Effect of L-buthionine sulfoximine on the radiation response of human renal carcinoma cell lines

BACKGROUND

Renal cell carcinoma (RCC) is a progressive and relatively radioresistant disease. Currently, no data are available on the in vitro radiobiologic characterization of renal tumor cells to the authors' knowledge.

METHODS

Two RCC were cultured from specimens from previously untreated patients after either surgical resection of the primary tumor or from the malignant ascites. These two cell lines were characterized with respect to cytogenetic abnormalities, gamma radiation survival response, intracellular levels of glutathione and its related detoxification enzymes, and the effect of glutathione depletion on radiation toxicity.

RESULTS

The two RCC grew as adherent monolayer cultures with a median doubling time of 29 hours and 37 hours, respectively. Histopathologic analysis of the tumor cells grown in the renal capsule of the athymic mice confirmed their epithelial neoplastic growth. Both cell lines were aneuploid (range, 65-100 chromosomes) and had several marker chromosomes, including those derived from chromosomes 3, 7, and 11. In vitro radiation survival analysis indicated the relative radioresistance (RR; Do, 2.35 Gy) and relative radiosensitivity (RS; Do, 1.42 Gy), respectively, of these tumor cell lines. The levels of intracellular glutathione (GSH) were higher in the RR cells compared with the RS cells. The enzymatic activities of GSH S-transferase, GSH reductase, and the levels of GSH peroxidase and superoxide dismutase were elevated in the RS cells compared with the RR cells. L-Buthionine sulfoximine (BSO) treatment (concentration, 20 microM, applied for 17 hours) resulted in 77% and 63% GSH depletion compared with the untreated RR and RS cells, respectively. Pretreatment with higher concentration of BSO (50 microM for 17 hours) caused a modest radiosensitization of the RR cells (Do, 1.78 Gy).

CONCLUSIONS

RCC have a differential pattern of radiosensitivity. BSO treatment causes moderate radiosensitization of the relatively radioresistant renal tumor cells.

The effect of high- and low-intensity warm-up on the physiological responses to a standardized swim and tethered swimming performance

This investigation was conducted to determine the effect of high- and low-intensity warm-ups on physiological responses, lactate accumulation, and high-intensity freestyle and tethered swimming performance. Ten male collegiate swimmers were tested for maximal oxygen uptake (VO2 max) followed by two series of three warm-up protocols performed in a randomized order at least 2 days apart. The warm-up protocols were: (1) no warm-up (NWU), (2) a 366-m swim at 70% VO2 max (LWU) and (3) four 46-m swims at 1-min intervals at a speed corresponding to 110% VO2 max (HWU). Five minutes after each warm-up in the first series, the swimmers swam a 183-m standardized freestyle swim at a velocity corresponding to 110% VO2 max, and 5 min after each warm-up in the second series the swimmers completed a tethered swim to exhaustion with a weight attached to the tether to elicit fatigue at about 2 min. Three minutes after each warm-up and 3 min after each standardized and tethered swim, a finger-prick blood sample for lactate measurement was obtained. Heart rate and VO2 were also measured during the warm-up and the standardized and tethered swims. The performance times in the tethered swim were not significantly different between the three conditions (116.8 +/- 46.8, 137 +/- 53.3 and 122.94 +/- 37.2 s for the NWU, LWU and HWU, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Building integrated systems--the holographic organization

Organized delivery systems will be the vehicle for addressing cost, technology, quality, chronic illness, and information management issues in the context of caring for defined populations.

The management of tracheal rupture using bilateral bronchial intubation

The management of a patient who required positive pressure ventilation following pharyngolaryngo-oesophagectomy during which tracheal injury was sustained is described. Ventilation with a tracheal tube resulted in a massive pneumoperitoneum. Bilateral bronchial intubation was employed with success.

Diffusion of new technologies in the treatment of the Medicare population. Implications for patient access and program expenditures

Patterns of diffusion were analyzed for three pairs of technologies: one of each pair represents a very recent innovation, and the other is more established. Tremendous growth in utilization from 1985-89 was documented, primarily due to more patients obtaining access to the technologies. Nevertheless, disturbing differences in levels of use remain between black and white patients.

