AKR-2B mouse embryo cells were incubated for 24 hr with [3H]benzo(a)pyrene, and the histones were isolated and analyzed using one- and two-dimensional gel electrophoresis and autoradiography. The results revealed that (a) histones H1, H2A, and H3 incorporated significant amounts of label whereas little or no label was associated with histones H2B and H4 and (b) electrophoresis of the histones in the Triton:acid:urea gel system caused labeled histones to have a slower migration than did the corresponding unlabeled histones. Additional studies such as incubation of (±)-7β,8α-[3H]dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene with nuclei resulted in radioactive labeling of histones H1, H2A, H2B, and H3 and of high-mobility-group proteins HMG1 and HMG2. The low levels of label associated with histone H4 in the whole-cell and nuclear studies were further investigated by incubating isolated histones with (±)-7β,8α-[3H]dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo-(a)pyrene. Under these conditions, negligible amounts of radioactivity were associated with H4, while significant labeling of H1, H2A, H2B, and H3 and other nuclear proteins was observed. The results suggest that factors other than the presence of suitable nucleophilic acceptor sites on the histones may be necessary for carcinogen binding.
This research was supported by Research Grant CA 22272 from the National Cancer Institute, the National Fraternal Order of the Eagles, and Mayo Clinic and Foundation.