BINDING OF METABOLICALLY ACTIVATED BENZO(A)PYRENE TO NUCLEAR MACROMOLECULES

Abstract

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. On incubation with isolated liver nuclei and NADPH [3H]BP bound to nuclear components. The binding appeared to be covalent in nature. Treatment of the rats with 3-methylcholanthrene induced nuclear aryl hydrocarbon hydroxylase (AHH) activity and also increased the level of carcinogen binding. The addition of rat liver microsomes to the incubation systems enhanced the level of [3H]BP binding to the macromolecules in the nuclei from control and 3-methylcholanthrene-treated rats, and the maximal levels of binding obtained with these 2 types of nuclei were similar. Binding was inhibited by 7,8-benzoflavone or glutathione. Lung nuclei from control rats and 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 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 NADPH added), the radioactivity associated with the macromolecule fractions is probably a background value rather than due to binding caused by a specific interaction between benzo(a)pyrene and cytochrome P-450. 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 endoplasmic reticulum and the nuclear envelope are potentially important sites of carcinogen activation.