Development and Preclinical Study of Free Radical Imaging Using Field-Cycling Dynamic Nuclear Polarization MRI.

Free radicals, such as metabolic intermediates, reactive oxygen species, and metal enzymes, are key substances in organisms, although they can also cause various oxidative diseases. Thus, in vivo free radical imaging should be considered as the ultimate form of metabolic imaging. Unfortunately, electron spin resonance (ESR) imaging has inherent disadvantages, such as free radicals with large linewidths generating blurred images and the presence of two or more free radicals resulting in a complicated imaging procedure. Dynamic nuclear polarization-magnetic resonance imaging (DNP-MRI) is a noninvasive imaging method to visualize in vivo free radicals, theoretically, with the same resolution as the MRI anatomical resolution, and fixed low-field DNP-MRI provides unique information on oxidative diseases and cancer. However, the large gyromagnetic ratio of the electron spin, which is 660-fold greater than that of a proton, requires field cycling, wherein the external magnetic field should be varied during DNP-MRI observations. This causes difficulties in developing a DNP-MRI system for clinical purposes. We developed a novel field-cycling DNP-MRI system for a preclinical study. In the said system, the magnetic field is switched by rotationally moving two magnets, with a magnetic flux density of 0.3 T for MRI and 5 mT for ESR. The image quality was examined using various pulse sequences and ESR irradiation using nitroxyl radical as the phantom, and the optimum conditions were established. Using the system, we performed a preclinical study involving free radical imaging by placing the free radicals under the palm of a human hand.

The Anticonvulsant Zonisamide Inhibits Hydroxyl Radical Generated from Methylguanidine.

Methylguanidine (MG) is a known nephrotoxin and neurotoxin, and an intracisternal injection of MG can induce convulsions in experimental animals. In this in vitro study, we examined the inhibitory effects of the antiepileptic agent zonisamide (ZNS) on hydroxyl radicals (•OH) generated from MG by using an electron spin resonance (ESR) technique. ZNS scavenged •OH generated from MG in a dose-dependent manner through direct scavenging during the auto-oxidation of MG. The rate constant of ZNS reacting with the •OH was at a near diffusion-controlled rate. These findings indicate that ZNS might detoxify MG and could thus protect against convulsive disorders.

Generation of radical species from dihydropyrazines having DNA strand-breakage activity and other characteristics.

The various biological activity of dihydropyrazines(DHPs)due to the radical generation potency has been described in previous papers. Detailed data about radical species generating be mentioned here. The electron spin resonance (ESR) spin-trapping technique revealed that DHPs generate free radical species such as ·OH, ·OOH, ·CHR(2) and ·CR(3). Oxygen radicals and two carbon-centered radicals were detected as adducts of the spin traps DMPO and DBNBS, respectively. All the 5,5-dimethyl-1-pyrroline-N-oxide (DMPO)- and 3,5-dibromo-4-nitrosobenzenesulfonate (DBNBS)-adducts of compounds DHP-1-8 exhibited approximately the same signal patterns, with various levels of intensity depending on the substituent of the dihydropyrazine ring. The ESR signal intensity of DHPs also increased remarkably upon addition of Cu(2+), resulting that the effects of DHPs were enhanced.

Enzymatic reduction-resistant nitroxyl spin probes with spirocyclohexyl rings.

To suppress enzymatic reduction of nitroxyl group of spin probes, this study designed two new nitroxyl probes, 4-hydroxy and 4-oxopiperidine-N-oxyls having 4'-hydroxyspirocyclohexyl groups at the 2- and 6-positions of the piperidine ring (hydroxy-DICPO and oxo-DICPO, respectively). The decay of the EPR signal of these probes in mouse liver homogenates was significantly suppressed compared with that of 4-hydroxy- and 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (hydroxy-TEMPO and oxo-TEMPO, respectively), although hydroxy-DICPO and oxo-DICPO showed no difference in the reactivities with ascorbic acid. While both hydroxy- and oxo-DICPO reacted with hydroxyl radicals, only hydoxy-DICPO lost its EPR signal by the reaction with superoxide anion radical in the presence of cysteine. This feature is similar to that observed for hydroxy- and oxo-TEMPO. These results suggest that the introduction of spirocyclohexyl groups to nitroxyl spin probes is effective for protecting the nitroxyl group against enzymatic reduction without changing the characteristics of the reaction with oxygen radicals.

Electron spin resonance assay of ascorbyl radical generation in mouse hippocampal slices during and after kainate-induced seizures.

