We studied in vitro the ability of nitecapone, 3-[(3,4-dihydroxy-5-nitrophenyl)methylene]-2,4-pentanedione, a novel water-soluble compound with antioxidative properties, to inhibit the LDL oxidation promoted by copper ions, the aqueous free radical generator 2,2'-azobis(2-amidinopropane) hydrochloride (AAPH), and mouse peritoneal macrophages. In these three oxidation systems, the extent of LDL oxidation was determined by measuring the formation of conjugated dienes, the formation of thiobarbituric acid-reactive substances, the change in the electrophoretic mobility of LDL, and the uptake of LDL by macrophages. When LDL oxidation was promoted by copper ions, the reaction was found to be inhibited by nitecapone added in a three- to five-molar excess of the concentration of copper ions. The mechanism by which nitecapone exerted its antioxidative effect in copper-mediated LDL oxidation depended on binding and redox inactivation of the copper ions. Moreover, nitecapone released LDL-bound copper ions and so rendered the LDL particles more resistant to oxidation. In contrast to a water-soluble alpha-tocopherol analogue that was rapidly consumed during the oxidative process, nitecapone retained its inhibitory effect for at least 2 days. Using immobilized metal ion affinity chromatography, we showed that nitecapone binds both copper and iron ions, whereas its affinity for zinc ions is low. Nitecapone also inhibited LDL oxidation in the free radical-mediated oxidation system (AAPH). In this system, nitecapone showed synergistic antioxidative action with ascorbic acid. Finally, nitecapone inhibited macrophage-mediated LDL oxidation. Accordingly, nitecapone appears to have a unique antioxidative profile in that it both selectively chelates pro-oxidative transition metals and scavenges free radicals.(ABSTRACT TRUNCATED AT 250 WORDS)