Direct effects of ionizing radiation on integral membrane proteins. Noncovalent energy transfer requires specific interpeptide interactions

Medicinally important aromatic plants with radioprotective activity

Aromatic plants are often used as natural medicines because of their remedial and inherent pharmacological properties. Looking into natural resources, particularly products of plant origin, has become an exciting area of research in drug discovery and development. Aromatic plants are mainly exploited for essential oil extraction for applications in industries, for example, in cosmetics, flavoring and fragrance, spices, pesticides, repellents and herbal beverages. Although several medicinal plants have been studied to treat various conventional ailments only a handful studies are available on aromatic plants, especially for radioprotection. Many plant extracts have been reported to contain antioxidants that scavenge free radicals produced due to radiation exposure, thus imparting radioprotective efficacy. The present review focuses on a subset of medicinally important aromatic plants with radioprotective activity.

Studies on black tea (Camellia sinensis) extract as a potential antioxidant and a probable radioprotector

Positive health effects of tea (Camellia sinensis) on a wide range of physiological problems and diseases are well known and are in part due to its copious antioxidant content. The effect of black tea extract (BTE), which is rich in polyphenolic antioxidants, against the consequences of radiation exposure has not been properly identified. The functional properties of BTE were analyzed and its radioprotective effect on V79 cells was explored in the present study. BTE scavenged free radicals and inhibited Fenton reaction-mediated 2-deoxyribose degradation and lipid peroxidation in a dose-dependent fashion, establishing its antioxidant properties. The radioprotective effects of BTE on strand break induction in pBR322 plasmid DNA were 100 % at 80 μg/ml and higher. In V79 cells, BTE was effective in decreasing the frequency of radiation-induced micronucleated cells and the yields of reactive oxygen species (ROS) and also in restoring the integrity of cellular mitochondrial membrane potential significantly. BTE exerted maximum protection against radiation-induced damage in V79 at a dose of 5 μg/ml. Due to the functional properties of BTE-flavonoids, which have been identified by HPLC, it is envisaged that the key player in radioprotection is elimination of ROS.

Amelioration of radiation-induced hematological and biochemical alterations in Swiss albino mice by Panax ginseng extract

BACKGROUND

This study was carried out to observe the radioprotective effect of Panax ginseng root extract (PGE) against radiation-induced hematological and biochemical alterations in blood and liver of mice. Materials and methods. Adult Swiss albino mice were exposed to 6 Gy gamma radiation in the presence (experimental) or absence (control) of PGE to study the quantitative and qualitative alterations in the blood and liver.

RESULTS

Radiation exposure resulted in a significant decline (P<.001) in erythrocyte count, hemoglobin (Hb), and hematocrit (Hct) in peripheral blood. Maximum changes in all the parameters were observed on day 3 after irradiation. In contrast, PGE-pretreated irradiated animals showed a significant increase in erythrocyte, Hct, and Hb values compared with irradiated controls. Furthermore, a significant elevation in lipid peroxidation level over normal was recorded in irradiated control mice, whereas this increase was considerably lesser in PGE pretreated animals. Likewise, pretreatment with PGE caused a significant increase in glutathione levels in serum as well as in liver in comparison to irradiated controls.

CONCLUSION

From this study, it is clearly evident that PGE provides protection against radiation-induced hematological and biochemical alterations in Swiss albino mice.

Protection of DNA and membranes from gamma-radiation induced damages by Centella asiatica

Objectives

The objective of the present study was to examine the ability of Centella asiatica extract to offer protection to DNA and membranes against the deleterious effects of ionizing radiation exposure.

Methods

Protection of DNA under in-vitro conditions of irradiation was estimated using plasmid relaxation assay. For in-vivo studies the extract was administered orally to mice exposed to whole-body γ-radiation. The ability of the extract to offer protection against whole-body γ-radiation exposure was analysed by performing an alkaline comet assay on mouse bone marrow cells. The extent of lipid peroxidation was estimated using the TBARS (thio-barbituric acid reacting substances) method, in order to monitor membrane damage. Radiation-induced mortality of the animals following a lethal dose of γ-radiation was also examined.

Key findings

Centella asiatica extract significantly reduced radiation-induced damage to DNA. The extent of radiation-induced mortality and lipid peroxidation was also found to be considerably reduced in animals administered with the extract.

