Biomedical subjects

D C Liebler

Publications and source records attributed to D C Liebler.

62 records · Page 4Linked to original sources

PubMedDate not supplied

Peroxyl radical trapping and autoxidation reactions of alpha-tocopherol in lipid bilayers.

A phospholipid liposome system was employed to model peroxyl radical trapping reactions of alpha-tocopherol (1) in biological membranes. Peroxyl radicals generated by thermolysis of 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN) at 37 degrees C oxidized 1 to 8a-[(2,4-dimethyl-1-nitrilopent-2-yl)dioxy]tocopherone (3a), 8a-(hydroperoxy)tocopherone (3b), alpha-tocopherol quinone (4), 4a,5-epoxy-8a-hydroperoxytocopherone (6), 2,3-epoxy-alpha-tocopherol quinone (7), and 5,6-epoxy-alpha-tocopherol quinone (8). The products were purified by high-performance liquid chromatography and characterized by UV-vis spectroscopy, mass spectrometry, and cochromatography with authentic standards. Products accumulated in approximately constant proportion as 1 was consumed. Tocopherones 3a/3b decomposed in the bilayer primarily by hydrolyzing to produce 4. Tocopherone decomposition also produced small amounts of epoxides 6-8, apparently by unimolecular tocopherone decomposition rather than by peroxyl radical dependent oxidation, since neither AMVN nor 1 affected the rate of 3a loss or the distribution of products. Epoxides 6-8 appear to be formed primarily by autoxidation reactions that compete with the peroxyl radical trapping reactions that form tocopherone 3a. Epoxide products may thus serve as biochemical markers for irreversible oxidation of 1 by peroxyl radicals in membranes.

Azo Compounds↗

PubMedDate not supplied

Peroxyl radical oxidation of beta-carotene: formation of beta-carotene epoxides.

A chemical model system was used to study peroxyl radical trapping reactions of beta-carotene (1) that may contribute to its antioxidant action in biological systems. Peroxyl radicals generated in hexane by thermolysis of azobis(2,4-dimethylvaleronitrile) (AMVN) at 37 degrees C oxidized 1 to 5,6-epoxy-beta,beta-carotene (2) and a previously unreported product, 15,15'-epoxy-beta,beta-carotene (6), in addition to several unidentified polar products. The epoxide products were purified by high-performance liquid chromatography and characterized by UV-vis spectroscopy, mass spectrometry, and 1H NMR. Epoxides 2 and 6 and the polar products were formed together from the initial stages of the reaction. As the reaction progressed, the epoxides were oxidized further to more polar products. Although epoxides 2 and 6 were formed at similar rates, 2 was oxidized more rapidly than 6. Incubations with [14C]-1 indicate that at their maximum concentrations 2 and 6 account for approximately 20% of the radiolabeled oxidation products. Epoxide formation may result from peroxyl radical addition to the polyene chain to form a resonance-stabilized peroxyl radical adduct. Peroxide bond scission would yield the epoxide and release an alkoxyl radical. Although this two-step sequence produces no net radical trapping, it could produce a kinetically significant inhibition of peroxyl radical propagation and account, in part, for the antioxidant properties of 1. Epoxides 2 and 6 are structurally distinct from retinoids, which are the metabolic products of 1, and therefore may be useful biochemical markers for its antioxidant actions.

Azo Compounds↗

PubMedDate not supplied

Reactions of vitamin E and its model compound 2,2,5,7,8-pentamethylchroman-6-ol with ozone.

Reaction of vitamin E [(R,R,R)-alpha-tocopherol] with ozone in acetonitrile yielded alpha-tocopheryl quinone and its precursor 8a-hydroxytocopherone, which accounted for approximately 30% of the products at < 50% alpha-tocopherol oxidation. In addition, two novel products were identified as epimers of 10-acetyl-7-(4',8',12'-trimethyltridecyl)-3,4,7-trimethyl-2-oxo- 1,6-dioxaspiro[4.5]-deca-3,9-diene. These spiro products were formed in equal amounts in a combined yield of approximately 33% after complete alpha-tocopherol oxidation. Ozonation of the vitamin E model compound 2,2,5,7,8-pentamethylchroman-6-ol yielded an analogous spiro product, 10-acetyl-3,4,7,7-tetramethyl-2-oxo-1,6-dioxaspiro[4.5]deca-3,9-di ene, whose structure was confirmed by X-ray crystallography. The spiro products may be formed by ozone addition to the chroman ring and subsequent rearrangement to ring-opened hydroxyacid products, which yield spiro products by ring closure due to dehydration. Novel spiro products formed by ozonation of vitamin E may be unique markers of ozone interaction with lipid structures that contain vitamin E.

Chromans↗

PubMedDate not supplied

Reactions of the vitamin E model compound 2,2,5,7,8-pentamethylchroman-6-ol with peroxyl radicals.

The vitamin E model compound 2,2,5,7,8-pentamethylchroman-6-ol (1b) was oxidized by peroxyl radicals generated by thermolysis of 2,2'-azobis(2,4- dimethylvaleronitrile) in oxygenated solvents. Oxidation of 1b yielded 4a,5-epoxy-4a,5-dihydro-8a-hydroperoxy- 2,2,5,7,8-pentamethylchroman-6(8aH)-one (5b), 7,8-epoxy-7,8-dihydro-8a-hydroperoxy-2,2,5,7,8-pentamethylchroman- 6(8aH)- one (6b), 8a-[(2,4-dimethyl-1-nitrilopent-2-yl)dioxy]-2,2,5,7,8- pentamethylchroman-6(8aH)-one (3b), and a 5,5'-spirodimer product (7b). In otherwise identical reactions, yields of chromanone 3b and epoxides 5b/6b increased with increasing solvent polarity, whereas the yield of spirodimer 7b decreased. Deuterium substitution at C5a inhibited oxidation of 1b to spirodimer 7b and favored the formation of a novel 5,7'-spirodimer 11. The results demonstrate that the reaction medium controls the balance between competing reactions of chromanoxyl radical 2b and are consistent with the formation of epoxides and 8a-substituted tocopherones as the predominant products of alpha-tocopherol oxidation in biological membranes.

