measured after PMN stimulation with three agents (N-fMetLeuPhe, phorbol myristate acetate (PMA), or opsonised zymosan) using two probes (lucigenin or luminol) with or without horseradish peroxidase (HRP) in order to derive specificity profiles for each test compound. The profiles of the phenolic acids and flavonoids were compared by a multivariate (correspondence) factor analysis. Overall, the phenolic acids were less specific than the flavonoids and, with a few exceptions, less potent. Phenolic acids had virtually no effect on the chemiluminescence related to O2- formation that is measured by lucigenin but inhibited luminol luminescence. Inhibition for all but two phenolic acids was sensitive to HRP and might be explained by a scavenger mechanism. Few structure-activity relationships emerged suggesting that simple properties such as radical scavenging and/or redox activity rather than overall structure might be the key determinants of chemiluminescence inhibition. Whatever the mechanism, however, we conclude that part of the in vivo pharmacological activity of flavonoids may readily be accounted for by phenolic acids.