Conclusions

Centella asiatica rendered radioprotection to DNA and membranes against radiation exposure, both in vitro and in vivo. We have earlier reported that administration of the extract can prevent a radiation-induced decline in antioxidant enzyme levels. This suggests that radioprotection by Centella asiatica extract could be mediated by mechanisms that act in a synergistic manner, especially involving antioxidant activity.

Melatonin as a radioprotective agent: a review

Melatonin (N-acetyl-5-methoxytryptamine), the chief secretory product of the pineal gland in the brain, is well known for its functional versatility. In hundreds of investigations, melatonin has been documented as a direct free radical scavenger and an indirect antioxidant, as well as an important immunomodulatory agent. The radical scavenging ability of melatonin is believed to work via electron donation to detoxify a variety of reactive oxygen and nitrogen species, including the highly toxic hydroxyl radical. It has long been recognized that the damaging effects of ionizing radiation are brought about by both direct and indirect mechanisms. The direct action produces disruption of sensitive molecules in the cells, whereas the indirect effects ( approximately 70%) result from its interaction with water molecules, which results in the production of highly reactive free radicals such as *OH, *H, and e(aq)- and their subsequent action on subcellular structures. The hydroxyl radical scavenging ability of melatonin was used as a rationale to determine its radioprotective efficiency. Indeed, the results from many in vitro and in vivo investigations have confirmed that melatonin protects mammalian cells from the toxic effects of ionizing radiation. Furthermore, several clinical reports indicate that melatonin administration, either alone or in combination with traditional radiotherapy, results in a favorable efficacy:toxicity ratio during the treatment of human cancers. This article reviews the literature from laboratory investigations that document the ability of melatonin to scavenge a variety of free radicals (including the hydroxyl radical induced by ionizing radiation) and summarizes the evidence that should be used to design larger translational research-based clinical trials using melatonin as a radioprotector and also in cancer radiotherapy. The potential use of melatonin for protecting individuals from radiation terrorism is also considered.

Regulation of FOS by different compartmental stresses induced by low levels of ionizing radiation

We irradiated different cellular compartments and measured changes in expression of the FOS gene at the mRNA and protein levels. [(3)H]Thymidine and tritiated water were used to irradiate the nucleus and the whole cell, respectively. (125)I-Concanavalin A binding was used to irradiate the cell membrane differentially. Changes in FOS mRNA and protein levels were measured using semi-quantitative RT-PCR and SDS-PAGE Western blotting, respectively. Irradiation of the nucleus or the whole cell at a dose rate of 0.075 Gy/h caused no change in the level of FOS mRNA expression, but modestly (1.5-fold) induced FOS protein after 0.5 h. Irradiation of the nucleus at a dose rate of 0.43 Gy/h induced FOS mRNA by 1.5-fold after 0.5 h, but there was no significant effect after whole-cell irradiation. FOS protein was transiently induced 2.5-fold above control levels 0.5 h after a 0. 43-Gy/h exposure of the nucleus or the whole cell. Irradiation of the cell membrane at a dose rate of 1.8 Gy/h for up to 2 h caused no change in the levels of expression of FOS mRNA or protein, but a dose rate of 6.8 Gy/h transiently increased the level of FOS mRNA 3-fold after 0.5 h. These data demonstrate the complexity of the cellular response to radiation-induced damage at low doses. The lack of quantitative agreement between the transcript and protein levels for FOS suggests a role for post-transcriptional regulation.

Antioxidative effects of melatonin in protection against cellular damage caused by ionizing radiation

Ionizing radiation is classified as a potent carcinogen, and its injury to living cells is, to a large extent, due to oxidative stress. The molecule most often reported to be damaged by ionizing radiation is DNA. Hydroxyl radicals (*OH), considered the most damaging of all free radicals generated in organisms, are often responsible for DNA damage caused by ionizing radiation. Melatonin, N-acetyl-5-methoxytryptamine, is a well-known antioxidant that protects DNA, lipids, and proteins from free-radical damage. The indoleamine manifests its antioxidative properties by stimulating the activities of antioxidant enzymes and scavenging free radicals directly or indirectly. Among known antioxidants, melatonin is a highly effective scavenger of *OH. Melatonin is distributed ubiquitously in organisms and, as far as is known, in all cellular compartments, and it quickly passes through all biological membranes. The protective effects of melatonin against oxidative stress caused by ionizing radiation have been documented in in vitro and in vivo studies in different species and in in vitro experiments that used human tissues, as well as when melatonin was given to humans and then tissues collected and subjected to ionizing radiation. The radioprotective effects of melatonin against cellular damage caused by oxidative stress and its low toxicity make this molecule a potential supplement in the treatment or co-treatment in situations where the effects of ionizing radiation are to be minimized.

