Synthesis of F-18 labeled resazurin by direct electrophilic fluorination

We present the synthesis and characterization of F18-labeled fluorinated derivatives of resazurin, a probe for cell viability. The compounds were prepared by direct fluorination of resazurin with diluted [F18]-F₂ gas under acidic conditions. The fluorination occurs into the ortho-positions to the hydroxyl group producing various mono-, di-, and trifluorinated derivatives. The properties of the fluorinated resazurins are similar to the parent compound with the addition of fluorine leading to decreased pK_a values and a bathochromic shift of the absorption maxima. The fluorinated resazurin derivatives can be used as probes for observation of cell viability in various cells, tissues and organs using a combination of positron emission tomography and direct optical imaging of Cerenkov luminescence.

Cerenkov-specific contrast agents for detection of pH in vivo

We report the design, testing, and in vivo application of pH-sensitive contrast agents designed specifically for Cerenkov imaging. Radioisotopes used for PET emit photons via Cerenkov radiation. The multispectral emission of Cerenkov radiation allows for selective bandwidth quenching, in which a band of photons is quenched by absorption by a functional dye. Under acidic conditions, (18)F-labeled derivatives emit the full spectrum of Cerenkov light. Under basic conditions, the dyes change color and a wavelength-dependent quenching of Cerenkov emission is observed.

METHODS

Mono- and di-(18)F-labeled derivatives of phenolsulfonphthalein (phenol red) and meta-cresolsulfonphthalein (cresol purple) were synthesized by electrophilic fluorination. Cerenkov emission was measured at different wavelengths as a function of pH in vitro. Intramolecular response was measured in fluorinated probes and intermolecular quenching by mixing phenolphthalein with (18)F-FDG. Monofluorocresol purple (MFCP) was tested in mice treated with acetazolamide to cause urinary alkalinization, and Cerenkov images were compared with PET images.

RESULTS

Fluorinated pH indicators were produced with radiochemical yields of 4%-11% at greater than 90% purity. Selective Cerenkov quenching was observed intramolecularly with difluorophenol red or monofluorocresol purple and intermolecularly in phenolphthalein (18)F-FDG mixtures. The probes were selectively quenched in the bandwidth closest to the indicator's absorption maximum (λmax) at pHs above the indicator pKa (the negative logarithm of the acid dissociation constant). Addition of acid or base to the probes resulted in reversible switching from unquenched to quenched emission. In vivo, the bladders of acetazolamide-treated mice exhibited a wavelength-dependent quenching in Cerenkov emission, with the greatest reduction occurring near the λmax. Ratiometric imaging at 2 wavelengths showed significant decreases in Cerenkov emission at basic pH and allowed the estimation of absolute pH in vivo.

CONCLUSION

We have created contrast agents that selectively quench photons emitted during Cerenkov radiation within a given bandwidth. In the presence of a functional dye, such as a pH indicator, this selective quenching allows for a functional determination of pH in vitro and in vivo. This method can be used to obtain functional information from radiolabeled probes using multimodal imaging. This approach allows for the imaging of nonfluorescent chromophores and is generalizable to any functional dye that absorbs at suitable wavelengths.

Synthesis of 18F-labeled phenolphthalein and naphtholphthalein

The fluorination of phenolphthalein and naphtholphthalein was performed with diluted fluorine gas under acidic conditions. For both compounds we observed an electrophilic fluorination into ortho position to the hydroxyl group. Through the use of this reaction we synthesized and characterized mono-and difluorinated derivatives of phenolphthalein and naphtholphthalein. The compounds were also prepared in the 18F labeled form, which are usable as a new type of probe for in vivo pH measurement in biological objects using Cerenkov imaging or combination of light absorption and PET.

Synthesis and characterization of fluorinated derivatives of cresolsulfonphthalein

The reaction between dilute fluorine gas and cresolsufonphthalein in acetic acid was investigated. The mono-, di-, and trifluorinated cresolsufonphthalein derivatives were isolated and characterized. These compounds possessed the properties of pH indicators with biologically relevant pK_a values (6.4-7.5) and the absorption maxima of the basic forms at 582-592 nm. This method can be used for synthesis of positron-emitting ¹⁸F-labeled pH indicators with potential application for non-invasive in vivo pH measurement in biological objects.