Targetted phototherapy with sensitizer-monoclonal antibody conjugate and light

Photodynamic therapy (PDT) was performed in vitro and in vivo using monoclonal antibody conjugated to hematoporphyrin (HP). The antibody (45-2D9) recognized a cell surface glycoprotein on cells derived from NIH 3T3 cells which were transformed with the ras oncogene (45-342). Radionuclide imaging with either In111 or I125 chelated to 45-2D9 or the isotype identical (IgG1) antibody MOPPC-21 revealed selectivity of 45-2D9 for 45-342 flank tumours in nude mice, and minimal targetting for a 45-342 clone which did not express the cell surface glycoprotein. The 45-2D9-HP conjugate resulted in selective killing of the 45-342 line compared with the parent line in vitro. At HP concentrations of 76 micrograms ml-1, the 45-2D9-HP conjugate resulted in significantly more long-term cures of PDT treated flank tumours compared with free HP at the same concentration. 45-2D9 alone had no effect on tumour growth. The antibody-HP conjugate resulted in significantly less local toxicity compared with standard Photofrin II PDT, and also achieved a greater number of long-term cures. This 'photoimmunotherapy' demonstrates the ability to treat established tumours with greater efficacy and decreased morbidity, probably due to specific sensitizer targetting which allows normal surrounding tissue to be spared upon illumination.

Adenosine-induced suppression of synaptic responses and the initiation and expression of long-term potentiation in the CA1 region of the hippocampus

The results of several previous studies have suggested that pretreatment with adenosine can block the induction of long-term potentiation (LTP), although other studies have found no effect of adenosine on the induction of LTP. The interaction of adenosine with the induction of LTP in the rat hippocampal slice was investigated. Inhibition of synaptic responses by adenosine either prior to or immediately after high-frequency or theta-burst stimulation did not affect LTP measured after washout of the adenosine. The only conditions under which adenosine blocked the development of LTP was when it was given 3-5 minutes prior to the stimulation train. To understand how it was possible to induce LTP, during the period 1-3 minutes following adenosine when synaptic responses were virtually eliminated, evoked responses during the 100 Hz stimulation train were recorded. Although synaptic responses to low-frequency stimulation were virtually eliminated by adenosine, they reappeared during high-frequency stimulation. These results suggest that although adenosine can depress synaptic responses, an increase in neurotransmission during a high-frequency train can partially overcome this effect of adenosine, and the hypothesis that adenosine can selectively block LTP is not supported.

Nitroxide-mediated protection against X-ray- and neocarzinostatin-induced DNA damage

The stable free radical Tempol (4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy) has been shown to protect against X-ray-induced cytotoxicity and hydrogen peroxide- or xanthine oxidase-induced cytotoxicity and mutagenicity. The ability of Tempol to protect against X-ray- or neocarzinostatin (NCS)-induced mutagenicity or DNA double-strand breaks (dsb) was studied in Chinese hamster cells. Tempol (50 mM) provided a protection factor of 2.7 against X-ray-induced mutagenicity in Chinese hamster ovary (CHO) AS52 cells, with a protection factor against cytotoxicity of 3.5. Using the field inversion gel electrophoresis technique of measuring DNA dsb, 50 mM Tempol provides a threefold reduction in DNA damage at an X-ray dose of 40 Gy. For NCS-induced damage, Tempol increased survival from 9% to 80% at 60 ng/mL NCS and reduced mutation induction by a factor of approximately 3. DNA dsb were reduced by a factor of approximately 7 at 500 ng/mL NCS. Tempol is representative of a class of stable nitroxide free radical compounds that have superoxide dismutase-mimetic activity, can oxidize metal ions such as ferrous iron that are complexed to DNA, and may also detoxify radiation-induced organoperoxide radicals by competitive scvenging. The NCS chromophore is reduced by sulfhydryls to an active form. Electron spin resonance (ESR) spectroscopy shows that 2-mercaptoethanol-activated NCS reacts with Tempol 3.5 times faster than does unactivated NCS. Thus, Tempol appears to inactivate the NCS chromophore before a substantial amount of DNA damage occurs.

Identification of nitroxide radioprotectors

The nitroxide Tempol, a stable free radical, has recently been shown to protect mammalian cells against several forms of oxidative stress including radiation-induced cytotoxicity. To extend this observation, six additional water-soluble nitroxides with different structural features were evaluated for potential radioprotective properties using Chinese hamster V79 cells and clonogenic assays. Nitroxides (10 mM) were added 10 min prior to radiation exposure and full radiation dose-response curves were determined. In addition to Tempol, five of the six nitroxides afforded in vitro radioprotection. The best protectors were found to be the positively charged nitroxides, Tempamine and 3-aminomethyl-PROXYL, with protection factors of 2.3 and 2.4, respectively, compared with Tempol, which had a protection factor of 1.3. 3-Carboxy-PROXYL, a negatively charged nitroxide, provided minimal protection. DNA binding characteristics as studied by nonequilibrium dialysis of DNA with each of the nitroxides demonstrated that Tempamine and 3-amino-methyl-PROXYL bound more strongly to DNA than did Tempol. Since DNA is assumed to be the target of radiation-induced cytotoxicity, differences in protection may be explained by variabilities in affinity of the protector for the target. This study establishes nitroxides as a general class of new nonthiol radioprotectors and suggests other parameters that may be exploited to find even better nitroxide-induced radioprotection.

