Effect of bleomycin on deoxynucleotide-polymerizing enzymes from human cells.

DNA polymerases alpha and beta from Molt-4 cells are inhibited by bleomycin, whereas DNA polymerase gamma assayed with poly-(A)-(dT)12-18 as the template primer or terminal deoxynucleotidyl transferase assayed with activated DNA, poly(dA), (dG)12-18 or (dA)12-18 as the initiator are not inhibited by this antibiotic. Inhibition by bleomycin increased the Km for template DNA but not that for dTTP. Increasing amounts of bleomycin did not affect the Vmax for DNA polymerase alpha or beta when the amount of template DNA was varied but it reduced the Vmax for these enzymes when dTTP was varied. Moreover, the addition of extra template reversed the bleomycin inhibition but the addition of extra enzyme did not. Although dithiothreitol was required for bleomycin inhibition of DNA polymerase activity, bleomycin preincubated with dithiothreitol (or beta-mercaptoethanol) at pH 6.5 to 9.0 lost its inhibitory activity. This was not the case when DNA was also included in the preincubation mixture. The results obtained in this study indicate that bleomycin inhibits DNA polymerases alpha and beta by a thiol reagent-dependent interaction with the template. Thus, the antitumor activity of bleomycin may be greatly influenced by the concentration of sulfhydryl compounds and their proximity to DNA in the target cells.