Biodistribution and dosimetry of (18)F-EF5 in cancer patients with preliminary comparison of (18)F-EF5 uptake versus EF5 binding in human glioblastoma

PURPOSE

The primary purpose of this study was to assess the biodistribution and radiation dose resulting from administration of (18)F-EF5, a lipophilic 2-nitroimidazole hypoxia marker in ten cancer patients. For three of these patients (with glioblastoma) unlabeled EF5 was additionally administered to allow the comparative assessment of (18)F-EF5 tumor uptake with EF5 binding, the latter measured in tumor biopsies by fluorescent anti-EF5 monoclonal antibodies.

METHODS

(18)F-EF5 was synthesized by electrophilic addition of (18)F(2) gas, made by deuteron bombardment of a neon/fluorine mixture in a high-pressure gas target, to an allyl precursor in trifluoroacetic acid at 0° then purified and administered by intravenous bolus. Three whole-body images were collected for each of ten patients using an Allegro (Philips) scanner. Gamma counts were determined in blood, drawn during each image, and urine, pooled as a single sample. PET images were analyzed to determine radiotracer uptake in several tissues and the resulting radiation dose calculated using OLINDA software and standard phantom. For three patients, 21 mg/kg unlabeled EF5 was administered after the PET scans, and tissue samples obtained the next day at surgery to determine EF5 binding using immunohistochemistry techniques (IHC).

RESULTS

EF5 distributes evenly throughout soft tissue within minutes of injection. Its concentration in blood over the typical time frame of the study (∼3.5 h) was nearly constant, consistent with a previously determined EF5 plasma half-life of ∼13 h. Elimination was primarily via urine and bile. Radiation exposure from labeled EF5 is similar to other (18)F-labeled imaging agents (e.g., FDG and FMISO). In a de novo glioblastoma multiforme patient, focal uptake of (18)F-EF5 was confirmed by IHC.

CONCLUSION

These results confirm predictions of biodistribution and safety based on EF5's characteristics (high biological stability, high lipophilicity). EF5 is a novel hypoxia marker with unique pharmacological characteristics allowing both noninvasive and invasive measurements.

The radiation response of cells from 9L gliosarcoma tumours is correlated with [F18]-EF5 uptake

PURPOSE

Tumour hypoxia affects cancer biology and therapy-resistance in both animals and humans. The purpose of this study was to determine whether EF5 ([2-(2-nitro-1-H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide]) binding and/or radioactive drug uptake correlated with single-dose radiation response in 9L gliosarcoma tumours.

MATERIALS AND METHODS

Twenty-two 9L tumours were grown in male Fischer rats. Rats were administered low specific activity (18)F-EF5 and their tumours irradiated and assessed for cell survival and hypoxia. Hypoxia assays included EF5 binding measured by antibodies against bound-drug adducts and gamma counts of (18)F-EF5 tumour uptake compared with uptake by normal muscle and blood. These assays were compared with cellular radiation response (in vivo to in vitro assay). In six cases, uptake of tumour versus muscle was also assayed using images from a PET (positron emission tomography) camera (PENN G-PET).

RESULTS

The intertumoural variation in radiation response of 9L tumour-cells was significantly correlated with uptake of (18)F-labelled EF5 (i.e., including both bound and non-bound drug) using either tumour to muscle or tumour to blood gamma count ratios. In the tumours where imaging was performed, there was a significant correlation between the image analysis and gamma count analysis. Intertumoural variation in cellular radiation response of the same 22 tumours was also correlated with mean flow cytometry signal due to EF5 binding.

CONCLUSION

To our knowledge, this is the first animal model/drug combination demonstrating a correlation of radioresponse for tumour-cells from individual tumours with drug metabolism using either immunohistochemical or non-invasive techniques.

Catalysis of fluorine addition to double bond: an improvement of method for synthesis of (18)F PET agents

Catalysis for reaction of electrophilic fluorine addition to a fluorinated double bond is described. The presence of small amounts of iodine, bromine, or boron trifluoride increases an overall product yield and degree of fluorine incorporation, making the reaction a more efficient method for preparation of (18)F positron emission tomography agents. A possible mechanism of catalytic action of iodine is discussed.

