Antioxidant potential of thymol determined by chemiluminescence inhibition in human neutrophils and cell-free systems

Thyme essential oil and thymol have antimicrobial, antifungal and antioxidant activities. Their antioxidant activity has been studied almost exclusively by means of chemical testing in order to be able to use it for food preservation purposes. The aim of this luminol amplified chemiluminescence (LACL) study was to investigate whether thymol can interfere with the production of reactive oxygen species, nitric oxide and the nitric oxide-derived peroxynitrite released by human neutrophils after activation by fMLP and PMA with and without the addition of the L-arginine (L-Arg) nitric oxide donor to the medium. The lowest thymol concentration that was still active in reducing LACL was 2.73 mu g/ml, and there was a progressive linear inhibition of LACL from this concentration to 21.87 mu g/ml, the highest thymol concentration investigated. This was also observed in the case of both fMLP and PMA stimulation with or without L-Arg. In cell-free systems using H2O2/HOCl- and SIN-1 as radical producers, a significant scavenging activity of thymol was present already at 0.08 and 0.68 mu g/ml respectively, and these are very low concentrations. These findings can be related to the phenolic structure of thymol, because phenolic compounds have redox properties and play an important role in adsorbing and neutralizing free radicals and peroxynitrite, and in decomposing peroxides. Our findings in human neutrophils are pharmacologically relevant as they imply that thymol is a potential antioxidant and anti-inflammatory agent in human cells. Copyright (C) 2006 S. Karger AG, Basel.