Preservative agents in foods. Mode of action and microbial resistance mechanisms.

After long postirradiation incubation periods, the residual frequency of prematurely condensed chromosome fragments following X-ray exposure of noncycling diploid human fibroblasts was found to be correlated with the frequency of chromosome aberrations observed under identical treatment conditions when the cells were subcultured and scored after they reached mitosis. Over a wide range of doses, the proportion of such cells without aberrations at their first metaphase was not significantly different from the proportion able to form macroscopic colonies. Further, the rate of rejoining of interphase chromosome breaks was the same as the rate of increase in survival due to the repair of potentially lethal damage (PLD). These results suggest that there is a one-to-one correspondence between the initial breakage and rejoining of G0 chromosomes and the induction and repair of PLD measured by delayed plating from plateau-phase cultures of these cells.

A quantitative comparison of potentially lethal damage repair and the rejoining of interphase chromosome breaks in low passage normal human fibroblasts

This paper presents data on modelling of DNA damage induced by electrons, protons and alpha-particles to provide an insight into factors which determine the biological effectiveness of radiations of high and low linear energy transfer (LET). These data include the yield of single- and double-strand breaks (ssb, dsb) and base damage in a cellular environment. We obtain a ratio of 4–15 for ssb:dsb for solid and cellular DNA and a preliminary ratio of about 2 for base damage to strand breakage. Data are also given on specific characteristics of damage at the DNA level in the form of clustered damage of varying complexity, that challenge the repair processes and if not processed adequately could lead to the observed biological effects. It is shown that nearly 30% of dsb are of complex form for low-LET radiation, solely by virtue of additional breaks, rising to about 70% for high-LET radiation. Inclusion of base damage increases the complex proportion to about 60% and 90% for low- and high-LET radiation, respectively. The data show a twofold increase in frequencies of complex dsb from low-LET radiation when base damage is taken into account. It is shown that most ssb induced by high-LET radiation have associated base damages, and also a substantial proportion is induced by low-energy electrons.

Quantitative modelling of DNA damage using Monte Carlo track structure method.

We suggest that the observed experimental data on relative double-strand break (dsb) yield as a function of radiation quality can act as valuable constraints in defining the type of energy deposition which causes this basic lesion in radiation biology. Both heavy-ion and α-particle data show sufficient trends for quantitative comparisons with calculation to be made. We use the technique of track-structure simulation and search for energy-deposition clusters (containing at least a given number of ionizations in a given diameter) whose relative frequencies (compared to sparsely ionizing radiation) correlate with the relative biological effects (RBEs) for dsb induction. We conclude that locally multiply damaged sites (LMDS) which cause dsb are probably energy depositions of at least two to five ionizations localized, respectively, in sites of diameters of 1–4 nm. Although our derived cluster sizes should be viewed in light of the quality of the experimental data and uncertainties in the computer simulations ...

Constraints on energy deposition and target size of multiply damaged sites associated with DNA double-strand breaks.

Studies of ionizing radiations of different quality are discussed with particular emphasis on damage to DNA of mammalian cells. Three related themes are followed. Firstly, inactivation and mutation experiments with ultrasoft X-rays and slow heavy ions, coupled with theoretical analyses of the structures of the radiation tracks, have emphasized the biological importance of localized track features over nanometre dimensions. This led to the suggestion that the critical physical features of the tracks are the stochastic clusterings of ionizations, directly in or very near to DNA, resulting in clustered initial molecular damage including various combinations of breaks, base damages, cross-links, etc. in the DNA. The quantitative hypotheses imply that final cellular effects from high-LET radiations are dominated by their more severe, and therefore less repairable, clustered damage, and that these are qualitatively different from the dominant low-LET damage. Second, relative effectiveness of different types of ...

Effects of Radiations of Different Qualities on Cells: Molecular Mechanisms of Damage and Repair

Induction of DNA double-strand breaks in diploid wild-type yeast cells, and inactivation of diploid mutant cells (rad54-3) unable to repair DNA double-strand breaks, were studied with aluminium K (1.5 keV) and carbon K (0.278 keV) characteristic X-rays. The induction of DNA double-strand breaks was found to increase linearly with absorbed dose for both characteristic X-rays. Carbon K X-rays were more effective than aluminium K X-rays. Relative to 60Co gamma-rays the r.b.e.-values for the induction of DNA double-strand breaks were found to be 3.8 and 2.2 for carbon K and aluminium K X-rays respectively. The survival curves of the rad54-3 mutant cells were exponential for both ultrasoft X-rays. For inactivation of rad54-3 mutant cells, the r.b.e.-values relative to 60Co gamma-rays were 2.6 and 2.4 for carbon K and aluminium K X-rays, respectively. The DNA double-strand break data obtained with aluminium K and carbon K X-rays are in agreement with the data obtained for gene mutation, chromosome aberrations and inactivation of mammalian cells, suggesting that DNA double-strand breaks are the possible molecular lesions leading to these effects.