Structural perspectives of phospholamban, a helical transmembrane pentamer

Phospholamban is a 52-amino-acid protein that assembles into a pentamer in sarcoplasmic reticulum membranes. The protein has a role in the regulation of the resident calcium ATPase through an inhibitory association that can be reversed by phosphorylation. The phosphorylation of phospholamban is initiated by beta-adrenergic stimulation, identifying phospholamban as an important component in the stimulation of cardiac activity by beta-agonists. In this role of phospholamban that has motivated studies in recent decades. There is evidence that phospholamban may also function as a Ca(2+)-selective ion channel. The structural properties of phospholamban have been studied by mutagenesis, modeling, and spectroscopy, resulting in a new view of the organization of this key molecule in membranes.

GLUT1 transmembrane glucose pathway. Affinity labeling with a transportable D-glucose diazirine

We synthesized a transportable diazirine derivative of D-glucose,3-deoxy-3,3-azi-D-glucopyranose (3-DAG), and studied its interaction with purified human erythrocyte facilitative glucose transporter, GLUT1. 3-DAG was rapidly transported into human erythrocytes and their resealed ghosts in the dark via a mercuric chloride-inhibitable mechanism and with a speed comparable with that of 3-O-methyl-D-glucose (3-OMG). The rate of 3-DAG transport in resealed ghosts was a saturable function of 3-DAG concentration with an apparent Km of 3.2 mM and the Vmax of 3.2 micromol/s/ml. D-Glucose inhibited the 3-DAG flux competitively with an apparent KI of 11 mM. Cytochalasin B inhibited this 3-DAG flux in a dose-dependent manner with an estimated KI of 2.4 x 10(-7) M. Cytochalasin E had no effect. These findings clearly establish that 3-DAG is a good substrate of GLUT1. UV irradiation of purified GLUT1 in liposomes in the presence of 3-DAG produced a significant covalent incorporation of 3-DAG into glut1, and 200 mM D-glucose abolished this 3-dag incorporation. Analyses of trypsin and endoproteinase Lys-C digestion of 3-DAG-photolabeled GLUT1 revealed that the cleavage products corresponding to the residues 115 183, 256 300, and 301 451 of the GLUT1 sequence were labeled by 3-DAG, demonstrating that not only the C-terminal half but also the N-terminal half of the transmembrane domain participate in the putative substrate channel formation. 3-DAG may be useful in further identification of the amino acid residues that form the substrate channel of this and other members of the facilitative glucose transporter family.

Molecular size of the functional complex and protein subunits of the renal phosphate symporter

The oligomeric structure of the rabbit renal brush-border membrane sodium/phosphate cotransporter was examined with the radiation inactivation and fragmentation technique. The size of its functional complex (its "radiation inactivation size") was estimated from the rate of decay of its sodium-dependent transport activity as a function of the radiation dose. A radiation inactivation size of 223 +/- 42 kDa was obtained. The polypeptide constituting the monomeric unit of the Na1+/Pi symporter was detected by immunoblotting with polyclonal anti-peptide antibodies directed against the 14 amino acid C-terminal portion of the symporter molecule. Its apparent molecular size estimated by comparison with standards following SDS-polyacrylamide gel electrophoresis was 64,000. This value is in good agreement with its known molecular mass of 51,797 Da calculated from the amino acid sequence deducted from the nucleotide sequence of its gene since this protein is probably glycosylated. The loss of labeling intensity of the polypeptide of M(r) = 64,000 was also measured as a function of radiation dose. The molecular size calculated from these data (its "target size") was 165 +/- 20 kDa. The target size estimated for the rat phosphate cotransporter was 184 +/- 46 kDa, and its previously reported radiation inactivation size was 234 +/- 14 kDa. These results strongly suggest that the renal Na1+/Pi cotransporter exists as an oligomeric protein, probably a homotetramer. The fact that the values obtained for the target size are about 3/4 those obtained for the radiation inactivation size of these cotransport proteins indicates that their subunits are closely associated since most of their subunits appear to be fragmented by a single ionizing radiation hit.

Damage to proteins due to the direct action of ionizing radiation

Proteins exposed to ionizing radiation suffer both reversible and irreversible effects. Reversible effects are defined as those which disappear in a short period of time after the removal of the radiation field and without further treatment of the sample. Irreversible effects are those which cause a permanent alteration in the structure of a protein.