Biomedical subjects
R C Sealy
Publications and source records attributed to R C Sealy.
Two sides to the shrew: dysfunctional relational patterns in The Taming of the Shrew and commentaries on the play.
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The psychology of the shrew and shrew taming: an object relations perspective.
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Characterization of semiquinone free radicals formed from stilbene catechol estrogens. An ESR spin stabilization and spin trapping study.
Electron spin resonance spectroscopy has been used to detect, characterize, and to infer structures of o-semiquinones derived from stilbene catechol estrogens. Radicals were generated enzymatically using tyrosinase and were detected as their Mg2+ complexes. It is suggested that initial hydroxylation of stilbene estrogen gives a catechol estrogen in situ; subsequent two-electron oxidation of the catechol to the quinone, followed by reverse disproportionation, leads to the formation of radicals. Consistent with this mechanism, o-phenylenediamine, a quinone trapping agent, inhibits formation of o-semiquinones. A competing mechanism of radical production involves autoxidation of the catechol. Hydroxyl radicals are shown to be produced in this system via a mechanism involving reduction of iron and copper complexes by stilbene catechols. Possible differences in the reactivity of stilbene ortho- and para-semiquinones are discussed.
Semiquinone anion radicals from addition of amino acids, peptides, and proteins to quinones derived from oxidation of catechols and catecholamines. An ESR spin stabilization study.
Either chemical or enzymatic oxidation of catechols or catecholamines in the presence of nucleophiles (amino acids, peptides, and proteins) leads to the production of ring-substituted o-semiquinones which have been detected by ESR spin stabilization techniques. In many cases, radicals have been completely characterized and structures assigned. Chemical considerations point to a mechanism involving addition of nucleophile to o-quinone, followed by oxidation of product to o-semiquinone. These results confirm that addition occurs in oxidizing polyhydroxy aromatic systems, probably via o-quinone, in a reaction considered to account for much of the toxicity found for catechols and catecholamines.
Tyrosinase-catalyzed oxidation of dopa and related catechol(amine)s: a kinetic electron spin resonance investigation using spin-stabilization and spin label oximetry.
The oxidation of four catechol(amine)s by tyrosinase has been studied by electron spin resonance and optical methods. Rates of oxygen consumption and of dopaquinone and dopachrome formation during the oxidation of dopa have been measured, and compared with rates of dopasemiquinone production measured using spin-stabilization procedures. In the presence of spin-stabilizing metal ions, production of semiquinone is approximately quantitative. Time-dependent ESR spectra obtained from dopa and dopamine show a slow regeneration of semiquinone, suggesting that a semiquinone precursor is slowly reformed. In contrast, time-dependent spectra for 4-methylcatechol and N-acetyldopamine show decay of the primary semiquinone together with buildup of a secondary semiquinone apparently derived from the corresponding 6-hydroxy-catechol(amine). Thus, catecholamines that give rise to a cyclizable quinone show a pattern of behavior that differs from those that produce a non-cyclizable quinone. These results are discussed in terms of their possible significance to melanogenesis and the toxicity of catechol(amine)s, which has been attributed to production of semiquinones and/or other oxygen radicals.
Analysis of the ESR spectrum of synthetic dopa melanin.
The 35 GHz ESR spectrum of frozen aqueous suspensions of synthetic melanin from autoxidation of dopa is asymmetric in the pH range 3-12. This asymmetry increases with increasing pH. A detailed computer analysis of second-derivative spectra suggests that the asymmetry is a result of two factors: approximately axial anisotropy of the g-tensor of the radical species; superposition of ESR spectra arising from a total of four radical species. The relative amounts of these individual spectra vary in a pH-dependent manner. Anisotropy of spectra varies with pH, probably due to pH-induced changes in the delocalization of the unpaired electrons. Thus the species present at high pH are suggested to be relatively localized radical anions, while the species detected at low pH are suggested to be protonated forms of the high pH species in which the unpaired electron is more extensively delocalized.
Reaction of superoxide anions with melanins: electron spin resonance and spin trapping studies.
Scavenging of superoxide radicals by melanin is a possible factor in the photoprotection afforded by melanin pigments. The reaction between superoxide anions and melanins has been studied by electron spin resonance and spin trapping methods. It was found that superoxide anions react to produce melanin free radicals in a reaction inhibited by superoxide dismutase but not by catalase. The rate of radical formation depends on the concentration of melanin and superoxide, the pH of the medium and the presence of diamagnetic metal ions. The melanin pigment competes with the enzyme superoxide dismutase for removal of superoxide radicals. It was found that the xanthine-xanthine oxidase system is not suitable for studying the reaction of superoxide with melanin, as the enzymatic activity of xanthine oxidase is considerably inhibited by melanin.
An electron spin resonance study of free radicals from catechol estrogens.
Electron spin resonance spectroscopy has been used to demonstrate production of semiquinone free radicals from the oxidation of the catechol estrogens 2- and 4-hydroxyestradiol and 2,6- and 4,6-dihydroxyestradiol. Radicals were generated by horseradish peroxidase/H2O2 or tyrosinase/O2, or by autoxidation, and were detected as their complexes with spin-stabilizing metal ions (Zn2+ and/or Mg2+). Radical production occurs via one- or two-electron oxidation of catechol estrogens, depending on the type of activating system. Autoxidation of catechol estrogens produces superoxide and H2O2 at physiological pH values. The present results also indicate a difference in the reactivity of quinones derived from 2- and 4-hydroxyestradiol. The toxicological significance of these reactions is discussed.
Mechanism of dismutation of superoxide produced during autoxidation of melanin pigments.
