Antioxidant and prooxidant activity of a-tocopherol in human plasma and low density lipoprotein

a-Tocopherol is a classical lipophilic antioxidant well known as a scavenger of free radicals in a hydrophobic milieu. However, it can develop both antiand prooxidant activity in isolated low density lipoprotein (LDL). It is unknown how these activities are balanced in vivo in human plasma. We studied oxidation of plasma and LDL isolated from healthy donors or from a patient with familial isolated vitamin E deficiency and supplemented with a-tocopherol in vivo or in vitro. We found that a-tocopherol supplementation decreased plasma and LDL oxidizability under strong oxidative conditions when oxidation was initiated by high amounts of C U ~ + or 2,2’-azobis-(2-amidinopropane) hydrochloride (AAPH). The effect was independent of the presence of ascorbate in the samples. Under conditions of mild oxidation by low amounts of Cu2+ or AAPH, a-tocopherol supplementation decreased plasma oxidizability only in the presence of physiological amounts of ascorbate. A prooxidant effect of a-tocopherol was found under mild oxidative conditions in highly diluted (150-fold) plasma and in isolated LDL.I These results indicate that the level of oxidative stress and concentration of co-antioxidants, such as ascorbate, capable of regenerating a-tocopherol in the oxidizing lipoprotein particle, appear to represent major factors determining a-tocopherol activity towards oxidation both in human plasma and LDL. In vivo, in the presence of high concentrations of co-antioxidants and under mild oxidative conditions, a-tocopherol should normally behave as an antioxidant. This antioxidant activity is also expected to prevail under strong oxidative conditions independently of the presence of co-antioxidants but it may evolve into prooxidant, when the co-antioxidants are exhausted under conditions of mild oxidation. It remains to be shown whether such a transformation is physiologically relevant and can occur in vivo.-Kontush, A., B. Finckh, B. Karten, A. Kohlschiitter, and U. Beisiegel. Antioxidant and prooxidant activity of a-tocopherol in human plasma and low density lipoprotein.]. Lipid Res. 1996. 37: 1436-1448. Supplementary key words dation blood plasma low density lipoprotein atherosclerosis a-tocopherol ascorbate lipid peroxiLDL undergoes oxidation in vivo is not yet fully understood but there is little doubt that it involves free radical oxidation of LDL lipids (3,4). Preventing the oxidation of LDL using antioxidants is widely discussed at present as a promising antiatherosclerotic therapy (2, 5). This implies that LDL-associated lipid-soluble antioxidants able to efficiently protect the lipoprotein against oxidation may represent important antiatherosclerotic agents. a-Tocopherol is a classical lipid-soluble antioxidant well known as a scavenger of free radicals in a hydrophobic milieu (6). It is well established that a-tocopherol is, on a molar base, the major antioxidant in LDL (3,4). All other antioxidants (y-tocopherol, carotenoids, ubiquinol-10) are present in LDL in much smaller amounts. Inhibiting LDL oxidation by increasing its a-tocopherol content would be a very attractive way to prevent atherogenesis. Data on the antioxidant function of a-tocopherol in lipoproteins are, however, contradictory. The rate of LDL oxidation is generally expected to be low in the presence and high in the absence of a-tocopherol (lag-phase and propagation phase of the oxidation, respectively) (3). However, LDL oxidizability by Cu2+ has only been found to correlate negatively with LDL a-tocopherol when the lipoprotein is enriched with this antioxidant (7-10) and when conditions of a relatively strong oxidative stress are used (11). No significant correlation between a-tocopherol content and susceptibility to oxidation in native LDL unsupplemented with antioxidants has been found under such oxidative conditions (7, 8, 12-18). When mild oxidative conditions There is increasing evidence that suggests that the oxidation of low density lipoprotein (LDL) in the arterial wall is an important step in the early development of atherosclerosis (1,2). The exact mechanism by which Abbreviations: AAPH, 2,2’-azobis-(2-amidopropane) hydrochloride; DMSO, dimethylsulfoxide; EDTA, ethylenediaminetetraacetic acid FIVE, familial isolated vitamin E deficiency; HPLC, high performance liquid chromatography; LDL, low density lipoprotein; PBS, phosphate-buffered saline; PUFA, polyunsaturated fatty acid. ’To whom correspondence should be addressed. 1436 Journal of Lipid Research Volume 37, 1996 by gest, on N ovem er 6, 2017 w w w .j.org D ow nladed fom TABLE 1. Antioxidant and PUFA content of plasma samples obtained from donors supplemented with vitamin E Plasma Concentration Donor, Sample a-Tocopherol Ubiquinol-10 PUFAs