Concentration-dependent antioxidant activity of probucol in low density lipoproteins in vitro: probucol degradation precedes lipoprotein oxidation.

The ability of probucol, a lipid-lowering drug with antioxidant properties, to prevent the Cu2'-induced oxidation of human plasma low density lipoproteins (LDL) was examined as a function of the concentration of probucol in LDL. In the absence of probucol, 3 p~ Cu*' induced half-maximal LDL lipid oxidation, as determined by the formation of thiobarbi- turic acid reactive substances (TBARS). Oxidation was asso- ciated with a loss of apolipoprotein B-100 and the appearance of higher molecular weight forms of the protein. In the presence of 0.6 mol% probucol (relative to phospholipid) and with 3 pM Cu", the time required to obtain half-maximal LDL lipid oxi- dation increased from 130 to 270 min and was explained by an increase in the lag time prior to LDL lipid oxidation. Once rapid oxidation of LDL had begun, the rate of TBARS formation was similar to that for LDL containing no probucol. At a probucol concentration of 4.2 mol %, the antioxidant prevented the oxida- tion of LDL-lipids. The delay in Cu2+-induced LDL oxidation with probucol corresponded to the time required for free radical- mediated processes to convert probucol to a spiroquinone and a diphenoquinone. These in vitro findings suggest that the potent antioxidant property of probucol is directly related to the amount of drug in the LDL particle and may have relevance to its antiatherosclerotic effects observed in vivo. - Barnhart, R. L., S. J. Busch, and R. L. Jackson. Concentration-depen- dent antioxidant activity of probucol in low density lipoproteins in vitro: probucol degradation precedes lipoprotein oxidation. J. Lipid Res. 1989. 30 1703-1710. not known with certainty. It is known that modification of LDL by lipid oxidation products (3-5), mono- cytes/macrophages, neutrophils (6-8), endothelial cells (9, 10-14), and smooth muscle cells (9, 14-16) enhances LDL uptake by macrophages and renders LDL cytotoxic to cells in tissue culture. The mechanisms of free radical- mediated modification of LDL and the pathological con- sequences as it relates to human diseases have recently been reviewed (17-20). In vivo, biologically modified LDL presumably results from free-radical formation in either the plasma or suben- dothelial space followed by nucleophilic attack of the radical on LDL lipid and protein. The antioxidant probu- col, (4,4'-(isopro~lidenedithio)bis(2,6-di-tert-bu~lphenol)), a drug used in the treatment of hypercholesterolemia, has recently been shown to inhibit oxidative modification of LDL (21, 22). Furthermore, probucol reduces atheroscler- osis in the Watanabe heritable hyperlipidemic rabbit (23-25) and the cholesterol-fed rabbit (26). In the present study, we provide in vitro evidence that the concentration of probucol in LDL determines the extent of oxidative modification of the lipoprotein.