Mutation in G6PD gene leads to loss of cellular control of protein glutathionylation: mechanism and implication

More than 400 million people are susceptible to oxidative stress due to glucose-6-phosphate dehydrogenase (G6PD) deficiency. Protein glutathionylation is believed to be responsible for loss of protein function and/or cellular signaling during oxidative stress. To elucidate the implications of G6PD deficiency specifically in cellular control of protein glutathionylation, we used hydroxyethyldisulfide (HEDS), an oxidant which undergoes disulfide exchange with existing thiols. G6PD deficient (E89) cells treated with HEDS showed a significant increase in protein glutathionylation compared to wild-type (K1) cells. In order to determine whether increase in global protein glutathionylation by HEDS leads to loss of function of an important protein, we compared the effect of HEDS on global protein glutathionylation with that of Ku protein function, a multifunctional DNA repair protein, using a novel ELISA. E89 cells treated with HEDS showed a significant loss of Ku protein binding to DNA. Cellular protein thiol and GSH, whose disulfide is involved in protein glutathionylation, were decreased by HEDS in E89 cells with no significant effect in K1 cells. E89 cells showed lower detoxification of HEDS, that is, conversion of disulfide HEDS to free sulfhydryl mercaptoethanol (ME), compared to K1 cells. K1 cells maintained their NADH level in the presence of HEDS but that of E89 cells decreased by tenfold following a similar exposure. NADPH, a cofactor required to maintain reduced form of the thiols, was decreased more in E89 than K1 cells. The specific role of G6PD in the control of such global protein glutathionylation and Ku function was further demonstrated by reintroducing the G6PD gene into E89 (A1A) cells, which showed a normal phenotype.

Detection of Reactive Oxygen Species via Endogenous Oxidative Pentose Phosphate Cycle Activity in Response to Oxygen Concentration

The oxidative pentose phosphate cycle (OPPC) is necessary to maintain cellular reducing capacity during periods of increased oxidative stress. Metabolic flux through the OPPC increases stoichiometrically in response to a broad range of chemical oxidants, including those that generate reactive oxygen species (ROS). Here we show that OPPC sensitivity is sufficient to detect low levels of ROS produced metabolically as a function of the percentage of O2. We observe a significant decrease in OPPC activity in cells incubated under severe and moderate hypoxia (ranging from <0.01 to 4% O2), whereas hyperoxia (95% O2) results in a significant increase in OPPC activity. These data indicate that metabolic ROS production is directly dependent on oxygen concentration. Moreover, we have found no evidence to suggest that ROS, produced by mitochondria, are needed to stabilize hypoxia-inducible factor 1alpha (HIF-1alpha) under moderate hypoxia. Myxothiazol, an inhibitor of mitochondrial electron transfer, did not prevent HIF-1alpha stabilization under moderate hypoxia. Moreover, the levels of HIF-1alpha that we observed after exposure to moderate hypoxia were comparable between rho0 cells, which lack functional mitochondria, and the wild-type cells. Finally, we find no evidence for stabilization of HIF-1alpha in response to the non-toxic levels of H2O2 generated by the enzyme glucose oxidase. Therefore, we conclude that the oxygen dependence of the prolyl hydroxylase reaction is sufficient to mediate HIF-1alpha stability under moderate as well as severe hypoxia.

