Abstract
A cell line resistant to γ-irradiation was isolated from the original mouse strain L cells after seven successive irradiations with total dose of 14, 000R and was designated Lγ7. The various biochemical and genetical properties of these resistant cells were investigated.
The Lγ7 cells manifested a resistance to radiation, although the degree of resistance was not so high. The mean generation time of the original L cells was 27 hours during the logarithmic phase of growth, whereas that of the Lγ7 cells was 84 to 90 hours. Their cell morphology also differed from that of the original L cells; the Lγ7 cells were spherical, whereas the original L cells were fibrous.
The lower metabolic activity of oxidative phosphorylation in the Lγ7 cells seems to have a relation to their reduced multiplication rate and radioresistance. Furthermore, the content of SH groups per cell in both protein and non-protein SH reactions were significantly greater in the radioresistant Lγ7 cells than in the original L cells. The high content of SH groups in the radioresistant cells also may have some relation to their radioresistance.
The original L cells had a mode of chromosome number at 63. The Lγ7 cells had a mode of 44 chromosomes. Thus there was a shift in chromosome number. This decrease in chromosome number in the latter cells was gradually derived by seven repeated γ-irradiations of 2, 000R each. Similar phenomenon was observed in the changes in number of chromosomes in irradiated Ehrlich ascites tumor cells. The original Ehrlich ascites tumor cells had a mode around 58 chromosomes, whereas EX4 cells, isolated after four successive irradiation with total dose of 8, 000R of X-rays, had a mode of 51 chromosomes.
In the present investigation it was indicated that a correlation may exist between chromosome number and radioresistance in mammalian cells. In addition, the gradual shift in the chromosome number of the radioresistant cells (Lγ7 cells and EX4 cells) from the original L cells and Ehrlich ascites tumor cells seems to suggest that radioresistant cells may be derived from the pre-existing resistant cells in the original unirradiated population.
