Chain length heterogeneity of nucleosomal DNA in mouse liver after dimethylnitrosamine administration

In vivo distribution of dimethylnitrosamine metabolites in mouse liver and their binding to micrococcal nuclease sensitive and resistant chromatin

Mice were injected i.p. with a single dose (5 mg/kg body wt) of [14C]dimethylnitrosamine and killed at time intervals between 15 and 120 min. Isolated nuclei were incubated with micrococcal nuclease and the chromatin separated into a 1100 g pellet P1 and supernatant fraction S1. The incorporation of 14C from [14C]dimethylnitrosamine into chromatin was significantly higher in the P1 than the S1 fraction. The purine bases of the P1 DNA showed a lower methylation than those of the S1 DNA. In contrast, radioactivity of the proteins was higher in the P1 than the S1 fraction. It is concluded that the open structures of the S1 chromatin were preferentially attacked by the hepatotoxin leading to a high 14C-labelling of the DNA relative to that of the proteins.

Non-random effect on RNA synthesis in liver chromatin by administration of dimethylnitrosamine to mice

The effect of dimethylnitrosamine on functional activities of liver chromatin was studied in mice. After a single dose of dimethylnitrosamine injected i.v. (25 mg/kg body wt, 45 min before sacrifice) liver nuclei were isolated and incubated with micrococcal nuclease (EC 3.1.4.7) to an acid-solubility of 2.5% of total DNA. Chromatin was fractionated into a 1,200 g pellet P1, 102,000 g pellet P2 and supernatant fraction S2. Chromatin-bound RNA polymerase I plus III activity decreased 15% in the P1 and 25% in the P2 fraction. No changes in activity were observed in the S2 fraction. Chromatin-bound RNA polymerase II activity decreased 19% in the P1, 49% in the P2 and 32% in the S2 fraction. Heparin stimulated RNA polymerase II activity decreased 10% in the P1 and 44% in the P2 fraction. Formation of initiation in nuclear lysates with RNA polymerase from Escherichia coli increased after administration of dimethylnitrosamine suggesting an increase in the number of sites available for the start of new RNA chains. The results show that limited digestion of nuclei with endonuclease cleaves chromatin regions which are more affected by dimethylnitrosamine than the total chromatin suggesting a non-random effect of the hepatotoxin on chromatin. Modifications of the DNA template by dimethylnitrosamine is indicated by the change in number of initiation complexes.