Selective isotopic labeling of a nitroxide spin label to enhance sensitivity for T2 oxymetry

Comparison of Antioxidant Properties of Dehydrolutein with Lutein and Zeaxanthin, and their Effects on Cultured Retinal Pigment Epithelial Cells

Dehydrolutein accumulates in substantial concentrations in the retina. The aim of this study was to compare antioxidant properties of dehydrolutein with other retinal carotenoids, lutein, and zeaxanthin, and their effects on ARPE-19 cells. The time-resolved detection of characteristic singlet oxygen phosphorescence was used to compare the singlet oxygen quenching rate constants of dehydrolutein, lutein, and zeaxanthin. The effects of these carotenoids on photosensitized oxidation were tested in liposomes, where photo-oxidation was induced by light in the presence of photosensitizers, and monitored by oximetry. To compare the uptake of dehydrolutein, lutein, and zeaxanthin, ARPE-19 cells were incubated with carotenoids for up to 19 days, and carotenoid contents were determined by spectrophotometry in cell extracts. To investigate the effects of carotenoids on photocytotoxicity, cells were exposed to light in the presence of rose bengal or all-trans-retinal. The results demonstrate that the rate constants for singlet oxygen quenching are 0.77 × 10¹⁰, 0.55 × 10¹⁰, and 1.23 × 10¹⁰ M^-1s^-1 for dehydrolutein, lutein, and zeaxanthin, respectively. Overall, dehydrolutein is similar to lutein or zeaxanthin in the protection of lipids against photosensitized oxidation. ARPE-19 cells accumulate substantial amounts of both zeaxanthin and lutein, but no detectable amounts of dehydrolutein. Cells pre-incubated with carotenoids are equally susceptible to photosensitized damage as cells without carotenoids. Carotenoids provided to cells together with the extracellular photosensitizers offer partial protection against photodamage. In conclusion, the antioxidant properties of dehydrolutein are similar to lutein and zeaxanthin. The mechanism responsible for its lack of accumulation in ARPE-19 cells deserves further investigation.

Products of Docosahexaenoate Oxidation as Contributors to Photosensitising Properties of Retinal Lipofuscin

Retinal lipofuscin which accumulates with age in the retinal pigment epithelium (RPE) is subjected to daily exposures to high fluxes of visible light and exhibits potent photosensitising properties; however, the molecules responsible for its photoreactivity remain unknown. Here, we demonstrate that autooxidation of docosahexaenoate (DHE) leads to the formation of products absorbing, in addition to UVB and UVA light, also visible light. The products of DHE oxidation exhibit potent photosensitising properties similar to photosensitising properties of lipofuscin, including generation of an excited triplet state with similar characteristics as the lipofuscin triplet state, and photosensitised formation of singlet oxygen and superoxide. The quantum yields of singlet oxygen and superoxide generation by oxidised DHE photoexcited with visible light are 2.4- and 3.6-fold higher, respectively, than for lipofuscin, which is consistent with the fact that lipofuscin contains some chromophores which do contribute to the absorption of light but not so much to its photosensitising properties. Importantly, the wavelength dependence of photooxidation induced by DHE oxidation products normalised to equal numbers of incident photons is also similar to that of lipofuscin—it steeply increases with decreasing wavelength. Altogether, our results demonstrate that products of DHE oxidation include potent photosensitiser(s) which are likely to contribute to lipofuscin photoreactivity.

In vivo EPR oximetry using an isotopically-substituted nitroxide: Potential for quantitative measurement of tissue oxygen

Variations in brain oxygen (O2) concentration can have profound effects on brain physiology. Thus, the ability to quantitate local O2 concentrations noninvasively in vivo could significantly enhance understanding of several brain pathologies. However, quantitative O2 mapping in the brain has proven difficult. The electron paramagnetic resonance (EPR) spectra of nitroxides are sensitive to molecular O2 and can be used to estimate O2 concentrations in aqueous media. We recently synthesized labile-ester-containing nitroxides, such as 3-acetoxymethoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl (nitroxide 4), which accumulate in cerebral tissue after in situ hydrolysis, and thus enable spatial mapping of O2 concentrations in the mouse brain by EPR imaging. In an effort to improve O2 quantitation, we prepared 3-acetoxymethoxycarbonyl-2,2,5,5-tetra((2)H3)methyl-1-(3,4,4-(2)H3,1-(15)N)pyrrolidinyloxyl (nitroxide 2), which proved to be a more sensitive probe than its normo-isotopic version for quantifying O2 in aqueous solutions of various O2 concentrations. We now demonstrate that this isotopically substituted nitroxide is ∼2-fold more sensitive in vivo than the normo-isotopic nitroxide 4. Moreover, in vitro and in vivo EPR spectral-spatial imaging results with nitroxide 2 demonstrate significant improvement in resolution, reconstruction and spectral response to local O2 concentrations in cerebral tissue. Thus, isotopic-substituted nitroxides, such as 2, are excellent sensors for in vivo O2 quantitation in tissues, such as the brain.