Partial dopamine depletion of the prefrontal cortex leads to enhanced mesolimbic dopamine release elicited by repeated exposure to naturally reinforcing stimuli

High-speed chronoamperometry was used to monitor the extracellular concentration of dopamine within the nucleus accumbens, a terminal field of the mesolimbic dopamine system, in freely behaving rats exposed daily, on 6 consecutive days, to one of two naturally reinforcing stimuli; a highly palatable food or sex-related olfactory cues. The animals either were intact or had previously received microinjections of 6-hydroxydopamine into prefrontal cortex to lesion dopamine terminals. Food reliably elicited increases in dopamine levels within the nucleus accumbens, and if prefrontal cortical dopamine had been depleted, the response to food increased with repeated testing. Animals exposed to the sexually relevant olfactory stimulus showed progressively enhanced dopamine release with repeated testing, and this enhancement was potentiated by prefrontal cortical dopamine depletion. These results indicate that repeated exposure to naturally reinforcing events can lead to a hyperresponsiveness of the mesolimbic dopamine system upon future activation, and suggest that the dopamine projection to prefrontal cortex exerts an indirect, inhibitory influence on mesolimbic dopamine neurotransmission.

Spin trap salvage from endotoxemia: the role of cytokine down-regulation

BACKGROUND

The spin trap alpha-phenyl-N-tert-butyl-nitrone (PBN) affords protection from the lethality of septic (lipopolysaccharide) shock. We hypothesized that PBN may work through down-regulation of the sepsis-induced cytokine cascade.

METHODS

C3H/HEN mice received 30 mg/kg lipopolysaccharide 15 minutes after pretreatment with PBN or vehicle. Animals were monitored for differences in behavior, histopathologic studies, survival, and serum levels of tumor necrosis factor (TNF-alpha), interferon-gamma (IFN-gamma), and interleukin-6 (IL-6) after lipopolysaccharide. Northern blot analyses of TNF, IFN-gamma, c-fos, and IL-6 transcripts were also performed.

RESULTS

Seventy-two-hour survival was significantly higher in the PBN-treated (59/60) compared with the saline-treated animals (13/60; p2 less than 0.005), and the PBN group exhibited a blunted endotoxemic response. TNF levels were significantly lower in the PBN-treated animals at 1 to 6 hours, whereas IFN-gamma levels were depressed at 8 hours. PBN down-regulated TNF transcription at 30 minutes, with maximum lowering of all cytokine transcripts at 6 hours. PBN depressed c-fos transcription within 15 minutes of lipopolysaccharide injection.

CONCLUSIONS

Spin trap protection from endotoxemia may be related to interruption of the cytokine network, with profound effects on transcription and protein elaboration. Such compounds may prove useful in not only sepsis but also other cytokine-free radical-related pathophysiologic alterations.

Towards an understanding of the arginine-aspartate interaction

We have made a comparison of the geometries of intra- and intermolecular arginine-aspartate interactions by extracting orientation information from protein co-ordinate data. The results show a pronounced difference, with both types of interaction preferring to form twin N-H . . . O = C hydrogen bonds, but involving different nitrogen atoms. In intramolecular interactions, the aspartate favours a "side on" geometry, forming hydrogen bonds with N epsilon and N eta 2; in the intermolecular case, however, "end on" contacts involving N eta 1 and N eta 2 of the arginine are preferred. We have used Distributed Multipole Analysis of the methylguanidinium-acetate system to model the electrostatic component of the arginine-aspartate ion pair interaction in vacuo. We find, in agreement with the experimental arginine-aspartate distribution, that side on and end on doubly N-H . . . O = C hydrogen-bonded configurations are clearly the most favourable, with the side on being marginally lower in energy. Thus, despite the many competing side-chain interactions in proteins, many arginine-aspartate pairs adopt one of the minimum electrostatic energy conformations, or one close to a minimum. Within each of the two regions (side on and end on) we find only a small energy gap between the "symmetric" doubly hydrogen-bonded and slightly displaced "staggered" structures, again in agreement with the crystal structure data. Further calculations of the total ab initio interaction energy show that this follows the electrostatic term in its orientational variation, this phenomenon of "electrostatic domination" being well known in hydrogen-bonded systems. The end on arginine nitrogen atoms are observed to be more surface-exposed than N epsilon, as demonstrated by their greater accessibilities over a large sample of proteins. This helps explain the side on and end on preferences of intra- and intermolecular interactions, respectively. We also note the effect of short sequence intervals, particularly i in equilibrium with i + 2 relationships, in forcing many intramolecular contacts to be side on.

