Binding of metabolically activated benzo(a)pyrene to nuclear macromolecules

The binding of metabolically activated [3H]benzo(a)pyrene ([3H]BP) to the DNA, RNA, histones, and nonhistones of isolated rat liver and lung nuclei was studied. Conditions for optimal binding to the nuclear components were determined. Upon incubation with isolated liver nuclei and reduced nicotinamide adenine dinucleotide phosphate, [3H]BP was able to bind to nuclear components. The binding appeared to be covalent in nature. Treatment of the rats with 3-methylcholanthrene induced the nuclear aryl hydrocarbon hydroxylase (AHH) activity and also increased the level of carcinogen binding. The addition of rat liver microsomes to the incubation systems greatly enhanced the level of [3H]BP binding to the macromolecules in the nuclei from both the control and 3-methylcholanthrene-treated rats, and the maximal levels of binding obtained with these two types of nuclei were similar. The binding was inhibited by 7,8-benzoflavone or glutathione. Lung nuclei from control rats had very low AHH activity and did not exhibit appreciable carcinogen binding, whereas those from 3-methylcholanthrene-pretreated animals had slightly higher AHH activity and caused low levels of binding. The binding of [3H]BP to lung nuclei was greatly enhanced by liver microsomes but only slightly by lung microsomes, which had rather low AHH activity. Several lines of evidence indicate that, in the control experiments (no reduced nicotinamide adenine dinucleotide phosphate added), the radioactivity associated with the macromolecule fractions is probably a background value rather than due to the binding caused by a specific interaction between benzo(a)pyrene and cytochrome P-450. The present study clearly demonstrates that a carcinogen activated at the microsomes can enter into the nucleus and react with its macromolecules; the carcinogen can also be activated by the monoxygenase system of the nuclear envelope. It appears that both the endoplasmic reticulum and the nuclear envelope are potentially important sites of carcinogen activation.