Studies on the mechanism of action of ionizing radiations; inhibition of enzymes by X-rays

Gold-Protein Composite Nanoparticles for Enhanced X-ray Interactions: A Potential Formulation for Triggered Release

Drug-delivery vehicles have been used extensively to modulate the biodistribution of drugs for the purpose of maximizing their therapeutic effects while minimizing systemic toxicity. The release characteristics of the vehicle must be balanced with its encapsulation properties to achieve optimal delivery of the drug. An alternative approach is to design a delivery vehicle that preferentially releases its contents under specific endogenous (e.g., tissue pH) or exogenous (e.g., applied temperature) stimuli. In the present manuscript, we report on a novel delivery system with potential for triggered release using external beam radiation. Our group evaluated Zein protein as the basis for the delivery vehicle and used radiation as the exogenous stimulus. Proteins are known to react with free radicals, produced during irradiation in aqueous suspensions, leading to aggregation, fragmentation, amino acid modification, and proteolytic susceptibility. Additionally, we incorporated gold particles into the Zein protein matrix to create hybrid Zein-gold nanoparticles (ZAuNPs). Zein-only nanoparticles (ZNPs) and ZAuNPs were subsequently exposed to kVp radiation (single dose ranging from 2 to 80 Gy; fractionated doses of 2 Gy delivered 10 times) and characterized before and after irradiation. Our data indicated that the presence of gold particles within Zein particles was correlated with significantly higher levels of alterations to the protein, and was associated with higher rates of release of the encapsulated drug compound, Irinotecan. The aggregate results demonstrated a proof-of-principle that radiation can be used with gold nanoparticles to modulate the release rates of protein-based drug-delivery vehicles, such as ZNPs.

Heavy-ion beam-induced reactive oxygen species and redox reactions

Quantification and local density estimation of radiation-induced reactive oxygen species (ROS) were described focusing on our recent and related studies. Charged particle radiation, i.e. heavy-ion beams, are currently utilized for medical treatment. Differences in ROS generation properties between photon and charged particle radiation may lead to differences in the quality of radiation. Radiation-induced generation of ROS in water was quantified using several different approaches to electron paramagnetic resonance (EPR) techniques. Two different densities of localized hydroxyl radical (•OH) generation, i.e. milli-molar and molar levels, were described. Yields of sparse •OH decreased with increasing linear energy transfer (LET), the yield total •OH was not affected by LET. In the high-density, molar level, •OH environment, •OH can react and directly make hydrogen peroxide (H₂O₂), and then possible to form a high-density H₂O₂ cluster. The amount of total oxidation reactions caused by oxidative ROS, such as •OH and hydroperoxyl radial (HO₂^•), was decreased with increasing LET. Possibilities of the sequential reactions were discussed based on the initial localized density at the generated site. Water-induced ROS have been well investigated. However, little is known about radiation-induced free radical generation in lipidic conditions. Radio-chemistry to understand the sequential radio-biological effects is still under development.

Evolution of the Knowledge of Free Radicals and Other Oxidants

Free radicals are chemical species (atoms, molecules, or ions) containing one or more unpaired electrons in their external orbitals and generally display a remarkable reactivity. The evidence of their existence was obtained only at the beginning of the 20th century. Chemists gradually ascertained the involvement of free radicals in organic reactions and, in the middle of the 20th century, their production in biological systems. For several decades, free radicals were thought to cause exclusively damaging effects . This idea was mainly supported by the finding that oxygen free radicals readily react with all biological macromolecules inducing their oxidative modification and loss of function. Moreover, evidence was obtained that when, in the living organism, free radicals are not neutralized by systems of biochemical defences, many pathological conditions develop. However, after some time, it became clear that the living systems not only had adapted to the coexistence with free radicals but also developed methods to turn these toxic substances to their advantage by using them in critical physiological processes. Therefore, free radicals play a dual role in living systems: they are toxic by-products of aerobic metabolism, causing oxidative damage and tissue dysfunction, and serve as molecular signals activating beneficial stress responses. This discovery also changed the way we consider antioxidants. Their use is usually regarded as helpful to counteract the damaging effects of free radicals but sometimes is harmful as it can block adaptive responses induced by low levels of radicals.

Studies on the mechanism of action of ionizing radiations; inhibition of sulfhydryl enzymes by alpha, beta, and gamma rays

The activity of crystalline phosphoglyceraldehyde dehydrogenase and urease was decreased when dilute solutions of these sulfhydryl enzymes were irradiated with small doses of alpha rays from Po, beta rays from Si⁸⁹, and gamma rays from Ra. Partial reactivation of the enzyme by addition of glutathione was obtained after inhibition with alpha rays. Evidence that these inhibitions are due to oxidation of the —SH groups of the enzymes was given by the irradiation of the mercury-mercaptide urease with gamma rays. This irradiated complex was completely reactivated by glutathione as was the non-irradiated enzyme. The ionic efficiency of all these ionizing radiations on inhibition of phosphoglyceraldehyde dehydrogenase was similar (ionic yield around 1). The sulfhydryl groups of crystalline phosphoglyceraldehyde dehydrogenase were titrated by enzyme activity measurements and by ferricyanide oxidation.