Oxoammonium cation intermediate in the nitroxide-catalyzed dismutation of superoxide

Dismutation of superoxide has been shown previously to be catalyzed by stable nitroxide compounds. In the present study, the mechanism of superoxide (.O2-) dismutation by various five-membered ring and six-membered ring nitroxides was studied by electron paramagnetic resonance spectrometry, UV-visible spectrophotometry, cyclic voltammetry, and bulk electrolysis. Electron paramagnetic resonance signals from the carbocyclic nitroxide derivatives (piperidinyl, pyrrolidinyl, and pyrrolinyl) were unchanged when exposed to enzymatically generated .O2-, whereas, in the presence of .O2- and reducing agents such as NADH and NADPH, the nitroxides underwent reduction to their respective hydroxylamines. The reaction of 4-hydroxy-2,2,6,6-tetramethyl-1-hydroxypiperidine (Tempol-H) with .O2- was measured and, in agreement with earlier reports on related compounds, the rate was found to be too slow to be consistent with a mechanism of .O2- dismutation involving the hydroxylamine as an intermediate. Voltammetric analyses of the carbocyclic nitroxide derivatives revealed a reversible one-electron redox couple at positive potentials. In contrast, oxazolidine derivatives were irreversibly oxidized. At negative potentials, all of the nitroxides studied exhibited a broad, irreversible reductive wave. The rate of .O2- dismutation correlated with the reversible midpoint redox potential. Bulk electrolysis at positive potentials was found to generate a metastable oxidized form of the nitroxide. The results indicate that the dismutation of .O2- is catalyzed by the oxoammonium/nitroxide redox couple for carbocyclic nitroxide derivatives. In addition to the one-electron mitochondrial reduction pathway, the present results suggest the possibility that cellular bioreduction by a two-electron pathway may occur subsequent to oxidation of stable nitroxides. Furthermore, the cellular destruction of persistent spin adduct nitroxides might also be facilitated by a primary univalent oxidation.

Mesolimbic dopamine release elicited by activation of the accessory olfactory system: a high speed chronoamperometric study

Male rats were anesthetized with chloral hydrate, and extracellular dopamine concentrations in the nucleus accumbens were measured using high speed chronoamperometry. Recordings were obtained during application of air passed through bedding obtained from cages that housed estrus female rats and application of depolarizing concentrations of potassium (K+) to the accessory olfactory system. Application of bedding-impregnated air increased the electrochemical signal obtained from within the nucleus accumbens, and this response was enhanced by treatment with the DA re-uptake blocker, GBR-12909. Application of K+ to the vomeronasal nerve layer of the accessory olfactory bulb and to the accessory olfactory bulb, itself, also elicited increases in the electrochemical signal recorded from the nucleus accumbens. These results indicate that the accessory olfactory system is able to activate the mesolimbic DA system.

Serotonergic regulation of type II corticosteroid receptor binding in hippocampal cell cultures: evidence for the importance of serotonin-induced changes in cAMP levels