Noninvasive imaging of tumor hypoxia in rats using the 2-nitroimidazole 18F-EF5

Tumor hypoxia is an important prognostic indicator for cancer therapy outcome. EF5 [2-(2-nitro-1[ H]-imidazol-1-yl)- N-(2,2,3,3,3-pentafluoropropyl)-acetamide] has been employed to measure tumor hypoxia in animals and humans using immunohistochemical methods. EF5 is a lipophilic molecule designed to have a very uniform biodistribution, a feature of obvious benefit for use in PET imaging. The present study represents the first demonstration of noninvasive PET imaging of rat tumors using fluorine-18 labeled EF5. Because of the small tumor size, partial volume effects may result in underestimation of concentration of the compound. Therefore, validation of the PET data was performed by gamma counting of the imaged tissue. The tumor models studied were the Morris 7777 (Q7) hepatoma (n=5) and the 9L glioma (n=2) grown subcutaneously in rats. Our previous studies have demonstrated that early passage 9L tumors are not severely hypoxic and that Q7 tumors are characterized by heterogeneous regions of tumor hypoxia (i.e., Q7 tumors are usually more hypoxic than early passage 9L tumors). The seven rats were imaged in the HEAD Penn-PET scanner at various time points after administration of 50-100 micro Ci (18)F-EF5 in 30 mg/kg carrier nonradioactive EF5. The carrier was used to ensure drug biodistribution comparable to prior studies using immunohistochemical methods. (18)F-EF5 was excreted primarily via the urinary system. Images obtained 10 min following drug administration demonstrated that the EF5 distributed evenly to all organ systems, including brain. Later images showed increased uptake in most Q7 tumors compared with muscle. Liver uptake remained relatively constant over the same time periods. Tumor to muscle ratios ranged from 0.82 to 1.73 (based on PET images at 120 min post injection) and 1.47 to 2.95 (based on gamma counts at approximately 180 min post injection). Tumors were easily visible by 60 min post injection when the final tumor to muscle ratios (based on gamma counts) were greater than 2. Neither of the 9L tumors nor the smallest Q7 tumor met this criterion, and these tumors were not seen on the PET images. These preliminary results suggest that (18)F-EF5 is a promising agent for noninvasive assessment of tumor hypoxia. Plans are underway to initiate a research project to determine the safety and preliminary evidence for the efficacy of this preparation in patients with brain tumors.

[18F]-EF5, a marker for PET detection of hypoxia: synthesis of precursor and a new fluorination procedure

There is a great deal of clinical and experimental interest in determining tissue hypoxia using non-invasive imaging methods. We have developed EF5, 2-(2-nitro-1[H]-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide, with both invasive and non-invasive hypoxia detection in mind. EF5 and other 2-nitroimidazoles are used to detect hypoxia, because the rate of their bioreductive metabolism is inversely dependent on oxygen partial pressure. Such metabolism leads to the formation of covalent adducts within the metabolizing cells. Previously, we have described the invasive detection of these adducts by highly specific monoclonal antibodies after tissue biopsy. In this report, we demonstrate the synthesis of 18F-labeled EF5, [18F]-2-(2-nitro-1[H]-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide, in greater than 10% yield by direct fluorination of the newly synthesized precursor 2-(2-nitro-1[H]-imidazol-1-yl)-N-(2,3,3-trifluoroallyl)-acetamide by [18F]-F2 in trifluoroacetic acid. Our objective was to optimize the electrophilic fluorination of the fluorinated alkene bond with fluorine gas, a new method of 18F-labeling of polyfluorinated molecules. Previous biodistribution studies in mice have demonstrated uniform access of EF5 to all tissues with bioelimination dominated by renal excretion. When [18F]-EF5 was injected into a rat followed by urine collection and analysis, we found no detectable metabolism to other radioactive compounds. Thus, [18F]-EF5 should be well suited for use as a non-invasive hypoxia marker with detection using positron emission tomography (PET).

Noninvasive detection of tumor hypoxia using the 2-nitroimidazole [18F]EF1

The noninvasive assessment of tumor hypoxia in vivo is under active investigation because hypoxia has been shown to be an important prognostic factor for therapy resistance. Various nuclear medicine imaging modalities are being used, including PET imaging of 18F-containing compounds. In this study, we report the development of 18F-labeled EF1 for noninvasive imaging of hypoxia. EF1 is a 3-monofluoro analog of the well-characterized hypoxia marker EF5, 2(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)acetami de, which has been used to detect hypoxia in tumor and nontumor systems using immunohistochemical methods.

METHODS

We have studied 2 rat tumor types: the hypoxic Morris 7777 (Q7) hepatoma and the oxic 9LF glioma tumor, each grown in subcutaneous sites. PET studies were performed using a pharmacological dose of nonradioactive carrier in addition to [18F]EF1 to optimize and assess drug biodistribution. After PET imaging of the tumor-bearing rats, tissues were obtained for gamma-counting of the 18F in various tissues and immunohistochemical detection of intracellular drug adducts in tumors. In one pair of tumors, Eppendorf needle electrode studies were performed.

