(1) The binding of a single oral dose of [3H]o-aminoazotoluene ([3H]o-AAT) to liver DNA, RNA and protein was examined in female C57 mice which had previously been fed unlabelled o-AAT in the diet for 2-8 weeks. A rapid decline in the level of binding of labelled material to DNA occurred during the first 2 weeks but thereafter the binding remained at a constant level. Binding to protein did not alter significantly during the experiment but there was a continual gradual decline in the binding of labelled material to RNA. (2) Examination of liver DNA, RNA and protein up to 84 days after a single dose of [3H]o-AAT showed that radioactivity associated with protein could no longer be detected after 28 days whereas thai in DNA and RNA was present at 84 days. These results suggest either the existence of a long-lived species of RNA or that o-AAT stabilises a species of mouse-liver RNA. (3) Chronic feeding of o-AAT in the diet for 4 weeks reduced the incorporation of [3H]uridine into liver RNA compared with that in untreated mice, although no alteration in the rate of breakdown of RNA was found.
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