Comparison of the aerobic photoreactivity of A2E with its precursor retinal

A2E (2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E, 3E,5E,7E-octatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E-hexatrienyl]pyridinium) is a blue-absorbing molecular constituent of human ocular lipofuscin and contributes to the golden-yellow emission of this pigment. Lipofuscin photoproduces toxic reactive oxygen intermediates (ROI), but the specific molecular components responsible for this phototoxicity remain unidentified. In this article the aerobic photoreactivity of A2E is quantified by comparison with its biosynthetic precursor, all-trans-retinal, and with other appropriate standards. Under blue-light exposure the efficacies for formation of cholesterol (Ch) hydroperoxides and the superoxide radical anion (O2*-) were determined using high-pressure liquid chromatography with electrochemical detection and electron spin resonance oximetry and spin trapping, respectively. Photogeneration of singlet oxygen after blue-light excitation of A2E was demonstrated unambiguously by the Ch peroxidation assay. After blue-light irradiation of A2E, O2*- were detected, but the concentration was insufficient to account for the measured production of O2*- by the solvent extract of lipofuscin granules. The collective data support the conclusion that A2E does not produce sufficient concentrations of ROI to be the primary phototoxic constituent of lipofuscin.

Dynamic in vivo oxymetry using overhauser enhanced MR imaging

A noninvasive method for in vivo measurement of the oxygen concentration has been developed. By introducing a novel contrast medium (CM) based on a single electron substance, it is possible to enhance the proton signal through the Overhauser effect. A low-field magnetic resonance scanner is used to image the proton nuclei of the object. The electron spin transition of the CM is saturated using rf irradiation. As a consequence, the nuclear polarization becomes enhanced through dipole-dipole interaction. The signal enhancement is a function of rf power and of the EPR line width of the substance, which is influenced by the oxygen concentration. The maximum in vivo enhancement has been measured to 60. Image data, generated with different scanning parameters, is used in a postprocessing method to generate images showing pO(2) and the contrast medium concentration, respectively. The mathematical foundation of the postprocessing algorithm is outlined. The results from phantom experiments and animal experiments, in which the oxygen content of the inspired gas was varied, are presented. The potential for human imaging is discussed. J. Magn. Reson. Imaging 2000;12:929-938.

The measurement of oxygen in vivo using EPR techniques

The measurement of pO2 in vivo using EPR has some features which have already led to very useful applications and this approach is likely to have increasingly wide and effective use. It is based on the effect of oxygen on EPR spectra which provides a sensitive and accurate means to measure pO2 quantitatively. The development of oxygen-sensitive paramagnetic materials which are very stable, combined with instrumental developments, has been crucial to the in vivo applications of this technique. The physical basis and biological applications of in vivo EPR oximetry are reviewed, with particular emphasis on the use of EPR spectroscopy at 1 GHz using particulate paramagnetic materials for the repetitive and non-invasive measurement of pO2 in tissues. In vivo EPR has already produced some very useful results which have contributed significantly to solving important biological problems. The characteristics of EPR oximetry which appear to be especially useful are often complementary to existing techniques for measuring oxygen in tissues. These characteristics include the capability of making repeated measurements from the same site, high sensitivity to low levels of oxygen, and non-invasive options. The existing techniques are especially useful for studies in small animals, where the depth of measurements is not an overriding issue. In larger animals and potentially in human subjects, non-invasive techniques seem to be immediately applicable to study phenomena very near the surface (within 10 mm) while invasive techniques have some very promising uses. The clinical uses of EPR oximetry which seem especially promising and likely to be undertaken in the near future are long-term monitoring of the status and response to treatment of peripheral vascular disease and optimizing cancer therapy by enabling it to be modified on the basis of the pO2 measured in the tumour.

In vivo and in vitro EPR oximetry with fusinite: a new coal-derived, particulate EPR probe

The peak-to-peak line width (LW) of the first derivative electron spin resonance (EPR) spectrum of the coal maceral fusinite is reversibly broadened by O2. The extent of broadening per unit of partial pressure of oxygen (pO2) is unusually large, exceeding that of nitroxides by almost two orders of magnitude. This paramagnetic property of fusinite, combined with its very stable physicochemical properties and low toxicity, is shown to be of utility in the measurement of pO2 in vitro and in vivo. Fusinite particles are endocytosed by chinese hamster ovary (CHO) cells in vitro; this is useful for intracellular O2 measurements with commercially available EPR spectrometers operating at 9.1-9.3 GHz. For measurement of oxygen in vivo using low frequency EPR (1.1-1.3 GHz), fusinite provides a sensitive and persistent means to measure pO2 in tissues. Particles implanted into the gastrocnemius muscle of A/J mice remained interstitially in the same position for months with undiminished sensitivity to pO2 and no specific toxic effects.