RESULTS

[18F]EF1 was excreted dominantly through the urinary tract. The tumor-to-muscle (T/M) ratio of [18F]EF1 in the Q7 tumors was 2.7 and 2.4 based on PET studies and 2.1, 2.5, and 3.0 based on gamma-counting of the tissues (n = 3). In contrast, the T/M ratio of [18F]EF1 in the 9LF glioma tumor was 0.8 and 0.5 based on PET studies and 1.0, 1.2, and 1.4 based on gamma-counting of the tissues (n = 3). Immunohistochemical analysis of drug adducts for the two tumor types agreed with the radioactivity analysis. In the Q7 tumor, substantial heterogeneous binding was observed throughout the tumor, whereas in the 9LF tumor minimal binding was found.

CONCLUSION

[18F]EF1 is an excellent radiotracer for noninvasive imaging of tumor hypoxia.

Synthesis of new hypoxia markers EF1 and [18F]-EF1

We report on the preparation of a hypoxia marker 2-(2-nitroimidazol-1[H]-yl)-N-(3-fluoropropyl)acetamide (EF1) and its 18F analog, 2-(2-nitroimidazol-1[H]-yl)-N- (3-[18F]fluoropropyl)acetamide ([18F]-EF1). Two methods for the preparation of 3-fluoropropylamine, the EF1 side chain, are described. [18F]-EF1 was prepared with a radiochemical yield of 2% by nucleophilic substitution of bromine in 2-(2-nitroimidazol-1[H]-yl)-N-(3-bromopropyl)acetamide (EBr1) by carrier-added 18F in DMSO at 120 degrees C. Our results demonstrate the preparation of clinically relevant amounts of [18F]-EF1 for use as a non-invasive hypoxia marker with detection using positron emission tomography (PET).

Photofrin-mediated photodynamic therapy induces vascular occlusion and apoptosis in a human sarcoma xenograft model

Photodynamic therapy (PDT) involves light activation of a photosensitizer, resulting in oxygen-dependent, free radical-mediated cell death. Little is known about the efficacy of PDT in treating human sarcomas, despite an ongoing clinical trial treating i.p. sarcomatosis. The present study evaluates PDT treatment of a human sarcoma xenograft in nude mice and explores the mechanism of PDT-mediated antitumor effect. Athymic nude mice, 6-8 weeks of age, were s.c. injected with 5 x 10(6) cells of the A673 human sarcoma cell line. Tumors were allowed to grow to a diameter of about 10 mm. Photofrin (PF), 10 mg/kg, was injected by tail vein, and 24 h later, 630 nm light was delivered to the tumor with fluences of 50, 100, 150, or 300 J/cm2 at a fluence rate of 250 mW/cm2. To assess the efficacy of PDT in the treatment of sarcomas, photosensitizer uptake/retention studies and dose-response studies were performed. Studies carried out to determine the mechanism of tumor response included tumor temperature measurements before, during, and after treatment; tumor vascular perfusion studies with laser Doppler; electron microscopic analysis of tumor sections for vascular occlusion; and analysis of tumor cryosections for endothelial cell damage, apoptosis, and necrosis. At all time points of analysis, photosensitizer levels were greater in tumor than in muscle. Dose-response studies showed that at 100 J/cm2, five of six mice had a complete response to treatment, one of six had a partial response, and no deaths occurred. Temperature measurements indicated that thermal injury did not contribute to tumor response. Vascular perfusion studies demonstrated a significant reduction in blood flow as early as 6 h after PDT. Electron micrographs revealed erythrostasis in tumor microvessels starting as early as 2 h after treatment and complete occlusion of blood vessels by 12 h. Starting as early as 4 h after PDT, apoptosis first appeared in endothelial cells lining the occluded blood vessels and became more widespread at later time points. PDT is an effective treatment for this human sarcoma xenograft in nude mice. The mechanism of tumor destruction in this model appears to be vascular damage with initial apoptosis in tumor endothelial cells and delayed tumor cell apoptosis. This therapy may be valuable in the treatment of patients with sarcomatosis.

Mechanism of copper-catalyzed autoxidation of cysteine

The kinetics of copper-catalyzed autoxidation of cysteine and its derivatives were investigated using oxygen consumption, spectroscopy and hydroxyl radical detection by fluorescence of a coumarin probe. The process has complex two-phase kinetics. During the first phase a stoichiometric amount of oxygen (0.25 moles per mole of thiol) is consumed without production of hydroxyl radicals. In the second reaction phase excess oxygen is consumed in a hydrogen peroxide-mediated process with significant *OH production. The reaction rate in the second phase is decreased for cysteine derivatives with a free aminogroup and increased for compounds with a modified aminogroup. The kinetic data suggest the catalytic action of copper in the form of a cysteine complex. The reaction mechanism consists of two simultaneous reactions (superoxide-dependent and peroxide-dependent) in the first phase, and peroxide-dependent in the second phase. The second reaction phase begins after oxidation of free thiol. This consists of a Fenton-type reaction between cuprous-cysteinyl complex and following oxidation of cysteinyl radical to sulfonate with the consumption of excessive oxygen and significant production of hydroxyl radicals.