Effectiveness of 1.5 keV aluminium K and 0.3 keV carbon K characteristic X-rays at inducing DNA double-strand breaks in yeast cells.

SummaryYeast cells were irradiated with 3·5 MeV α-particles and 30 MeV electrons, as reference radiation. The kinetics of DNA double-strand break (dsb) rejoining during incubation of cells under non-growth conditions (PLDR conditions) were measured using the neutral sedimentation technique. A monophasic kinetic was found after irradiation of cells with α-particles, with a dose-independent t1/2 value of about 13 h. The kinetics of rejoining of dsb induced by 30 MeV electrons was found to be biphasic, with dose-independent t1/2 values of 3·8 h for the initial and of about 11 h for the slow component. The fraction of the slow component was, however, dose-dependent. These kinetics were measured for both types of radiation at doses yielding high surviving fractions (5% up to 100%). Dsb are induced linearly with dose of both radiations. The RBE value of α-particles was found to be 2·5 for initial dsb. The RBE of α-particles increased as a consequence of dsb rejoining. This increase in RBE value suggests that DS...

A comparative study of rejoining of DNA double-strand breaks in yeast irradiated with 3.5 MeV alpha-particles or with 30 MeV electrons.

DNA double-strand breaks are the molecular lesions the repair of which leads to the reappearance of the shoulder observed in split-dose experiments. This conclusion is based on results obtained with the help of a diploid yeast mutant rad 54-3 which is temperature-conditional for the repair of DNA double-strand breaks. Two repair steps must be met to yield the reappearance of the shoulder on a split-dose survival curve: the repair of double-strand breaks during the interval between two doses and on the nutrient agar plate after the second dose. In yeast lethality may be attributable to either an unrepaired double-strand break (i.e. a double-strand break is a potentially lethal lesion) or to the interaction of two double-strand breaks (misrepair of double-strand breaks). Evidence is presented that the two cellular phenomena of liquid holding recovery (repair of potentially lethal damage) and of split-dose recovery (repair of sublethal damage) are based on the repair of the same molecular lesion, the DNA double-strand break.

Split-dose recovery is due to the repair of DNA double-strand breaks.

Evidence is presented that in yeast cells one DNA double-strand break (dsb) may be considered as one potentially lethal lesion (PLL). Using a temperature conditional radiosensitive diploid yeast mutant (rad 54-3) it is demonstrated that the shoulder of survival curves, for cells plated immediately, is due to repair of dsb (PLL) within a restricted time period. Split dose experiments with the mutant rad 54-3 show that the reappearance of a shoulder is observed when two conditions are met: (1) repair of dsb (PLL) during the time interval between doses and (2) repair of mainly those dsb (PLL) which are induced by the second dose on the nutrient agar plate. Irradiation of wild type yeast cells at high dose rate followed by liquid holding treatment for 72 h (delayed plating, DP) or at low dose rate show that the bending of DP-survival curves is due to the accumulation of dsb (PLL) leading to lethal lesions probably by misrepair of dsb.

Interpretation of the shape of survival curves in terms of induction and repair/misrepair of DNA double-strand breaks.

DNA double-strand break (dsb) induction in diploid yeast was measured by neutral sucrose sedimentation after exposure to very heavy ions with values of linear energy transfer (LET) ranging from about 300 to 11500 ke V/microns. Linear fluence dependencies were found in all cases from which dsb production cross-sections (sigma dsb) could be calculated. Corresponding cross-sections for cell killing (sigma i) were derived from final slopes of survival curves measured in parallel and for the same fluence range. A close correlation was found between sigma i and sigma dsb. It is calculated that over the entire LET range, including 30 MeV electron irradiation, about 22 dsb are induced per lethal event when high exposures are considered.

M. Frankenberg-Schwager

Papers

Effectiveness of 1.5 keV aluminium K and 0.3 keV carbon K characteristic X-rays at inducing DNA double-strand breaks in yeast cells.

A comparative study of rejoining of DNA double-strand breaks in yeast irradiated with 3.5 MeV alpha-particles or with 30 MeV electrons.

Split-dose recovery is due to the repair of DNA double-strand breaks.

Interpretation of the shape of survival curves in terms of induction and repair/misrepair of DNA double-strand breaks.

Heavy ion-induced DNA double-strand breaks in yeast.