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The possibility of polarity breakage in sister-chromatid exchanges (SCEs) — Analyses of SCEs in ring chromosomes and diplochromosomes
381 61 Sofuni, T., M. Hatanaka and M. Ishidate Jr., National Institute of Hygienic Sciences, Tokyo (Japan) Induction of chromosomal aberrations in cultured Chinese hamster cells in a superoxide-generating system The induction of chromosomal aberrations in a superoxide-generating system using xanthine oxidase and hypoxanthine was investigated in cultured Chinese hamster cells. The production of chromosomal aberrations in this system was inhibited by the addition of cytochrome c. This finding indicates that the generation of superoxide was primarily essential for inducing the chromosomal aberrations. On the other hand, superoxide dismutase showed no effect on the frequency of chromosomal aberrations, whereas catalase was effective in preventing the aberrations. It is conceivable, therefore, that the induction of chromosomal aberrations in the superoxide-generating system may be directly or indirectly due to hydrogen peroxide that formed in the cultured medium as a result of spontaneous dismutation reaction of superoxide.
62 Sutou, S., and S. Sato, NRI Life Science, 4-7-1 Kajiwara, Kamakura, Kanagawa (Japan) The possibility of polarity breakage in sister-chromatid exchanges (SCEs) - - Analyses of SCEs in ring chromosomes and dipiochromosomes A cell line containing a ring chromosome was cultured in the presence of colcemid and BUdR for 2 cell cycles and morphological changes of rings and SCEs were analyzed. More than one-third of symmetric dicentric rings had no or asymmetric SCEs, suggesting that polarity was not always conserved and that, even if inverted polarity sequences were repaired, repair was not perfect.
Twin and single SCEs can be counted separately in diplochromosomes. Observed ratios of singles to twins were not 2 but around 3. Induction treatment for endoreduplication caused no increment of single SCEs. BUdR-substituted DNAs showed slightly increased numbers of both twin and single SCEs and had nothing to do with the ratios, about 3. Lesions in S1 slightly increased twin SCEs but mainly contributed to the increment of single SCEs, indicating that at least some SCEs were formed in two distinct steps, initiation and manifestation. Overall, results are consistent with the random-rejoining and random-repair hypothesis of SCEs.
63 Suzuki, E., M. Osabe, M. Mochizuki and M. Okada, Tokyo Biochemical Research Institute, Tokyo (Japan) Correlation between mutagenicity and hepatocytemediated metabolic activation of N-nitroso- and N-nitrodialkylamines After coincubation of N-nitroso- or Nnitrodialkylamines (alkyl = ethyl, butyl) and hepatocytes (isolated from Sprague-Dawley rats by a collagenase perfusion technique) with E. coli WP2 uvrA/pkMlO1 up to 2 h, their mutagenicity and metabolic activation by a-hydroxylation were studied. A good correlation was observed between mutagenic potency and a-hydroxylation rate of the N-nitrodialkylamines, both being higher with the ethyl compound than with the butyl homolog. The m u t a g e n i c i t y was m o r e p o t e n t with Nnitrosodiethylamine than with the butyl homolog although disappearance of the substrate is faster in the latter. Since a reverse relationship was observed between the mutagenicity and metabolic activation with respect to the alkyl group when microsomes or $9 mix were used as an activation system in both series of substrates, the hepatocyte-mediated assay correlated better with carcinogenic potency of the N-nitroso- and Nnitrodialkylamines tested.
62 Sutou, S., and S. Sato, NRI Life Science, 4-7-1 Kajiwara, Kamakura, Kanagawa (Japan) The possibility of polarity breakage in sister-chromatid exchanges (SCEs) - - Analyses of SCEs in ring chromosomes and dipiochromosomes A cell line containing a ring chromosome was cultured in the presence of colcemid and BUdR for 2 cell cycles and morphological changes of rings and SCEs were analyzed. More than one-third of symmetric dicentric rings had no or asymmetric SCEs, suggesting that polarity was not always conserved and that, even if inverted polarity sequences were repaired, repair was not perfect.
Twin and single SCEs can be counted separately in diplochromosomes. Observed ratios of singles to twins were not 2 but around 3. Induction treatment for endoreduplication caused no increment of single SCEs. BUdR-substituted DNAs showed slightly increased numbers of both twin and single SCEs and had nothing to do with the ratios, about 3. Lesions in S1 slightly increased twin SCEs but mainly contributed to the increment of single SCEs, indicating that at least some SCEs were formed in two distinct steps, initiation and manifestation. Overall, results are consistent with the random-rejoining and random-repair hypothesis of SCEs.
63 Suzuki, E., M. Osabe, M. Mochizuki and M. Okada, Tokyo Biochemical Research Institute, Tokyo (Japan) Correlation between mutagenicity and hepatocytemediated metabolic activation of N-nitroso- and N-nitrodialkylamines After coincubation of N-nitroso- or Nnitrodialkylamines (alkyl = ethyl, butyl) and hepatocytes (isolated from Sprague-Dawley rats by a collagenase perfusion technique) with E. coli WP2 uvrA/pkMlO1 up to 2 h, their mutagenicity and metabolic activation by a-hydroxylation were studied. A good correlation was observed between mutagenic potency and a-hydroxylation rate of the N-nitrodialkylamines, both being higher with the ethyl compound than with the butyl homolog. The m u t a g e n i c i t y was m o r e p o t e n t with Nnitrosodiethylamine than with the butyl homolog although disappearance of the substrate is faster in the latter. Since a reverse relationship was observed between the mutagenicity and metabolic activation with respect to the alkyl group when microsomes or $9 mix were used as an activation system in both series of substrates, the hepatocyte-mediated assay correlated better with carcinogenic potency of the N-nitroso- and Nnitrodialkylamines tested.