The measurement of bioreductive capacity of tumor cells using methylene blue

PURPOSE

Methylene blue (MB) can be used as an intracellular electron acceptor. The purpose of this study was to demonstrate the usefulness of MB for the determination of total bioreductive capacity of cell suspensions.

METHODS AND MATERIALS

We measured oxygen consumption by Clark electrode and pentose cycle activity by release of 14CO2 from 1-14C-glucose.

RESULTS

Methylene blue catalyzes the reaction of intracellular reductants NADPH, NADH, and reduced glutathione (GSH) with oxygen, causing the production of hydrogen peroxide. The reaction rate correlates with the negative charge of molecule (NADPH(-4) > NADH(-2) > GSH(-1)), suggesting that reaction with positively charged oxidized MB is the limiting step of the reaction. In a cellular system MB causes the electron flow from cellular endogenous substrates to oxygen. It is activated by the disruption of the NADP+/NADPH ratio due to several processes. These are direct oxidation of NADPH and GSH, the GSH peroxidase catalyzed reaction of GSH with H2O2, followed by NADPH oxidation by oxidized glutathione (GSSG). This results in increased cellular oxygen consumption and stimulation of the oxidative limb of pentose cycle (PC) in the presence of MB. The cellular effect of MB differs from other electron accepting drugs. Diamide and tert-butylhydroperoxide act as direct oxidants, while MB is an electron carrier to oxygen. Accordingly, MB shows the highest effect on PC activation and oxygen consumption.

CONCLUSIONS

Our results indicate that MB may be used for the determination of the total bioreductive capacity of the cells, measured by oxygen consumption and PC activation.

Autoxidation of ferrous ion complexes: a method for the generation of hydroxyl radicals

The kinetics of the production of hydroxyl radicals during the autoxidation of ferrous ion complexes at pH 7.4 was investigated using the fluorescent probe coumarin-3-carboxylic acid. Polyphosphates (tri- and tetrapolyphosphate and their adenosine derivatives), citrate, and acetic derivatives of ethyleneamine ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), triethylenetetraminehexaacetic acid (TTHA), ethylenediamine-(N,N')-diacetic acid (EDDA) and nitrilotriacetic acid (NTA) were used as iron chelators. Production of hydroxyl radical in these chemical systems was compared with that by radiation to determine the equivalent doses of radiation that produced equal amounts of .OH. The amount of .OH formed during ferrous ion autoxidation is determined by the concentration of the complex, its structure and the radical scavenging by the chelator molecule. Production of .OH for homologous ethylenamine acetates increases with increased complex stability: NTA < EDDA < TTHA < EDTA < DTPA. The radiation dose equivalence for 0.1 mM complexes increased from 5 Gy for NTA to 25 Gy for DTPA. The radiation dose equivalence for polyphosphates was 15 Gy for tripolyphosphate and 32 Gy for tetrapolyphosphate. The dose equivalences for adenosine phosphates are lower, 5 Gy for ATP and 10 Gy for adenosine tetraphosphate, due to intramolecular .OH scavenging. The rate of generation of .OH shows an inverse correlation with the charge of the ferrous ion complex, varying from 2 cGy/s for DTPA to 1.2 Gy/s for EDTA. The data presented indicate the usefulness of autoxidation of ferrous ion complexes for generation of .OH in chemical systems. The ability to control the amount and the rate of production of .OH may prove useful for examining the cytotoxic effects of .OH generated in biological systems.

