Monte Carlo simulation of the production of short DNA fragments by low-linear energy transfer radiation using higher-order DNA models

A realistic DNA target model has been developed and implemented in the biophysical simulation code PARTRAC. It describes five levels of the B-DNA structure (nucleotides, DNA helices, nucleosomes, chromatin fiber structure and chromatin fiber loops) on an atomic level for the whole genome inside a mammalian cell nucleus. The model is capable of describing regular solenoidal, crossed-linker or zigzag structures as well as repeating stochastic arrangements of nucleosomes in the chromatin fiber. Electron tracks resulting from monoenergetic electrons with energies up to 100 keV and from 220 kVp X rays, starting at random positions in the cell, were superimposed on four DNA target models with different chromatin fiber structures. The yields of SSBs, DSBs and short single- and double-stranded DNA fragments were determined from spatial coincidences with strand atoms. Two parameters of the model-the energy necessary to create an SSB and the distance between two breaks that would be scored as a DSB-were adapted to equate simulated and measured strand break yields after X irradiation of human fibroblast cells. The integral fractions of short single- and double-stranded fragments were rather similar for all condensed chromatin fiber structures; they agreed with experimental data for DNA fragments below 2 kbp. The simulated fragment size distributions in the range from 0.1 to 1.5 kbp reflected the fiber structure irrespective of strandedness or electron energy. The distributions using a stochastic arrangement of nucleosomes in the chromatin fiber were found to be in better accordance with experimental data than those obtained with regular fiber structures.

Short-term effects of chlorophenols on the function and viability of primary cultured rat hepatocytes

Primary cultured rat hepatocytes were used as an experimental model to detect adverse effects of five chlorophenols (CP) in vitro (penta-CP, 2,3,4,5-tetra-CP, 2,4,5,-tri-CP, 2,4-di-CP, and 4-mono-CP). Monolayer cultures were exposed to the test compounds for 1 h, and concentration-response curves were established with respect to the effects on phase I and phase II metabolism of 7-ethoxycoumarin (7-EC) and on cellular ATP content. All CP tested inhibited the O-dealkylation of 7-EC, with half-maximum effective concentrations (EC50) ranging from about 36 microM for the three highest chlorinated phenols to 215 microM for 4-mono-CP, which proved to be least effective. The subsequent conjugation of the primary metabolite 7-hydroxycoumarin was even more sensitive towards CP exposure than the O-deethylation process. The concentrations which reduced the percentage of conjugated metabolite to 50% of the respective control cultures ranged from 7 microM for penta-CP to 48 microM for 4-mono-CP. Treatment of cultured hepatocytes with CP additionally resulted in a depletion of cellular ATP at EC50 concentrations ranging from 6 microM for penta-CP to 1330 microM for 4-mono-CP. Cellular viability, as measured by the leakage of lactate dehydrogenase from the cells, was not affected by any of the CP within the 1-h exposure period.