As an index of oxidative status, we analyzed ascorbyl radical generation during and after kainate-induced seizures in mouse hippocampus, using an ESR spectrometer equipped with a special tissue-type quartz cell. A specific doublet ESR spectrum was observed after seizures, and the g value and the hyperfine coupling constant (hfcc) of the spectrum were identical with those of ascorbyl radical itself. Antiepileptic zonisamide inhibited the generation of ascorbyl radical accompanying the seizures.

Assessment of ESR-CT imaging by comparison with autoradiography for the distribution of a blood-brain-barrier permeable spin probe, MC-PROXYL, to rodent brain.

Blood-brain-barrier (BBB)-permeable, 3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-1-yloxy (MC-PROXYL) and BBB-impermeable carbamoyl-PROXYL were used to assess the ESR imaging technique by comparing with autoradiography. For this purpose, spin probes, 14C-labeled at their five-membered ring, [14C]MC-PROXYL and [14C]carbamoyl-PROXYL, were newly synthesized. These probes were i.p. or i.v. injected into rats and autoradiograms were recorded. The autoradiograms of rat head showed that [14C]MC-PROXYL distributed well in the brain compared to [14C]carbamoyl-PROXYL. In vivo ESR spectra and two-dimensional ESR images of isolated rat brain treated with MC- or carbamoyl-PROXYL also indicated the extensive distribution of MC-PROXYL but not carbamoyl-PROXYL in the rat brain. The three-dimensional ESR images of the head of rats and mice were consistent with the fact that MC-PROXYL but not carbamoyl-PROXYL is incorporated into the brain. The ESR-CT images were better for mice than rats. However, the quality of the ESR-CT images was still not satisfactory. Although the resolution and sensitivity of the ESR-CT images were worse than those of the autoradiographic images, the former technique has unique features and advantages; e.g., functional, noninvasive and three-dimensional detection.

Levels of active oxygen species are controlled by ascorbic acid and anthocyanin in Arabidopsis.

Stabilization of the levels of active oxygen species (AOS) is important to the survival of organisms. To clarify the system controlling levels of AOS in plants, this study used an electron spin resonance (ESR) method to directly measure superoxide radical (O(2)(.-)) scavenging activities in the wild-type Arabidopsis thaliana (Col and Ler ecotypes), two anthocyanin mutants (tt3 and ttg1), and an ascorbic acid mutant (vtc1). Under ordinary growth conditions, Arabidopsis contained superoxide-scavenging activity (SOSA) of approximately 300-500 SOD units/g of fresh weight. The ESR pattern indicated that most (40-50%) of this activity was due to ascorbic acid. For the analysis of SOSA under conditions of oxidative stress, synthesis of AOS was induced by gamma-irradiation. The radical scavenging activity in irradiated plants increased approximately 10-fold following an associated increase in the accumulation of ascorbic acid and anthocyanin. The accumulation of ascorbic acid and anthocyanin was suppressed by treatment with an antioxidant before irradiation and was induced by treatment with a radical-generating reagent. The contributions of ascorbic acid and anthocyanin to the total superoxide radical scavenging activity differed among ecotypes. In the Ler ecotype, ascorbic acid accumulated at twice the level of that in the Col ecotype, and induction of anthocyanin was half that in Col. To confirm the activity of ascorbic acid and anthocyanin against AOS stress, the viability of the wild type and mutants (tt2, tt3,tt5, ttg1, and vtc1) was examined after gamma-irradiation. Only the plants in which ascorbic acid and anthocyanin were induced had the ability to grow and flower.

Inhibition of BPA degradation by serum as a hydroxyl radical scavenger and an Fe trapping agent in Fenton process.

Identification of reactive oxygen species (ROS) that contribute to bisphenol-A (BPA) degradation and monitoring of BPA at various concentrations in human serum under Fenton reaction conditions were carried out using electron spin resonance (ESR) spectrophotometry and high performance liquid chromatography with electrochemical detection (HPLC-ECD). BPA recovery decreased with increasing Fe concentration and time, both with a Fenton reaction using Fe(II), and with a Fenton-like reaction using Fe(III). In these reactions, BPA dose-dependently decreased the intensity of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO)-*OH, up to 1 microg/ml BPA, and no change in DMPO-O(2)(?-) intensity was observed. The decrease in BPA recovery was inhibited strongly by addition of serum under Fenton-like reaction conditions, and there was a negative correlation between turbidity and BPA recovery. To clarify the mechanism by which serum inhibits BPA degradation, the relationship between BPA recovery and sample turbidity, and characteristics of the precipitates were investigated using spectrophotometry and X-ray analysis. The precipitate formed in the serum-containing sample consisted of C, S, O, P and Fe. BPA degradation was also inhibited under Fenton-like reaction conditions in phosphate buffered saline (PBS), and a precipitate consisting of O, P, and Fe appeared. Precipitates also appeared in authentic albumin and gamma-globulin when sulfate was added with Fenton reagents. After precipitate removal, both Fe and protein concentrations in the supernatant of the protein solutions with sulfate decreased with increasing Fe addition. We demonstrate here that hydroxyl radical generation from Fenton or Fenton-like reactions can degrade BPA, and that serum strongly inhibits BPA degradation, not only by competing with BPA for hydroxyl radicals, but also by trapping Fe with oxidative components present in the serum.

