Radiation damage to human erythrocytes. Relative contribution of hydroxyl and chloride radicals in N2O-saturated buffers

Radiation activates myeloperoxidase (MPO) to generate active chlorine species (ACS) via a dephosphorylation mechanism - inhibitory effect of LGM2605

BACKGROUND

Radiation exposure of tissues is associated with inflammatory cell influx. Myeloperoxidase (MPO) is an enzyme expressed in granulocytes, such as neutrophils (PMN) and macrophages, responsible for active chlorine species (ACS) generation. The present study aimed to: 1) determine whether exposure to γ-irradiation induces MPO-dependent ACS generation in murine PMN; 2) elucidate the mechanism of radiation-induced ACS generation; and 3) evaluate the effect of the synthetic lignan LGM2605, known for ACS scavenging properties.

METHODS

MPO-dependent ACS generation was determined by using hypochlorite-specific 3'-(p-aminophenyl) fluorescein (APF) and a highly potent MPO inhibitor, 4-aminobenzoic acid hydrazide (ABAH), and confirmed in PMN derived from MPO-/- mice. Radiation-induced MPO activation was determined by EPR spectroscopy and computational analysis identified tyrosine, serine, and threonine residues near MPO's active site.

RESULTS

γ-radiation increased MPO-dependent ACS generation dose-dependently in human MPO and in wild-type murine PMN, but not in PMN from MPO-/- mice. LGM2605 decreased radiation-induced, MPO-dependent ACS. Protein tyrosine phosphatase (PTP) and protein serine/threonine phosphatase (PSTP) inhibitors decreased the radiation-induced increase in ACS. Peroxidase cycle results demonstrate that tyrosine phosphorylation blocks MPO Compound I formation by preventing catalysis on H2O2 in the active site of MPO. EPR data demonstrate that γ-radiation increased tyrosyl radical species formation in a dose-dependent manner.

CONCLUSIONS

We demonstrate that γ-radiation induces MPO-dependent generation of ACS, which is dependent, at least in part, by protein tyrosine and Ser/Thr dephosphorylation and is reduced by LGM2605. This study identified for the first time a novel protein dephosphorylation-dependent mechanism of radiation-induced MPO activation.

Gamma-irradiation produces active chlorine species (ACS) in physiological solutions: Secoisolariciresinol diglucoside (SDG) scavenges ACS - A novel mechanism of DNA radioprotection

Background

Secoisolariciresinol diglucoside (SDG), the main lignan in whole grain flaxseed, is a potent antioxidant and free radical scavenger with known radioprotective properties. However, the exact mechanism of SDG radioprotection is not well understood. The current study identified a novel mechanism of DNA radioprotection by SDG in physiological solutions by scavenging active chlorine species (ACS) and reducing chlorinated nucleobases.

Methods

The ACS scavenging activity of SDG was determined using two highly specific fluoroprobes: hypochlorite-specific 3′-(p-aminophenyl) fluorescein (APF) and hydroxyl radical-sensitive 3′-(p-hydroxyphenyl) fluorescein (HPF). Dopamine, an SDG structural analog, was used for proton ¹H NMR studies to trap primary ACS radicals. Taurine N-chlorination was determined to demonstrate radiation-induced generation of hypochlorite, a secondary ACS. DNA protection was assessed by determining the extent of DNA fragmentation and plasmid DNA relaxation following exposure to ClO⁻ and radiation. Purine base chlorination by ClO⁻ and γ-radiation was determined by using 2-aminopurine (2-AP), a fluorescent analog of 6-aminopurine. Results: Chloride anions (Cl⁻) consumed >90% of hydroxyl radicals in physiological solutions produced by γ-radiation resulting in ACS formation, which was detected by ¹H NMR. Importantly, SDG scavenged hypochlorite- and γ-radiation-induced ACS. In addition, SDG blunted ACS-induced fragmentation of calf thymus DNA and plasmid DNA relaxation. SDG treatment before or after ACS exposure decreased the ClO⁻ or γ-radiation-induced chlorination of 2-AP. Exposure to γ-radiation resulted in increased taurine chlorination, indicative of ClO⁻ generation. NMR studies revealed formation of primary ACS radicals (chlorine atoms (Cl^•) and dichloro radical anions (Cl₂^−•)), which were trapped by SDG and its structural analog dopamine.

