DNA damage by various radiations

Inactivation kinetics of Escherichia coli by pulsed electron beam

A novel and compact low-energy (keV) high-power pulsed electron beam (e-beam) that utilizes a secondary emission electron gun (SEEG) was designed and constructed. Escherichia coli JM 109 at a concentration of 10(6) CFU/mL was spread-plated on Luria-Bertani (LB) medium and subjected to the SEEG e-beam. The e-beam was administered as 1 or 5 pulses. The duration of a single pulse was constant at 5 micros, e-beam current density was constant at 25 mA/cm2, and e-beam energy varied between 60 and 82.5 keV. Following treatment with the SEEG e-beam, survivors of the irradiated E. coli samples were enumerated by a standard 10-fold dilution and spread-plated. The survivor curves were plotted on logarithmic scale as a function of e-beam dose. The D10-values were calculated as a negative reciprocal of the slope of the survivor curves. The D10-values for E. coli inactivated with 1- and 5-pulse SEEG e-beam were 0.0026 and 0.0217 Gy, respectively. These D10-values were considerably lower than published D10-values for E. coli inactivated with conventional high-energy continuous e-beam, likely due to shorter exposure time (t), greater current density (J), and a pulse mode of the SEEG e-beam. The SEEG e-beam showed promising results for microbial inactivation in a nonthermal manner; however, due to low energy of the SEEG e-beam, current applications are limited to surface decontamination. The SEEG e-beam may be an efficient processing step for surface inactivation of food-borne pathogens on ready-to-eat products, including fresh and leafy vegetables.

A novel method of measuring hydroxyl radical-scavenging activity of antioxidants using gamma-irradiation

A new method using ESR spin trapping was proposed for measuring the scavenging activity of antioxidants for the hydroxyl (OH) radical. (-)-Epigallocatechin gallate (EGCg) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were used as the antioxidant and spin trapping agent, respectively. The conventional method using a Fenton reaction had problems associated with the estimation of activity, because the antioxidant disturbs the system for generating OH radical by coordinating on Fe2+ and by consuming H2O2, besides scavenging the spin adduct (DMPO-OH). Intense gamma-irradiation was therefore used to generate OH radicals, and the intensity decrease in DMPO-OH after irradiation was followed to obtain the rate constant for the scavenging of DMPO-OH by EGCg. The intensities were extrapolated to zero time to estimate the quantity of DMPO-OH formed during gamma-irradiation. By using these values, the reaction rate constant between OH radical and EGCg was calculated as a ratio to that of DMPO. It was shown that this method is useful for comparing the OH radical-scavenging activity of various antioxidants.

Spin-trapping study on the hydroxyl radical formed from a tea catechin-Cu(II) system

A spin-trapping method was applied to examine the formation of the hydroxyl (OH) radical from a tea catechin-Cu(II) system to elucidate a previous result that some tea catechin-Cu(II) systems induced DNA scission. Three tea catechins, (-)-epigallocatechin (EGC), (-)-epigallocatechin gallate (EGCg) and (-)-epicatechin (EC), were used. The spin-trapping agent, 5,5'-dimethyl-pyrroline-1-oxide (DMPO), was dissolved in a pH 9 phosphate buffer solution, then a catechin and Cu(II) were added in that order, and the ESR spectral change was monitored for one hour. The order of adding the catechin and Cu(II) was then reversed, and the ESR spectral change was again monitored to examine the coordinating activity of each catechin toward the Cu(II) ion and the effect on OH radical generation. The intensity changes of the spin adducts, DMPO-OH, DMPO-CH3 and DMPO-H, were analyzed, the results suggesting that the OH radical generated in the system decomposed DMPO, resulting in the formation of DMPO-CH3 and DMPO-H. The results show that EGC formed a stable complex with Cu(II) and generated the OH radical. EGCg seemed to have this activity, but the OH radical that was generated was scavenged by the gallate group existing in the complex. EC did not show strong coordinating and OH-generating activities. These characteristics of the three catechins are consistent with the results shown for DNA scission.