The oxidation of thiols by ionizing radiations

Thiol compounds, such as glutathione, 2,3-dimercaptopropanol (BAL), propane-1,3-dithiol, and N-phenylaminopropanedithiol, were readily oxidized by x-rays, beta rays, and gamma rays. The ionic yield for this oxidation was about the same, 3 at pH 7, on irradiation with x-rays and with beta rays; it was 23 per cent less on irradiation with gamma rays. The ionic yield varied with the hydrogen ion concentration, increasing as the pH value increased. There was no reduction of oxidized glutathione on irradiation with dosages of x-rays and gamma rays which produced oxidation of the reduced compound. In the absence of oxygen, the oxidation of thiols by ionizing radiations was only 33 per cent of that obtained in the presence of dissolved oxygen. When the thiol solutions were irradiated in the presence of dissolved oxygen, catalase protected them from oxidation by 17 to 27 per cent.

The inactivation of dilute solutions of crystalline trypsin by x-radiation. II. Effects of enzyme concentration, medium, pH, and temperature

The proteolytic activity of dilute solutions of clystalline trypsin is destroyed by x-rays, the amount of inactivation being an exponential function of the radiation dose. The reaction yield increases steadily with increasing concentration of trypsin, varying, as the concentration of enzyme is increased from 1 to 300 µM, from 0.068 to 0.958 micromole of trypsin per liter inactivated per 1000 r with 0.005 N hydrochloric acid as the solvent, from 0.273 to 0.866 with 0.005 N sulfuric acid as the solvent, and from 0.343 to 0.844 with 0.005 N nitric acid as the solvent. When the reaction yields are plotted as a function of the initial concentration of trypsin, they fall on a curve given by the expression Y α X^K, in which Y is the reaction yield, X is the concentration of trypsin, and K is a constant equal to 0.46, 0.20, and 0.16, respectively, with 0.005 N hydrochloric, sulfuric, and nitric acids as solvents. The differences between the reaction yields found with chloride and sulfate ions in I to 10 µM trypsin solutions are significant only in the pH range from 2 to 4. The amount of inactivation obtained with a given dose of x-rays depends on the pH of the solution being irradiated and the nature of the solvent. The reaction yield-pH curve is a symmetrical one, with minimum yields at about pH 7. Buffers such as acetate, citrate, borate and barbiturate, and other organic molecules such as ethanol and glucose, in concentrations as low as 20 µM, inhibit the inactivation of trypsin by x-radiation. Sigmoid inactivation-dose curves instead of exponential ones are obtained in the presence of ethanol. The reaction yields for the inactivation of trypsin solutions by x-rays are approximately 1.5 times greater when the irradiation is done at 26°C. than when it is done at 5°C., when 0.005 N hydrochloric acid is the solvent. The dependence on temperature is less when 0.005 N sulfuric acid is used, and is negligible with 0.005 N nitric acid. The difficulties involved in interpreting radiation effects in aqueous systems, and in comparing the results obtained under different experimental conditions, are discussed.

Mechanism of the bactericidal action produced by electrohydraulic shock

Electrohydraulic shock was shown to produce oxidation reactions which inactivated certain compounds important in cellular metabolism. Enzymes that were inactivated included lactic dehydrogenase, trypsin, and proteinases of Bacillus subtilis. Free sulfhydryl groups and reduced nicotinamide adenine dinucleotide were oxidized. Adenosine triphosphate was destroyed, but deoxyribonucleic acid was not affected. Intracellular material of Escherichia coli lost its ability to absorb at 260 mmu after electrohydraulic shock. The bactericidal mechanism involved appeared to be due to nonselective oxidation reactions produced by high-voltage discharges in water. These oxidation reactions were probably mediated by free radicals produced in the water.

THE ROLE OF THIOL GROUPS IN THE DEVELOPMENT OF RADIATION DAMAGE IN THYMOCYTES AND EHRLICH ASCITES CARCINOMA CELLS

The effect of 800–1000 rads of X-irradiation on the thiol content of thymocytes and Ehrlich ascites carcinoma cells has been compared. Four hours after irradiation there was a decrease in the non-protein thiol (NP.SH) content of thymus and thymocytes but no change in ascites cells. In both cells the main NP.SH compound was glutathione. There was no significant effect of irradiation on the protein thiol (P.SH) content of thymus or ascites cells, but there was a slight decrease in P.SH in thymocytes after 4 hours incubation. Isolated thymus nuclei showed an immediate small decrease in P.SH content following 800 rads in vitro. Nuclei isolated from rat thymus 1 hour after 1000 rads in vivo showed an increase in the SH content of the globulin fraction and a decrease in the SH content of the nucleohistones. The total SH content of thymocytes and ascites cells was reduced by slow diffusion of H2O2into the cell suspension, but no effect of prior irradiation on this decrease of SH was found. Inhibition of catalase in vivo and in vitro did not produce any of the morphological signs of irradiation damage in thymocytes. There was no effect of irradiation on the copper content of thymus, thymocytes, or ascites cells. The ratio of NP.SH/P.SH is higher in thymocytes than in ascites cells, but, allowing for the difference in cell size, the overall total thiol concentration was the same. Anoxia produced only a small increase in NP.SH content in both cells and a small and doubtful increase in P.SH. It is concluded that, if thiol groups are involved in cell sensitivity to radiation, only a small fraction of the total SH groups are involved at critical sites.