Nar S. Dalal

ANTIOXIDANT BEHAVIOUR OF CAFFEINE : EFFICIENT SCAVENGING OF HYDROXYL RADICALS

Considerable controversy exists in the literature regarding the toxicity of coffee, including its possible carcinogenic and anticarcinogenic properties. This study reports on the reaction of 1,3,7-trimethylxanthine (caffeine) with the hydroxyl radical (.OH), as investigated by electron spin resonance (ESR) spin trapping. The .OH was generated by the Fenton reaction (Fe2+ + H2O2) as well as by the reaction of chromium(V) with H2O2. The results show that caffeine effectively scavenges .OH with a reaction rate constant of approximately 5.9 x 10(9) M-1 sec-1 that is comparable with those of other efficient .OH radical scavengers. ESR measurements provide evidence that a caffeine-derived oxygen-centred radical is formed in the reaction of caffeine with .OH and suggest a biochemical basis for the understanding of the reported anticarcinogenic properties of caffeine and related methylxanthine compounds.

Chromate-mediated free radical generation from cysteine, penicillamine, hydrogen peroxide, and lipid hydroperoxides

The Cr(VI)-mediated free radical generation from cysteine, penicillamine, hydrogen peroxide, and model lipid hydroperoxides was investigated utilizing the electron spin resonance (ESR) spin trapping technique. Incubation of Cr(VI) with cysteine (Cys) generated cysteinyl radical. Radical yield depended on the relative concentrations of Cr(VI) and Cys. The radical generation became detectable at a cysteine:Cr(VI) ratio of about 5, reached its highest level at a ratio of 30, and declined thereafter. Cr(VI) or Cys alone did not generate a detectable amount of free radicals. Similar results were obtained with penicillamine. Incubation of Cr(VI), Cys or penicillamine and H2O2 led to hydroxyl (.OH) radical generation, which was verified by quantitative competition experiments utilizing ethanol. The mechanism for .OH radical generation is considered to be a Cr(VI)-mediated Fenton-like reaction. When model lipid hydroperoxides such as t-butyl hydroperoxide and cumene hydroperoxide were used in place of H2O2, hydroperoxide-derived free radicals were produced. Since thiols, such as Cys, exist in cellular systems at relatively high concentrations, Cr(VI)-mediated free radical generation in the presence of thiols may participate in the mechanisms of Cr(VI)-induced toxicity and carcinogenesis.

Generation of Reactive Oxygen Species by Quartz Particles and Its Implication for Cellular Damage

Abstract Using wet analytical chemistry and electron spin resonance spin trapping, we have shown that aqueous suspensions of freshly fractured quartz particles generated hydrogen peroxide, hydroxyl radical, superoxide radical, and singlet oxygen. Superoxide dismutase partially inhibited the hydroxyl radical yield, whereas catalase suppressed it. Hydrogen peroxide enhanced the hydroxyl radical generation, while deferoxamine decreased it. Oxygen consumption measurements showed that freshly fractured quartz particles consumed molecular oxygen. Electrophoretic assays showed that freshly fractured quartz particles induced DNA double-strand breakage, which was inhibited by catalase. In an argon atmosphere DNA damage was suppressed, showing that molecular oxygen is required for quartz-induced DNA damage. Quartz particles also caused dose-dependent lipid peroxidation as measured by malondialdehyde formation. Superoxide dismutase, catalase, and sodium benzoate inhibited quartz-induced lipid peroxidation by 49, 52,...

Generation of Free radicals in reactions of Ni(II)-thiol complexes with molecular oxygen and model lipid hydroperoxides

The generation of free radicals from reactions of nickel(II)-thiol complexes with molecular oxygen and model lipid hydroperoxides was investigated by electron spin resonance (ESR) utilizing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap. Incubation of nickel(II) [Ni(II)] with cysteine in an aerobic environment generated hydroxyl (.OH) radical, which then reacted with cysteine to generate a carbon-centered alkyl (.R) radical. Radical generation was inhibited under a nitrogen atmosphere. Model lipid hydroperoxides, cumene hydroperoxide, and t-butyl hydroperoxide enhanced the yield of these radicals and also generated an alkoxyl (.OR) radical. Radical yield decreased by approximately half under a nitrogen atmosphere. Although histidine did not cause radical formation in the reaction between Ni(II) and cumene hydroperoxide under aerobic conditions, the addition of histidine to a mixture containing Ni(II), cysteine, and cumene hydroperoxide under the same experimental conditions increased the yield of .R radical but lowered the yield of .OR and .OH radical adducts. It thus appears that histidine caused the .OH attack to be more site-specific. Similar results were obtained utilizing t-butyl hydroperoxide. Penicillamine or N-acetylcysteine yielded similar results except that under aerobic conditions, reaction between Ni(II) and N-acetylcysteine without hydroperoxide did not generate a significant concentration of free radicals. Under the same experimental conditions, cystine did not generate any detectable free radicals, suggesting an important role of the -SH group in Ni(II)-mediated free radical generation. The results indicate that free radical generation from the reaction of Ni(II)-thiol complexes and molecular oxygen, and/or lipid hydroperoxides, may play an important role in the mechanism(s) of Ni(II) toxicity and carcinogenesis.

One-electron reduction of chromate by NADPH-dependent glutathione reductase

Electron spin resonance (ESR) measurements provide evidence for the formation of Cr(V) intermediates in the enzymatic reduction of Cr(VI) by glutathione reductase (GSSG-R) in the presence of NADPH, indicating an initial single-electron transfer step in the reduction mechanism. Depending on the pH, at least two different Cr(V) species are generated which are relatively long-lived. In addition, we have detected the hydroxyl (.OH) radical formation during the GSSG-R catalyzed reduction of Cr(VI) by spin trapping, employing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) as spin traps. Superoxide dismutase (SOD) causes only a minor effect on the .OH radical and Cr(V) formation, indicating that the O2- is not significantly involved in the reaction mechanism. Catalase enhances the Cr(V) formation and substantially inhibits the .OH radical formation, indicating the involvement of hydrogen peroxide (H2O2) in the reaction mechanism. Addition of H2O2 suppresses Cr(V) and enhances the .OH radical formation. Measurements involving N-ethylmaleimide show that the Cr(V) species, produced enzymatically by the reduction of Cr(VI) by GSSG-R, react with H2O2 to generate .OH radicals, which might participate in the initiation of Cr(VI) carcinogenicity.

Preparation and characterization of semiconducting copper phthalocyanine dimers

Abstract Copper phthalocyanine (CuPc) dimers have been prepared as gels and powders from monomeric CuPc in several mixed solvents containing H 2 SO 4 . The isolated powders are moisture insensitive and thermally stable up to 260°C. The observed e.p.r. spectra, characteristic of a triplet state with axial symmetry and exhibiting seven-line CuCu hyperfine patterns, provide strong evidence for the face-to-face dimer configuration with an average CuCu distance of 4.4 ± 0.1 A . Magnetic susceptibility data obtained from 1.8 to 280 K support the e.p.r. results and yield an average singlet-triplet spacing of ≈ 1.2 cm −1 , with the singlet as the ground state. Absorption spectra of the dimer showed a 30–40 nm shift to the blue of the monomer π−π ∗ transition, suggesting the cofacial coupling of the CuPc species. Elemental analysis suggests that the dimer has a molecular formula of (CuPc·H 2 SO 4 ) 2 . The room temperature conductivities of the dimers were determined to be 10 −10 to 10 −7 S/cm, which are about four orders of magnitude higher than that of the monomer.