3-hydroxyanthranilic acid is an efficient, cell-derived co-antioxidant for alpha-tocopherol, inhibiting human low density lipoprotein and plasma lipid peroxidation

alpha-Tocopherol (alpha-TSH) can promote lipid peroxidation in human low density lipoprotein (LDL) unless co-antioxidants are present that eliminate the chain-carrying alpha-tocopheroxyl radical (alpha-TO.) (Bowry, V. W., Mohr, D., Cleary, J., and Stocker, R. (1995) J. Biol. Chem. 270, 5756-5763). Interferon-gamma inhibits human monocyte/macrophage-facilitated LDL lipid peroxidation via induction of cellular tryptophan degradation and production and release of 3-hydroxyanthranilic acid (3HAA) (Christen, S., Thomas, S.R., Garner, B., and Stocker, R. (1994) J. Clin. Invest. 93, 2149-2158). We now report on the mechanism of antioxidant action of 3HAA. 3HAA directly reduced alpha-TO. in UV-exposed micellar dispersions of alpha-TOH or in LDL incubated with soybean 15-lipoxygenase (SLO), as assessed by electron paramagnetic resonance spectroscopy. 3HAA did not inhibit SLO enzyme activity. Anthranilic acid, which lacks the phenoxyl group, was incapable of reducing alpha-TO.. 3HAA dose-dependently inhibited the peroxidation of surface phospholipids and core cholesteryl esters in LDL exposed to SLO, peroxyl radicals (ROO(.)), or Cu2+; oxidants that convert alpha-TOH to alpha-TO.. In all cases, sparing of LDL's alpha-TOH, but not ubiquinol-10 (CoQ(10)H(2)), was observed until the majority of 3HAA was consumed. Addition of 3HAA or ascorbate prevented further consumption of alpha-TOH and accumulation of lipid hydroperoxides when added to aqueous or lipophilic ROO(.)-oxidizing LDL after complete and partial consumption of CoQ(10)H(2) and alpha-TOH, respectively. In contrast, addition of urate, an efficient ROO(.) scavenger incapable of scavenging alpha-TO., did not efficiently inhibit ongoing lipid peroxidation. Oxidation of 3HAA-supplemented human plasma by aqueous ROO(.) resulted in the successive consumption of ascorbate, CoQ(10)H(2), 3HAA, bilirubin, alpha-TOH, alpha-TOH, and urate. Lipid peroxidation was prevented as long as ascorbate, CoQ(10)H(2), and 3HAA were present, but subsequently proceeded as a free-radical chain reaction concomitant with alpha-TOH, bilirubin, and urate immediately and as efficiently as did ascorbate. These findings demonstrate that 3HAA is a highly efficient co-antioxidant for plasma lipid peroxidation by virtue of its ability to interact with alpha-TO. in lipoproteins. Since interferon-gamma is the principal inducer of tryptophan degradation and release of 3HAA by monocytes/macrophages, this may represent a localized extracellular antioxidant defense against LDL oxidation in inflammation.