beta-carotene and dihydrolipoic-acid

Oxidative modification of low density lipoproteins (LDL) and their unrestricted scavenger receptor-dependent uptake is believed to account for cholesterol deposition in macrophage-derived foam cells. It has been suggested that vitamin E that is transported by LDL plays a critical role in protecting against LDL oxidation. We hypothesize that the maintenance of sufficiently high vitamin E concentrations in LDL can be achieved by reducing its chromanoxyl radicals, i.e., by vitamin E recycling. In this study we demonstrate that: i) chromanoxyl radicals of endogenous vitamin E and of exogenously added alpha-tocotrienol, alpha-tocopherol or its synthetic homologue with a 6-carbon side-chain, chromanol-alpha-C6, can be directly generated in human LDL by ultraviolet (UV) light, or by interaction with peroxyl radicals produced either by an enzymic oxidation system (lipoxygenase + linolenic acid) or by an azo-initiator, 2,2'-azo-bis(2,4-dimethylvaleronitrile) (AMVN; ii) ascorbate can recycle endogenous vitamin E and exogenously added chromanols by direct reduction of chromanoxyl radicals in LDL; iii) dihydrolipoic acid is not efficient in direct reduction of chromanoxyl radicals but recycles vitamin E by synergistically interacting with ascorbate (reduces dehydroascorbate thus maintaining the steady-state concentration of ascorbate); and iv) beta-carotene is not active in vitamin E recycling but may itself be protected against oxidative destruction by the reductants of chromanoxyl radicals. We suggest that the recycling of vitamin E and other phenolic antioxidants by plasma reductants may be an important mechanism for the enhanced antioxidant protection of LDL.

Topics: Ascorbic Acid; beta Carotene; Carotenoids; Chromans; Drug Synergism; Electron Spin Resonance Spectroscopy; Free Radical Scavengers; Humans; Linolenic Acids; Lipid Peroxidation; Lipoproteins, LDL; Lipoxygenase; Thioctic Acid; Ultraviolet Rays; Vitamin E

Vitamin E (alpha-tocopherol) is the major lipid-soluble chain-breaking antioxidant of membranes. Its UV-absorbance spectrum (lambda max 295 nm) extends well into the solar spectrum. We hypothesize that in skin alpha-tocopherol may absorb solar UV light and generate tocopheroxyl radicals. Reduction of tocopheroxyl radicals by other antioxidants (e.g. ascorbate, thiols) will regenerate (recycle) vitamin E at the expense of their own depletion. Hence, vitamin E in skin may act in two conflicting manners upon solar illumination: in addition to its antioxidant function as a peroxyl radical scavenger, it may act as an endogenous photosensitizer, enhancing light-induced oxidative damage. To test this hypothesis, we have illuminated various systems (methanol-buffer dispersions, liposomes and skin homogenates) containing alpha-tocopherol or its homologue with a shorter 6-carbon side chain, chromanol-alpha-C6 with UV light closely matching solar UV light, in the presence or absence of endogenous or exogenous reductants. We found that: (i) alpha-tocopheroxyl (chromanoxyl) radicals are directly generated by solar UV light in model systems (methanol-water dispersions, liposomes) and in skin homogenates; (ii) reducing antioxidants (ascorbate, ascorbate+dihydrolipoic acid) can donate electrons to alpha-tocopheroxyl (chromanoxyl) radicals providing for vitamin E (chromanol-alpha-C6) recycling; (iii) recycling of UV-induced alpha-tocopheroxyl radicals depletes endogenous antioxidant pools (accelerates ascorbate oxidation); (iv) beta-carotene, a non-reducing antioxidant, is not active in alpha-tocopherol recycling, and its UV-dependent depletion is unaffected by vitamin E.

Topics: Animals; Antioxidants; Ascorbic Acid; beta Carotene; Carotenoids; Electron Spin Resonance Spectroscopy; Free Radicals; Liposomes; Methanol; Mice; Mice, Hairless; Models, Biological; Neoplasms, Radiation-Induced; Oxygen; Phosphatidylcholines; Radiation Tolerance; Skin; Skin Neoplasms; Suspensions; Thioctic Acid; Ultraviolet Rays; Vitamin E; Water

Reactive free radicals contained in cigarette smoke (CS) and compromised phagocytic antimicrobial activities including those of polymorphonuclear leukocytes (PMNs) have been implicated in the pathogenesis of severe CS-related pulmonary disorders. In CS-exposed buffer solutions, O2-. was the predominant generated reactive oxygen species, as demonstrated by lucigenin-amplified chemiluminescence and electron spin resonance (ESR) spin-trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO). When PMNs were incubated in this buffer, phorbol 12-myristate 13-acetate (PMA)-stimulated active oxygen production and coupled O2 consumption were strongly impaired without appreciably affecting PMN viability (1-min exposure inhibited active oxygen production by 75%). Superoxide dismutase (SOD) totally protected and an iron chelator, diethylenetriaminepentaacetic acid (DETAPAC), also protected the CS-exposed PMNs, suggesting that generated O2-. was an initiating factor in the impairment and OH. generation was a subsequent injurious factor. Pretreatment of PMNs with antioxidants such as alpha-tocopherol and dihydrolipoic acid (DHLA) was partially protective. The results suggest that (i) O2-. is probably generated in the upper and lower respiratory tract lining fluid when they come in contact with CS; (ii) such generated O2-. can primarily impair PMN capabilities to generate reactive oxygen species; and (iii) since these effects may contribute to the pathogenesis of CS-related lung diseases, prior supplementation with antioxidants such as alpha-tocopherol or DHLA might be successful in preventing these deleterious effects.

Topics: Animals; Antioxidants; beta Carotene; Carotenoids; Cyclic N-Oxides; Electron Spin Resonance Spectroscopy; Kinetics; Luminescent Measurements; Male; Neutrophils; Nicotiana; Plants, Toxic; Rats; Rats, Wistar; Smoke; Spin Labels; Superoxides; Tetradecanoylphorbol Acetate; Thioctic Acid; Vitamin E