Antioxidants↗

PubMedDate not supplied

Isolation and identification of singlet oxygen oxidation products of beta-carotene.

Singlet oxygen is a highly reactive form of oxygen produced by many toxic photosensitizers. beta-Carotene quenches singlet oxygen catalytically through a very efficient physical reaction. However, concomitant chemical reactions during photosensitized oxidations consume beta-carotene. To investigate the hypothesis that chemical reactions with singlet oxygen consume beta-carotene, we characterized products of the photosensitized oxidation of beta-carotene. beta-Carotene and the photosensitizer rose bengal were dissolved in toluene/methanol (85:15 v/v), which was bubbled with O2 and illuminated with a quartz-halogen lamp for 30 min at 5 degrees C. Reaction products were analyzed by reverse-phase HPLC, UV-vis spectrophotometry, and mass spectrometry. beta-Carotene oxidation products were identified as beta-ionone, beta-apo-14'-carotenal, beta-apo-10'-carotenal, beta-apo-8'-carotenal, and beta-carotene 5,8-endoperoxide. Formation of these products was dependent on the presence of the photosensitizer. The products apparently were formed from the action of singlet oxygen rather than by photochemically-initiated beta-carotene autoxidation, since suppression of autoxidation by equimolar alpha-tocopherol did not diminish product formation. beta-Carotene autoxidation initiated by 2,2'-azobis(2,4-dimethylvaleronitrile), which generates peroxyl radicals, yielded a different product distribution than that from photosensitized oxidation. Specific products formed by singlet oxygen oxidation of beta-carotene may serve as markers for singlet oxygen quenching in biological systems.

Carotenoids↗

PubMedDate not supplied

A rapid method for profiling the products of antioxidant reactions by negative ion chemical ionization mass spectrometry.

The antioxidant nutrients beta-carotene and alpha-tocopherol are thought to prevent oxidation of important biomolecules in vivo by trapping reactive free radicals. Conventional analytical methods for analyzing antioxidants and their products rely on time consuming chromatographic isolation followed by spectroscopic characterization. This approach is complicated by the instability of many of these products and the large amount of time required for isolation and characterization of multiple products. We have developed a negative ion chemical ionization mass spectrometry method to detect beta-carotene, alpha-tocopherol, and their reaction products. The method involves minimal sample handling and does not require compound isolation. Direct probe negative ion chemical ionization produces molecular anions with little or no fragmentation. Each ion signal in the resulting mass spectrum represents a compound or group of isomeric compounds in the original reaction mixture. Thus, a rapid "snapshot" of the reaction product profile is obtained within seconds. Application of this methodology to the analysis of beta-carotene oxidation in model chemical reaction systems and in microsomal membranes in vitro identified intact chain beta-carotene oxygenation products, carbonyl-containing chain cleavage products, and their oxygenation products. Similar analyses of alpha-tocopherol oxidations identified 8a-substituted tocopherones, alpha-tocopherolquinone and alpha-tocopherolquinone epoxides. Full-scan detection limits for the antioxidants and their products are in the picogram range. This method offers the first means for comprehensively profiling the fate of antioxidants in tissue samples.

Animals↗

PubMedDate not supplied

Antioxidant reactions of beta-carotene: identification of carotenoid-radical adducts.

beta-Carotene and other carotenoids are thought to exert disease preventive actions by scavenging reactive free radicals, but the mechanisms of these reactions are poorly understood. We detected products formed by reaction of beta-carotene with alkoxyl, and alkylperoxyl free radicals generated by thermolysis of azobis(2,4-dimethylvaleronitrile) (AMVN) in benzene. Analyses by atmospheric pressure chemical ionization mass spectrometry identified two previously unknown classes of beta-carotene oxidation products. Substitution products contain one AMVN-derived radical adduct group and result from hydrogen transfer from the carotenoid polyene followed by radical recombination. Addition products contain two AMVN-derived adduct groups and result instead from sequential radical additions to the polyene. These product structures provide the first mechanistic explanation for the radical scavenging reactions of carotenoids.

Antioxidants↗

PubMedDate not supplied

Photoprotective actions of topically applied vitamin E.

Topical application of vitamin E has been shown to decrease the incidence of ultraviolet (UV)-induced skin cancer in mice. Vitamin E provides protection against UV-induced skin photodamage through a combination of antioxidant and UV absorptive properties. Topical application of alpha-tocopherol on mouse skin inhibits the formation of cyclobutane pyrimidine photoproducts. However, topically applied alpha-tocopherol is rapidly depleted by UVB radiation in a dose-dependent manner. The photooxidative fate of the alpha-tocopherol depends on the local environment of the vitamin E. alpha-Tocopherol quinone and alpha-tocopherol quinone epoxides are principal photoproducts of vitamin E that has penetrated into the epidermal layer of the skin, whereas tocopherol dimers and trimers are formed from alpha-tocopherol in a bulk phase at the skin surface. Dimer and trimer products may participate in prevention of UV-induced photodamage.

Administration, Topical↗