Introduction ! The natural quinone compounds are produced by a number of plants, fungi, and also by animals. Some of them found application in industry as colorants and in medicine as laxatives. However, natural quinone compounds also exert other var- ious biological (particularly antibacterial, fungici- dal, and cytotoxic) effects including anti-inflam- matory activity [1–3]. As the best examples can be mentioned diacerein and its active metabolite rhein, the anthraquinones with interleukine-1β inhibitory activity used as symptomatic, slow- acting drugs in osteoarthritis [4]. The naphtho- quinones shikonin and alkannin as well as the benzoquinone thymoquinone exhibit in vivo anti-inflammatory activity when administered to rats [5,6]. It has been shown that thymoquinone is able to inhibit catalytic activity of cyclooxygen- ase-1 (COX-1) and -2 (COX-2) in our previous study [7]. However, no systematic study on qui- nones and their derivatives regarding their COX- inhibiting activity and mechanism of action has been reported yet. Cyclooxygenases (COXs, called also prostaglandin H 2 synthases) belong to the oxidoreductase class of enzymes and carry out two distinct biochemi- cal reactions converting arachidonic acid to un- stable intermediate prostaglandin G 2 (PGG 2 ; cy- clooxygenase function) and subsequently convert PGG 2 to prostaglandin H 2 (hydroperoxidase func- tion), a common precursor of all the prostanoids. Prostanoids play many important physiological (mainly mediated by COX-1) as well as pathologi- cal roles (mainly mediated by inducible COX-2). Because prostaglandins produced by COX-2 medi- ate inflammatory reactions in tissues, COXs are well-known targets of nonsteroidal anti-inflam- matory drugs (NSAIDs), which are widely used to treat acute and chronic inflammatory disorders [8]. The aim of this study was to test a panel of natural quinones for their ability to inhibit COX-1 and COX-2 activity in vitro. Further, we assessed the pro-oxidant effect of quinones as one of the possi- ble in vitro mechanisms of COX inhibition. For the quinone-type COX inhibitors with none or a low pro-oxidant effect, we applied an in silico model- ing approach to propose the non-redox mecha- Abstract ! In this study, ten anthra-, nine naphtho-, and five benzoquinone compounds of natural origin and five synthetic naphthoquinones were assessed, using an enzymatic in vitro assay, for their poten- tial to inhibit cyclooxygenase-1 and -2 (COX-1 and COX-2), the key enzymes of the arachidonic acid cascade. IC 50 values comparable with COX reference inhibitor indomethacin were recorded for several quinones (primin, alkannin, diospyrin, juglone, 7-methyljuglone, and shikonin). For some of the compounds, we suggest the redox po- tential of quinones as the mechanism responsible for in vitro COX inhibition because of the quanti- tative correlation with their pro-oxidant effect. Structure-relationship activity studies revealed that the substitutions at positions 2 and 5 play the key roles in the COX inhibitory and pro-oxi- dant actions of naphthoquinones. In contrast, the redox mechanism alone could not explain the ac- tivity of primin, embelin, alkannin, and diospyrin. For these four quinones, molecular modeling sug- gested similar binding modes as for conventional nonsteroidal anti-inflammatory drugs (NSAIDs). Supporting information available online at http://www.thieme-connect.de/ejournals/toc/ plantamedica Redox and Non-Redox Mechanism of In Vitro Cyclooxygenase Inhibition by Natural Quinones Authors Premysl Landa 1 , Zsofia Kutil 1, 2 , Veronika Temml 3 , Anna Vuorinen 3 , Jan Malik 4 , Marcela Dvorakova 1 , Petr Marsik 1 , Ladislav Kokoska 2,4 , Marie Pribylova 1 , Daniela Schuster 3 , Tomas Vanek 1 Affiliations The affiliations are listed at the end of the article Key words l " inflammation l " prostaglandin synthase l " structure‑activity relationship l " cytotoxicity l " free radicals l " pharmacophore modeling received April 4, 2011 revised Nov. 14, 2011 accepted Nov. 20, 2011 Bibliography DOI http://dx.doi.org/ 10.1055/s-0031-1280430 Published online December 15, 2011 Planta Med 2012; 78: 326–333 © Georg Thieme Verlag KG Stuttgart · New York · ISSN 0032‑0943 Correspondence Dr. Premysl Landa Laboratory of Plant Bio- technologies Institute of Experimental Botany AS CR, v. v. i. Rozvojova 263 165 02 Prague 6 – Lysolaje Czech Republic Phone: + 42 0 23 30 22 21 31 Fax: + 42 02 33 02 24 79 landa@ueb.cas.cz 326 Landa P et al. Redox and Non-Redox … Planta Med 2012; 78: 326–333 Original Papers Downloaded by: Wageningen UR. Copyrighted material.