The development of the hypothalamic-pituitary-adrenal response to stress is profoundly altered by environmental events. One target for environmental regulation within the hypothalamic-pituitary-adrenal axis is the hippocampal type II corticosteroid (or glucocorticoid) receptor system, which mediates the negative-feedback effects of glucocorticoids on hypothalamic-pituitary-adrenal activity. Thus, adult rats handled early in life show increased hippocampal type II corticosteroid receptor density and increased sensitivity to the inhibitory effects of circulating glucocorticoids on post-stress hypothalamic-pituitary-adrenal activity. Both effects persist throughout life. The effects of handling on type II corticosteroid receptor development are, at least in part, mediated by changes in hippocampal 5-hydroxytryptamine turnover. Moreover, 5-hydroxytryptamine can regulate type II corticosteroid receptor density in cultured hippocampal cells, providing a paradigm for examining the neurochemical mechanisms by which environmental stimuli might regulate neural differentiation. In the present studies, we examined the intracellular mechanisms underlying the effects of 5-hydroxytryptamine on type II corticosteroid receptors ([3H]RU 28362 binding) in hippocampal cell cultures. cAMP, but not cGMP, levels in cultured hippocampal cells were significantly increased by the addition of 5-hydroxytryptamine to the medium. The cAMP response to 5-hydroxytryptamine was biphasic: an initial increase in cAMP levels occurred in response to nanomolar 5-hydroxytryptamine concentrations (EC50 = 7.2 nM), while a second increase was apparent at low micromolar concentrations. 5-Hydroxytryptamine also increased [3H]RU 28362 binding (EC50 = 4.5 nM) with a maximal effect at a concentration of 10 nM. There was no further increase in [3H]RU 28362 binding even with higher, micromolar concentrations of 5-hydroxytryptamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Tempol, a stable free radical, is a novel murine radiation protector

Nitroxide compounds are stable free radicals which were previously investigated as hypoxic cell radiosensitizers. The stable nitroxide 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (Tempol) has recently been shown to protect aerated cells in culture against superoxide generated from hypoxanthine/xanthine oxidase, hydrogen peroxide, and radiation-induced cytotoxicity and to modestly sensitive hypoxic cultured cells. To extend these observations from the cellular level to the whole animal, the toxicity, pharmacology, and in vivo radioprotective effects of Tempol were studied in C3H mice. The maximum tolerated dose of Tempol administered i.p. was found to be 275 mg/kg, which resulted in maximal Tempol levels in whole blood 5-10 min after injection. Mice were exposed to whole-body radiation in the absence or presence of injected Tempol (275 mg/kg) 5-10 min after administration. Tempol treatment provided significant radioprotection (P less than 0.0001); the dose of radiation at which 50% of Tempol-treated mice die at 30 days was 9.97 Gy, versus 7.84 Gy for control mice. Tempol represents a new class of in vivo, non-sulfur-containing radiation protectors. Given the potential for hypoxic radiosensitization and aerobic cell radioprotection, Temporal or other analogues may have potential therapeutic application.

The catecholic metal sequestering agent 1,2-dihydroxybenzene-3,5-disulfonate confers protection against oxidative cell damage

Tiron (1,2-dihydroxybenzene-3,5-disulfonate), a nontoxic chelator of a variety of metals, is used to alleviate acute metal overload in animals. It is also oxidized to the EPR-detectable semiquinone radical by various biologically relevant oxidants, such as .OH, O2-., alkyl, and alkoxyl radicals. Since Tiron reacts with potentially toxic intracellular species and is also a metal chelator, we evaluated its protective effects in V79 cells subjected to various types of oxidative damage and attempted to distinguish the protection due to direct detoxification of intracellular radicals from that resulting from chelation of redox-active transition metals. We found that Tiron protects Chinese hamster V79 cells against both O2.(-)-induced (and H2O2 via dismutation of O2.-) and H2O2-induced cytotoxicity as measured by clonogenic assays. In experiments where Tiron was incubated with V79 cells and rinsed prior to exposure to HX/XO or H2O2, cytoprotection was observed, indicating that it protects against intracellular oxidative damage. On the other hand, Tiron did not protect V79 cells against the damage caused by ionizing radiation under aerobic conditions, which is predominantly mediated by H., .OH, and hydrated electrons in a metal-independent fashion. We demonstrate also that in in vitro studies, Tiron protects supercoiled DNA from metal-mediated superoxide-dependent strand breaks. We conclude that Tiron is a potentially useful protecting agent against the lethal effects of oxidative stress and suggest that it offers protection by chelating redox-active transition metal ions, in contrast to earlier reports where the protection by this compound in cellular systems subjected to oxidative damage has been interpreted as due to radical scavenging alone.

Long-term follow-up on National Cancer Institute Phase I/II study of glioblastoma multiforme treated with iododeoxyuridine and hyperfractionated irradiation

PURPOSE

We report the results of the final phase I/II program in glioblastoma (GBM) multiforme patients using only hyperfractionated irradiation and intravenous iododeoxyuridine (IdUrd).