Superoxide anion radical scavenging activities of herbs and pastures in northern Japan determined using electron spin resonance spectrometry.

Free radicals are not only destructive to the living cells but also reduce the quality of animal products through oxidation. As a result the superoxide anion radical (O2-), one of the most destructive reactive oxygen species, is a matter of concern for the animal scientists as well as feed manufacturers to ensure the quality of product to reach consumers demand. The superoxide anion radical scavenging activities (SOSA) of water and MeOH extracts of 2 herbs and 9 pasture samples collected from lowland and highland swards were determined against a 5,5-dimethyl-1-pyroline-N-oxide-O2-spin adduct based on a hypoxanthine-xanthine oxidase reaction using electron spin resonance spectrometry. Both the water and MeOH extracted SOSA differed among the herbs and pastures. Species and altitudinal variations were observed between extraction methods. The herbs were higher in both water and MeOH extracted SOSA than the pastures except for water extracts of one pasture, white clover (Trifolium repens L.). Among the pastures, quackgrass (Agrophyron repens L.) showed higher SOSA in both the MeOH and water extracts, and timothy (Phleum pretense L.) showed higher MeOH extracted SOSA. It is apparent that the kind and amount of antioxidants differ among herbs and pastures. Animal health and quality of animal products could be improved by adequate selection and combining of herbs and pastures having higher SOSA.

Growth inhibitory factor prevents neurite extension and the death of cortical neurons caused by high oxygen exposure through hydroxyl radical scavenging.

Growth inhibitory factor (GIF), a brain-specific member of the metallothionein family (MT-III), has been characterized as a inhibitory substance for neurotrophic factors in Alzheimer's disease brains. However, the function of GIF, other than the inhibition of neurotrophic factors, remains unknown. We demonstrate here that exogenous GIF prevents neurite extension of cortical neurons in the early period of differentiation and the death of differentiated neurons caused by high oxygen exposure. Down-regulation of GIF in cortical neurons with antisense S-oligonucleotides promoted neuronal death under high oxygen conditions. ESR spin-trapping studies demonstrated that GIF at 2-6 microm scavenged hydroxyl radicals generated by a Fenton-type reaction or the photolysis of hydrogen peroxide much more effectively than the same concentration of metallothionein I+II. GIF did not scavenge either superoxide produced by the xanthine/xanthine oxidase reaction or NO generated from 1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene. Moreover, GIF at 40-80 microm inhibited tyrosine nitration by peroxynitrite as efficiently as metallothionein I+II at the same concentration. These results indicate that GIF prevents neurite extension of neurons in the early period of differentiation and supports the survival of differentiated neurons by scavenging hydroxyl radicals.

Active oxygen species generated from photoexcited fullerene (C60) as potential medicines: O2-* versus 1O2.

To characterize fullerenes (C(60) and C(70)) as photosensitizers in biological systems, the generation of active oxygen species, through energy transfer (singlet oxygen (1)O(2)) and electron transfer (reduced active oxygen radicals such as superoxide anion radical O(2)(-)* and hydroxyl radical *OH), was studied by a combination of methods, including biochemical (DNA-cleavage assay in the presence of various scavengers of active oxygen species), physicochemical (EPR radical trapping and near-infrared spectrometry), and chemical methods (nitro blue tetrazolium (NBT) method). Whereas (1)O(2) was generated effectively by photoexcited C(60) in nonpolar solvents such as benzene and benzonitrile, we found that O(2)(-)* and *OH were produced instead of (1)O(2) in polar solvents such as water, especially in the presence of a physiological concentration of reductants including NADH. The above results, together with those of a DNA cleavage assay in the presence of various scavengers of specific active oxygen species, indicate that the active oxygen species primarily responsible for photoinduced DNA cleavage by C(60) under physiological conditions are reduced species such as O(2)(-)* and *OH.