Conclusion

We demonstrate that γ-radiation induces the generation of ACS in physiological solutions. SDG treatment scavenged ACS and prevented ACS-induced DNA damage and chlorination of 2-aminopurine. This study identified a novel and unique mechanism of SDG radioprotection, through ACS scavenging, and supports the potential usefulness of SDG as a radioprotector and mitigator for radiation exposure as part of cancer therapy or accidental exposure.

Simple radiosensitizing of hypoxic tumor tissues by N2O/Br(-) mixture

The radiosensitization model of hypoxic tumor tissues based on the N2O/Br(-) mixture is described. The well-documented radiolysis of water in the presence of N2O and Br(-) ions at a low concentration supports this model. An aqueous solution saturated with N2O gas during the radiolysis generates OH radicals in a large extent. In N2O/Br- media at pH<9, Br2 is formed. Br2 hydrolyzes in an aqueous solution to form a very reactive hypobromous (HOBr) acid. Such process is described by the following chemical reaction: H2O + Br(-) + N2O + ionizing radiation (IR) --> HOBr + OH(-). In vivo formed HOBr as a long-lived product with a high biological activity induces the hypoxic tumor cell damage via many unique mechanisms. A local application or inhalation of an N2O-O2 mixture before or during the radiotherapy to enhance the saturation of tissues with N2O is a key prerequisite. Since the extracellular concentration of Br(-) ions is very low (0.02-0.05 mM), an oral or local application of NaBr should be used to shift the extracellular concentration of Br(-) ions to the mM region.

ROS production and their influence on the cellular antioxidative system in human erythrocytes incubated with daunorubicin and glutaraldehyde

This study examined the effects of daunorubicin and glutaraldehyde on some parameters of erythrocytes. The aim of the article was to present the results of research aiming to identify the level of glutaraldehyde at which the hemoglobin oxidation, externalization of phosphatidylserine and the changes in the viability (hemolysis) of erythrocytes are not statistically significant and therefore this level of glutaraldehyde can be used for the drug carriers' preparation. Glutaraldehyde was used as a crosslinking agent to enhance the uptake of the drug within red blood cells and to prevent its leakage from the cells. Fluorescence microscopy, flow cytometry and fluorimetric measurements confirmed higher levels of the drug in glutaraldehyde-treated human erythrocytes. Unfortunately, substantial damage to the red blood cells was also noted. DNR increased oxidative processes in the cell, which in turn led to an increase in the reactive oxygen species (ROS) level. When the red blood cells were also treated with glutaraldehyde, ROS production was significantly higher. We also observed loss of both the reduced and the total glutathione. Moreover the decreased activity of glutathione reductase (GR) and glucose-6-phosphate dehydrogenase (G6PD) was also observed. As hemoglobin, the erythrocytes' main component plays an essential role in the erythrocytes, the level of its oxidized form (metHb) in the erythrocytes and the phosphatidylserine exposure on the erythrocyte surface were also investigated. When higher concentrations of glutaraldehyde (0.0025-0.005%) were used for the uptake of DNR the elevated level of metHb was observed. Only at 0.0005% the level of oxidized form of Hb was within the physiological level and at that level the increase in the exposure of phosphatidylserine at the cell surface was not observed to be statistically significant. Moreover the percent of released hemoglobin was less than 1%. Based on these results it was concluded that glutaraldehyde can be used as a cross-linker between the drug (DNR) and the erythrocytes only at low concentration of about 0.0005%.