METHODS

For a decade we investigated halogenated pyrimidine radiosensitizers in an effort to exploit the potential for differential uptake of thymidine analogs between proliferating tumor and normal brain tissues. Trials began with bromodeoxyuridine (BrdUrd) but were changed to IdUrd when the latter proved less photosensitizing. A series of dose-escalating pilot trials led to treatment at a maximum-tolerated dose (MTD) of IdUrd of 1,000 mg/m2/d for two separate 14-day courses, one during the initial radiation field and one during the cone down. The radiotherapy also evolved over time and was hyperfractionated in all cases reported. Over 5 years we accrued 45 patients into the final hyperfractionated, 1,000 mg/m2/d scheme. We report here results on only the patients with minimum follow-up of 1 year (90% had at least 2 years of follow-up) or until death.

RESULTS

The results do not indicate a significant benefit for use of sensitizers, as compared with other contemporary and aggressive types of radiation treatment. The median survival has been 11 months, with a 2-year actuarial survival of 9%. As yet, there are no survivors at 3 years. Tumor biopsies at craniotomy showed relatively low sensitizer incorporation.

CONCLUSION

The failure of radiosensitizers combined with radiation therapy to show major benefit may be due to patient selection but appears also to be related to the combined problems of poor drug penetration/uptake into tumor, tumor-cell heterogeneity, and a high inherent cellular radioresistance of GBM.

Measurement of thymidine replacement in patients with high grade gliomas, head and neck tumors, and high grade sarcomas after continuous intravenous infusions of 5-iododeoxyuridine

Based upon the radiation sensitization properties of the halogenated pyrimidines, 5-iododeoxyuridine (IdUrd) and 5-bromodeoxyuridine, long term i.v. infusions of halogenated pyrimidines in conjunction with fractionated radiation therapy have been evaluated in the treatment of a variety of human malignancies. While clinical studies have attempted to measure the halogenated pyrimidine incorporation, few have successfully related tumor response to the incorporation of IdUrd by the tumor. The present study reports the continuous IdUrd labeling index (number of cells labeled) and the IdUrd corrected replacement (percentage of thymidine replacement in the labeled cells of the population) from the tumors of 17 patients who received continuous infusions of IdUrd (1000 mg/m2/24 h). The tumors treated included four high grade gliomas, five head and neck tumors, four high grade sarcomas, and five other tumors of varying types. Less than 25% of the cells in three of four gliomas incorporated IdUrd after 5-7-day IdUrd infusion time. Corrected replacement for the gliomas ranged from 0 to 4%. In contrast, 63-85% of the cells in the head and neck biopsies were labeled with IdUrd after 3-7-day IdUrd infusions suggesting that these large tumors (3-12 cm diameter) have a high fraction of dividing cells. Corrected replacements values for the head and neck tumor patients ranged from 2.9 to 26.3%. The high grade sarcomas also demonstrated a high percentage of IdUrd labeled cells (57-79%) with three patients having corrected replacements of 7.5-14.2%. The continuous labeling and thymidine replacement data for four patients from whom serial biopsies were taken during IdUrd infusion demonstrated both an increasing IdUrd replacement and continuous labeling index with an increasing duration of IdUrd infusion. The clinical response of both the high grade glioma and head and neck tumor patients indicate that the IdUrd replacement and labeling data may provide some important predictive information with regard to the successful use of the halogenated pyrimidines in clinical radiation trials.

The importance of peroxide and superoxide in the X-ray response

Radiation produces a number of damaging radicals as well as peroxide. The chief cellular protection against these radicals, their secondary reactants and peroxide is the cellular glutathione (GSH), GSH peroxidase, GSH-S-transferase (GSHTase), and catalase enzymes. Inhibition of cellular catalase alone does not enhance the aerobic radiation response because cellular GSH peroxidase is equally effective in reducing peroxide. However, inhibition of GSHTase, and partial inhibition of peroxidase by L-buthionine sulfoximine (LBSO)-linked GSH depletion, results in an increased aerobic radiation response. The major pathway for peroxide reduction is the GSH peroxidase. The enzyme is accountable for 70% inactivation of low peroxide concentrations. Catalase accounts for the remaining inactivation. However, it is difficult to assess the relative contributions of GSHTase and peroxidase to the inactivation of radiation-produced hydroperoxides. Our data suggest that GSH depletion results in the inhibition of cellular GSHTase before it inhibits GSH peroxidase. Therefore, part of the increased aerobic radiation response maybe due to cellular inability to reduce hydroperoxides. Peroxide is not a substrate for GSHTase. However, total inhibition of peroxidase by L-BSO plus N-ethylmaleimide (NEM) treatment maximizes the aerobic radiation response. Total inhibition of GSH-S-transferase and peroxidase would block both peroxide and hydroperoxide reduction.