The 14C activity of [14C]bleomycin bound to DNA in bleomycin-sensitive rat ascites hepatoma cells (AH-66) was 8.7 times higher than in resistant cells (AH-66F) when the cells were incubated with [14C]bleomycin. The difference in permeability to bleomycin was not significant; uptake of [14C]bleomycin by the sensitive cells was only 1.2 times larger than that by the resistant cells, and the radioactivity incorporated into the nuclei of sensitive cells was only 1.3-fold greater. The bleomycin-inactivating enzyme level in the resistant cells was 3.5 times higher than in the sensitive cells, indicating that the antibiotic incorporated into the resistant cells was reduced in DNA-binding activity to a large extent. The level of protein-free thiol compound in the sensitive cells was 1.8-fold higher than in the resistant cells, suggesting a possible enhancement of bleomycin action by intracellular thiol compound as is found in vitro. These factors probably affect the DNA strand scission and the sensitivity of cells to this antibiotic.

Binding of [14C]bleomycin to DNA in vitro was studied in the presence and the absence of dithiothreitol. A large portion of the radioactivity bound in the presence of dithiothreitol was unstable to acid, but the acid-resistant binding was also enhanced by this thiol compound.

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This work was supported in part by a research grant for cancer research from the Ministry of Education of Japan.

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