Mechanism of copper-catalyzed oxidation of glutathione

The mechanism of copper-catalyzed glutathione oxidation was investigated using oxygen consumption, thiol depletion, spectroscopy and hydroxyl radical detection. The mechanism of oxidation has kinetics which appear biphasic. During the first reaction phase a stoichiometric amount of oxygen is consumed (1 mole oxygen per 4 moles thiol) with minimal .OH production. In the second reaction phase, additional (excess) oxygen is consumed at an increased rate and with significant hydrogen peroxide and .OH production. The kinetic and spectroscopic data suggest that copper forms a catalytic complex with glutathione (1 mole copper per 2 moles glutathione). Our proposed reaction mechanism assumes two parallel processes (superoxide-dependent and peroxide-dependent) for the first reaction phase and superoxide-independent for the second phase. Our current results indicate that glutathione, usually considered as an antioxidant, can act as prooxidant at physiological conditions and therefore can participate in cellular radical damage.

The generation of hydroxyl radicals in the reaction of molecular oxygen with polyphosphate complexes of ferrous ion

The reaction of Fe2+ with molecular oxygen (autoxidation) was investigated in 20 mM phosphate buffer (pH 7.4) at 37 degrees C using a fluorescent OH probe, coumarin-3-carboxylic acid. The autoxidation of unchelated Fe2+ produces OH radicals. Polyphosphatic chelators (pyrophosphate and tri- and tetrapoly phosphate) enhanced the generation of radicals. This effect was explained by an alteration of the reaction mechanism. The two-electron reduction of the oxygen molecule and the generation of hydrogen peroxide intermediates are the major reactions during Fe2+ autoxidation. The polyphosphatic complexes of ferrous ion reduce molecular oxygen and reactive oxygen intermediates by a one-electron mechanism. The chelation of ferrous ion increases the generation of the superoxide radical and production of OH during ferrous ion autoxidation and in the Fenton reaction. The results consider the ferrous ion-polyphosphate system as a convenient model for the generation of hydroxyl radical in biological systems.

Effect of purine nucleoside phosphates on OH-radical generation by reaction of Fe2+ with oxygen

The influence of various purine nucleotides, nucleosides and nucleoside phosphates on the generation of OH-radicals by the reaction of Fe2+ with oxygen was investigated. Coumarin-3-carboxylic acid was used as a fluorescent detector of OH.. Nucleoside triphosphates caused the enhancement of OH. production due to chelation of ferrous ion by the phosphate moiety. About 30% of produced OH. are intramolecularly scavenged by the nucleoside moiety of the chelator molecule. Nucleoside diphosphates cause a slight enhancement of OH. yield. Nucleotides, nucleosides and nucleoside monophosphates decrease the OH. production. Rate constants of reaction between OH. and nucleoside derivatives were determined from the competitive scavenging of OH radicals, produced by oxidation of Fe(2+)-EDTA complex. Derivatives of guanosine and xanthine are more efficient scavengers in comparison to adenine and inosine. Phosphate groups do not affect the constant of reaction of nucleoside with OH.. Our results suggest that the yield of OH. in the presence of the nucleotide derivatives is determined by chelation of ferrous with polyphosphates and preferential OH. scavenging by the organic portion of molecule. We propose that the generation of active oxygen intermediates in the reaction between nucleoside triphosphate complexes of iron and molecular oxygen is involved in iron-related cellular injury.

Mechanism of production of hydroxyl radicals in the copper-catalyzed oxidation of dithiothreitol

We have undertaken detailed studies of the mechanisms involved in the production of OH radicals during the copper-catalyzed oxidation of dithiothreitol (DTT). Most of these studies were conducted in real time, detecting .OH based on its reaction with coumarin-3-carboxylic acid to produce the fluorescent derivative 7-hydroxycoumarin-3-carboxylic acid (7-OHCCA). Studies of the kinetics of oxidizing DTT show that production of .OH occurs in two stages: an initial lag period during which there is little production of 7-OHCCA, and a second reaction phase during which there is more rapid generation of .OH. The duration of the initial reaction period depends on the concentrations of both DTT and Cu2+. During this initial stage, oxygen consumption is high, although an increasing concentration of Cu2+ decreases the oxygen consumption. The rate of production of .OH during the second phase of the reaction depends on the concentration of Cu2+, but little oxygen is consumed. A mechanism is proposed whereby a Cu2+-DTT complex is formed and catalyzes oxidation of free DTT via formation of an oxygen-containing intermediate. The second phase of the reaction begins after complete oxidation of free DTT and involves production of O2.- and H2O2 followed by generation of .OH via reduction of H2O2 by cuprous ion.