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

Abstract

α-Tocopherol (α-TOH) can promote lipid peroxidation in human low density lipoprotein (LDL) unless co-anti-oxidants are present that eliminate the chain-carrying α-tocopheroxyl radical (α-TO·) (Bowry, V. W., Mohr, D., Cleary, J., and Stocker, R. (1995) J. Biol. Chem. 270, 5750-5763). Interferon-γ 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 α-TO· in UV-exposed micellar dispersions of α-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 a-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 α-TOH to α-TO·. In all cases, sparing of LDL's α-TOH, but not ubiquinol-10 (CoQ10H2), was observed until the majority of 3HAA was consumed. Addition of 3HAA or ascorbate prevented further consumption of α-TOH and accumulation of lipid hydroperoxides when added to aqueous or lipophilic ROO·-oxidizing LDL after complete and partial consumption of CoQ10H2 and α-TOH, respectively. In contrast, addition of urate, an efficient ROO· scavenger incapable of scavenging α-TO·, did not efficiently inhibit ongoing lipid peroxidation. Oxidation of 3HAA- supplemented human plasma by aqueous ROO· resulted in the successive consumption of ascorbate, CoQ10H2, 3HAA, bilirubin, α-TOH, and urate. Lipid peroxidation was prevented as long as ascorbate, CoQ10H2, and 3HAA were present, but subsequently proceeded as a free-radical chain reaction concomitant with α-TOH, bilirubin, and urate consumption. Addition of 3HAA to aqueous ROO·-oxidizing plasma, after complete consumption of ascorbate and CoQ10H2, strongly inhibited ongoing lipid peroxidation and consumption of α-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 α-TO· in lipoproteins. Since interferon-γ, 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.