The hydrogen peroxide produced during the autoxidation of melanin pigments has been measured using an oxidase electrode. The autoxidation has been shown to occur via the superoxide intermediate. The melanin pigment competes with superoxide dismutase for the scavenging of superoxide radicals. However, superoxide dismutase at high concentrations caused a substantial increase in the production of hydrogen peroxide, formed during melanin autoxidation. The implications of this finding are discussed in light of melanin's ability to function as a pseudo-dismutase.
Radical anions from one-electron-reduced adrenochrome. Detection and identification by electron spin resonance spectroscopy.
Free radicals from the one-electron reduction of adrenochrome have been studied in aqueous solutions. These radicals have been detected and identified by electron spin resonance spectroscopy, using spin stabilization methods (complexation with diamagnetic metal ions) to enhance radical concentrations. It is shown that the radicals have a characteristic ESR spectrum enabling their identification in complex systems. The spin density distribution in the radicals has been studied as a function of complexing metal ions and solvent composition. In the presence of oxidants (e.g., oxygen) the spectrum of the radical is replaced by that derived from the one-electron exidation of adrenochrome.
Nitroxides as redox probes of melanins: dark-induced and photoinduced changes in redox equilibria.
The interaction of nitroxide free radicals and their reduced products (hydroxylamines) with synthetic and natural melanins has been studied. Electron spin resonance spectroscopy was used to measure changes in radical concentration in the dark and during irradiation with visible or uv light. Some reduction of nitroxide occurs in the dark, and is reversible: the nitroxide can be completely regenerated by the one-electron oxidant ferricyanide. The kinetics of the process depend strongly on radical charge and pH. For positively charged nitroxides the rate is much faster than for either neutral or anionic radicals. At pH 10 the rate is about 20 times faster than at pH 5. Oxidation of hydroxylamine also can occur so that a redox equilibrium is established. The equilibrium constant has been estimated for the reaction between a nitroxide and melanin from autoxidation of 3,4-dihydroxyphenylalanine. Results are also dependent upon the type of melanin used and chemical modification (oxidation or reduction) of the melanin. Redox equilibria are altered during irradiation with either visible or uv light. Rapid oxidation of hydroxylamine to nitroxide is apparent, together with a slower reduction of nitroxide. Action spectra for these processes are related to those for melanin radical production and oxygen consumption in nitroxide-free melanin systems. Reduction of nitroxide is inhibited by oxygen, suggesting a competition between nitroxide and oxygen for photoinduced reducing equivalents.
Photosensitization of melanins: a comparative study.
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Free radicals from the photodecomposition of bleomycin.
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Semiquinone anion radicals of catechol(amine)s, catechol estrogens, and their metal ion complexes.
The characterization and identification of semiquinone radicals from catechol(amine)s and catechol estrogens by electron spin resonance spectroscopy is addressed. The use of diamagnetic metal ions, especially Mg2+ and Zn2+ ions, to detect transient semiquinone radicals in biological systems and to monitor their reactions, is discussed. A brief account of the identification and reactions of quinones is also presented.
An electron spin resonance study of o-semiquinones formed during the enzymatic and autoxidation of catechol estrogens.
Electron spin resonance spectroscopy has been used to demonstrate production of semiquinone-free radicals from the oxidation of the catechol estrogens 2- and 4-hydroxyestradiol and 2,6- and 4,6-dihydroxyestradiol. Radicals were generated either enzymatically (using horseradish peroxidase-H2O2 or tyrosinase-O2) or by autoxidation, and were detected as their complexes with spin-stabilizing metal ions (Zn2+ and/or Mg2+). In the peroxidase system, radicals are produced by one-electron oxidation of the catechol estrogen and their decay is by a second-order pathway, consistent with their disproportionation to quinone and catechol products. With tyrosinase-O2, radical generation occurs indirectly. Initial hydroxylation of phenolic estrogen (at either the 2- or 4-position) gives a catechol estrogen in situ; subsequent two-electron oxidation of the catechol to the quinone, followed by reverse disproportionation, leads to the formation of radicals. A competing mechanism for radical production involves autoxidation of the catechol. Results obtained from the estrogen systems have been compared with those from the model compound 5,6,7,8-tetrahydro-2-naphthol.
Identification by electron spin resonance spectroscopy of free radicals produced during autoxidative melanogenesis.
Free radicals produced during the autoxidation of 3,4-dihydroxyphenylalanine (DOPA) and other catechol(amine)s to melanins have been studied using electron spin resonance spectroscopy. Magnetic parameters for the radical intermediates have been determined, allowing the radicals to be unambiguously identified. Three types of radical are formed: the primary radical from one-electron oxidation of the parent catechol(amine); and two secondary radicals, one formed via OH- substitution, the other via cyclization. The formation of these radical species can be linked to molecular products formed during catecholamine oxidation and melanin formation.
Free radicals from eumelanins: quantum yields and wavelength dependence.
Formation of light-induced free radicals from natural eumelanin (from bovine eyes) and synthetic melanin (from oxidation of 3,4-dihydroxyphenylalanine) has been studied by electron spin resonance spectroscopy. Action spectra measured for natural melanins are very similar to that found for synthetic melanin, and are unaffected by the removal of associated protein. A comparison of action spectra with optical absorbance spectra shows that the former has a more marked wavelength dependence, suggesting that the chromophore that is most active in free-radical production is not the major melanin chromophore that absorbs visible light. Measurements of quantum yields for free-radical production have been made over a wavelength range from 600 to 230 nm. The efficiency of radical production from natural eumelanin is about three times greater than from the synthetic material. Although production of the melanin radicals detected is independent of oxygen, some correlation with oxygen consumption is evident; quantum yields for radical production are approximately three times those for oxygen consumption obtained under similar conditions. Possible reasons for this are discussed.